WO2010003629A2

WO2010003629A2 - Thermoelectric apparatus and methods of manufacturing the same

Info

Publication number: WO2010003629A2
Application number: PCT/EP2009/004897
Authority: WO
Inventors: Volker Schmidt; Stephan Senz; Ulrich GÖSELE
Original assignee: Max-Planck-Gesellschaft Zur Förderung Der Wissenschaft E. V.
Priority date: 2008-07-08
Filing date: 2009-07-07
Publication date: 2010-01-14
Also published as: WO2010003629A3

Abstract

A thermoelectric apparatus comprises at least one pair of adjacent portions of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions. Said first surface regions of each said pair are coupled either directly, or indirectly via a metallic conductor, or indirectly via a doped electrically conductive semiconductor and said second surface regions are electrically connectable to an external circuit. The p-type and n-type semiconductor material comprises porous semiconductor material which allows a high dimensionless figure of merit to be achieved for either thermo-electrical power generation or thermo electrical cooling.

Description

Thermoelectric Apparatus And Methods Of Manufacturing The Same

The present invention relates to a thermoelectric apparatus such as a thermoelectric power generating apparatus or a thermoelectric cooling apparatus and to methods of manufacturing the same.

Thermoelectrics deals with materials that can convert heat into electric power or vice versa. This mainly comprises two effects, the Peltier effect and the Seebeck effect. The Peltier effect describes the observation that when an electric current is driven through a thermoelectric material electrically contacted at two points, then a temperature difference is established between the two contacts. This effect is mostly employed for thermoelectric cooling.

The Seebeck effect is in some sense the inverse effect. It relates to the observation that when a temperature difference is applied to a thermoelectric material that is electrically contacted at two points, a voltage drop occurs between the two contacts. The proportionality factor relating the temperature difference to the voltage drop is called the Seebeck coefficient S (in units V/ K).

The Seebeck effect is mostly employed in thermoelectric power generation. Although such a direct conversion of heat into electric power seems very attractive from an ecological point of view, especially when it concerns the conversion of waste heat, it is often economically not viable. Today, thermoelectric power generation is basically restricted to niche applications, where local power generation is much favored over non-local solutions, that is under conditions where establishing an electrical connection to the outside world is impossible or too expensive (e.g. self-sustaining sensor apparatuses) and/or where cost does not matter (e.g. space or military applications).

A large scale use of thermoelectric power generators is hampered by the poor conversion efficiency of these modules and the corresponding high costs per kWh. Typical total conversion efficiencies are of the order of 10% or below. In this context, the so called dimensionless figure of merit ZT has proven to be a very useful quantity. It is defined as

ZT=S2 _σ T/ K,

with T being the absolute temperature, and K and σ being the specific thermal conductivity and the specific electrical conductivity, respectively. The higher the figure of merit, the higher is the conversion efficiency. This is the most important challenge; to develop materials with a maximum thermoelectric figure of merit, best would be ZT=2-3 or even larger.

A basic description of a thermoelectric apparatus is given in the document "Introduction to Thermoelectrics" which was available on the Internet under http:/ /www.thermoelectrics.com/introduction.htm on June 24, 2008.

That document explains how the Seebeck effect, which was discovered in 1821, can be used for power generation. It is stated that Seebeck recognized that the naturally found semiconductors ZnSb and PbS could be used to construct a thermoelectric generator having an efficiency of around 3 %. It explains how the Seebeck coefficient is defined as the open circuit voltage produced between two points on the conductor, when a uniform temperature difference of I⁰K exists between those two points.

Furthermore, that document describes, under the heading "Thermoelectric materials" that Altenkirch recognized that good thermoelectric materials should have large Seebeck coefficients, high electrical conductivity and low thermal conductivity, with a high electrical conductivity being necessary to minimize Joule heating, whereas a low thermal conductivity helps to retain heat at the conjunctions and maintain a large temperature gradient.

The document also shows a simple thermoelectric power generating apparatus consisting of a thermocouple comprising p-type and n-type thermoelements connected electrically in series and thermally in parallel. It explains how heat is pumped into one side of the thermocouple and rejected from the opposite side. The electrical current produced is proportional to temperature gradient between the hot and the cold conjunctions. The reference also describes a simple thermoelectric cooling apparatus which is essentially similar to the thermoelectric power generating apparatus except that there is now an electrical power input at the hot side of the device where heat is dissipated and this results in heat being extracted from the cold junction where the p-type and n-type semiconductor materials meet at a metallic conductor.

Finally, the reference also describes how a thermoelectric module can be constructed which consists of pairs of p-type and n-type semiconductor thermoelements forming thermocouples which are connected electrically in series and thermally in parallel. This module can be used either for heating or for cooling. In the cooling mode an electrical current is supplied to the module, heat is pumped from one side to the other in accordance with the Peltier effect so that one side of the module becomes cold.

In the power generating mode a temperature gradient is maintained across the module and the heat flux passing through the module is converted into electrical power in accordance with the above mentioned Seebeck effect.

The most commonly used thermoelectric (TE) material nowadays is probably Bi2Te3. B.2Te3 has a good figure of merit ZT of around 0.7 at room temperature (as can be seen from Fig. 1).

Thus, for many practical applications like cooling or low temperature power generation, Bi2Tβ3 or related compounds like (Bio.25Sbo.7s)2Te3 or Bi2(Teo.9Seo.i)3 are currently the material of choice. Considering power generation, with the hot side of the thermoelectric module heated up to 400- 500 ⁰C, PbTe is most efficient. However, all those materials have one serious drawback, which is the use of tellurium. This is because Te is an extremely rare element. According to the U.S. Geological Survey (minerals. usgs.gov/ds/2005/ 140/-tellurium.pdf), annual worldwide Te production was merely 132 tons in 2006. An annual production of 132 tons sets severe limits for a mass production of bulk Te-compound based thermoelectric generators. In particular, an increased Te demand at limited supply will inevitably boost Te prices even further. Te prices already increased sharply" in the last 5 years.

Other critical issues in particular with regard to PbTe concern toxicity and environmental issues, as the RoHS regulation (directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, Regulation 2002 /95/ EC of the European Union). This regulation severely restricts the use of Pb-containing materials.

Silicon in contrast is the second most abundant element on earth and it is non-toxic. However, at low temperatures (say below 400K), Si is a poor thermoelectric material with a figure of merit of about ZT=O.01 at room temperature. This is because Si has a high thermal conductivity at low temperatures (about 150 W/mK as published by M. Ashegi et al. in Appl. Phys. Lett 71 (13) 1798- 1800 (1997)).

A reduction of the thermal conductivity by about one order of magnitude can be achieved by adding a few percent of Ge. Correspondingly, ZT increases by about one order of magnitude, but is still very poor at low temperatures. Only at very high temperatures (1200⁰C as can be seen from Fig. 1) SiGe reaches ZT values of around one. However, considering ther- moelectrics such high temperature applications are technically challenging and of rather low commercial interest.

It is a principal object of the present invention to propose materials with a large Seebeck coefficient S, a large electrical conductivity σ and a low thermal conductivity K, which are well suited for use in thermoelectric apparatus and which have a large thermoelectric figure of merit.

In order to satisfy this object there is provided, in accordance with a first embodiment of the present invention, a thermoelectric apparatus comprising at least two adjacent portions of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions, said portions of p-type and n-type semiconductor material electrically connected in series at said first and second surface regions either directly, or indirectly via a metallic conductor, or indirectly via a doped electrically conductive semiconductor and said electrical serial connection of said portions of p-type and n-type semiconductor material being electrically connectable to an external circuit, characterised in that the p-type and n-type semiconductor material comprises porous semiconductor material.

In this apparatus the second surface regions, or alternatively the first surface regions, or alternatively one second surface region and one first surface region are electrically connectable to the external circuit.

The present invention is based on the recognition that mesoporous semiconductor materials such as silicon (mesoporous meaning structure sizes smaller than 50 nm) fulfill all the requirements for a useful material for thermoelectric power generation and thermoelectric cooling. It is a spongy material, (for mechanical stability) which is structured on the nanoscale (for low thermal conductivity) and it is non-toxic. It can be made out of single or polycrystalline silicon. For thermoelectric applications, polycrys- talline material with large grains (of the order of 100 microns) is probably sufficient.

Mesoporous silicon can be obtained by an electrochemical etching process using an HF containing etchant solution. The manufacture of mesoporous silicon is known per se, for further details, please see for example R. L. Smith and S. D. Collins J. Appl. Phys. 71, Rl (1992), A. G. Cullis et al. J. Appl. Phys. 82 909 (1997); or V. Lehmann, et al. Mater. Sci Eng. B69-70, 11 (2000).

The fact that the thermal conductivity of porous silicon is strongly reduced has been reported in several publications as shown in Fig. 3. It is substantially lower than the thermal conductivity of bulk silicon (150 W/mK). The adjacent portions of p-type and n-type semiconductor material can be considered to have side regions between said surface regions and are either spaced apart at said side regions or separated by resistive or insulating material at said side regions. If the porous material is, for example, silicon then porous silicon, which is resistive and insulating, can be provided between the p-type and n-type regions so that a continuous block of porous silicon can be used. The porous silicon between the p-type and n- type regions can also be filled or lined with insulating material such as Siθ2 to increase the resistivity if necessary.

In a thermoelectric power generating apparatus, wherein said at least one pair of p-type and n-type semiconductor material portions is electrically connected in series at said first surface regions, said first surface regions form a heat source side and said second surface regions form a heat sink side and are electrically connectable to the external circuit to supply the power generated to the external circuit.

Alternatively, also the second surface regions can form a heat source side, in which case the first surface regions correspondingly form the heat sink side.

Alternatively, instead of the second surface regions, the first surface regions, or one second surface region and one first surface region can be electrically connectable to an external circuit.

Moreover, the thermoelectric power generating apparatus may comprise an odd number of p-type and n-type semiconductor material portions.

In a preferred embodiment, such as a thermoelectric power generating apparatus, a plurality of pairs of spaced apart p-type and n-type semicon- ductor material portions are typically provided. The first surface regions of each pair of semiconductors are connected together by respective first metallic layers and the second surface regions of adjacent pairs of first and second semiconductor material portions of different conductivity types are connected together either by direct connections, or indirectly by respective second metallic layers, or indirectly by electrically conducting semiconductor material, whereby said plurality of pairs of semiconductor material portions are connected electrically in series, with said first metallic layers being provided at said heat source side and said direct connections, or said second metallic layers, or said electrically conducting semiconductor material being provided at said heat sink side and the first and last ones of said second metallic layers of said pairs of semiconductor material portions included in said series circuit are connectable to said external circuit, with said plurality of pairs of semiconductor material portions being connected thermally in parallel between said heat source side and said heat sink side.

In this embodiment the first metallic layers produce a good thermal connection to the heat source. In the same way direct connections or the sec- ond metallic connections or the electrically conductive semiconductor material provide a good thermal connection to the heat sink.

To realize a thermoelectric cooling apparatus, said at least one pair of p- type and n-type semiconductor material portions is electrically connected in series at said first surface regions, with said first surface regions forming a heat extraction side and said second surface regions forming a heat dissipation side and being electrically connectable to an external circuit to extract heat from said heat extraction side.

Alternatively, also the second surface regions can form a heat extraction side, in which case the first surface regions correspondingly form the heat dissipation side.

Moreover, the thermoelectric apparatus may comprise an odd number of p-type and n-type semiconductor material portions.

In a preferred thermoelectric cooling apparatus of this kind a plurality of pairs of p-type and n-type semiconductor material portions are typically provided. The first surface regions of each pair of semiconductor material portions are connected together by respective first metallic layers and the second surface regions of adjacent pairs of first and second semiconductors of different conductivity types are connected together either by direct connections, or indirectly by respective second metallic layers, or indirectly via a doped electrically conductive semiconductor, whereby said plurality of pairs of semiconductors are connected electrically in series, with said first metallic layers being provided at said heat extraction side and said direct connections , or said second metallic layers, or said doped electrically conductive semiconductor are provided at said heat dissipation side and the first and last ones of said second metallic layers of said pairs of semiconductor material portions included in said series circuit being connectable to said external circuit, with said plurality of pairs of semiconductor material portions being connected thermally in parallel between said heat extraction side and said heat dissipation side.

In practice said plurality of pairs of spaced apart p-type and n-type semiconductor material portions are preferably connected electrically in series in an array, for example in a linear array or a rectangular array, to form a thermoelectric power generating module or a cooler module.

A plurality of modules can then be connected together to form an array of modules.

In an alternative realization of the present invention a thermoelectric apparatus comprises a stack of semiconductor material portions of alternating p-type and n-type conductivity, with the semiconducting material portions of opposite conductivity types at ends of said stack being contacted for connection to an external circuit and with thermally conductive spac- ers of semiconductor material being provided between each two adjacent semiconductor material portions of different conductivity types, with every second spacer contacting a heat source or heat extraction medium at one side of said stack and the alternating spacers contacting a heat sink or heat dissipating medium at an opposite side of said stack and wherein said p-type and n-type semiconductor material portions comprise porous semiconductor material.

It can be useful to provide a thermally conductive, electrically isolating material at at least one of said heat source side and said heat sink side or at at least one of said heat extraction side and said heat dissipation side. Said thermally conductive, electrically isolating material is preferably Al₂O₃.

As indicated above a thermoelectric apparatus in accordance with the present teaching can utilize porous silicon for the porous semiconductor material portions.

The porous silicon is usually obtained by an electrochemical process (for more information see e.g. "Porous Silicon" Zhe Chuan Feng and Raphael Tsu, World Scientific 1994 ISBN:9810216343 and references therein.). Porous silicon may also be obtained by so-called stain etching (see e.g. A. J. Steckl, J. Xu H. C. Mogul Appl. Phys. Lett. 62, 2211 (1993), V. A. MeI- nikov et al. J. Micromech. Microeng. 18, 025019 (2008), K. W. Kolasinski Current Opinion in Solid State and Materials Science 9, 73 (2005)) or by metal induced etching (see e.g. X. Li and P. W. Bohn Appl. Phys. Lett. 77 2572 (2000), K. Peng et al. Adv. Mater. 14, 1164 (2002), K. Peng et al. Angew. Chem. Int. Ed. 44 2737 (2005)) A thermoelectric apparatus in accordance with the present teaching can also utilize porous SiGe for the porous semiconductor material portions. That is, the porous semiconductor material portions comprise a porous composition of silicon and germanium Si_xGe₁-X, wherein the proportion of Ge lies in the range 1% to 10% and is typically 5% (atomic percent).

Porous Si_xGe₁-X structures very much like porous silicon can be obtained by a electrochemical process similar to the ones employed for porous silicon formation. In this connection, reference can be made to M. Schoisswohl et al. Phys. Rev. B 52 11898-11903 (1995).

The porous semiconductor preferably has an average structure size in the range from lnm to lOOnm, preferably in the range from 5 nm to 50nm.

The average structure size relates to the mean structure thickness of said porous semiconductor structures as it can be inferred from cross section electron microscopy images. By average structure size a measure is meant that is defined in the following way:

In a cross section electron microscope image, showing a perpendicμlar cut through the porous semiconductor layer, a multitude of partially connected semiconductor structures can be seen. One can define the local structure thickness to be measured at a certain point of the structure surface, as the closest distance from this surface point to the next surface point measured across said semiconductor structure. This process can be repeated for an arbitrary number of measurement points, e.g. measurement points lying on the structure closest to the intersection points of a notional square grid having a side length of each square of say 20 or 30nm. The arithmetic mean of the measurements can then be formed and this is the average structure size. Semiconductor structures that are not fully displayed because they are e.g. located at the edge of the electron micrograph such that the local structure thickness as described above can not reasonably determined are omitted in the determination of the average structure size.

The porosity of the porous material is usually determined by weighing the semiconductor material before and after the electrochemical etching process resulting in the creation of the porous semiconductor material layer. The weight difference (before and after etching) together with the volume of the material layer can be used to calculate the volume-fraction of the pores within the porous layer in contrast to the volume-fraction of the porous structures within the porous layer. The volume-fraction of the pores is usually called the porosity of the porous material. Concerning thermoelectric applications the porosity itself is not so important as long as the material still shows sufficient mechanical stability. Mechanical stability can be a problem if the porosity exceeds values of about 60%. Thus for thermoelectric applications, porosities in the range 20% to 50% are probably best.

The porous semiconductor material can be doped with at least one of B, Al, Ga, In for p-type conductivity and with at least one of P, As, Sb, Bi for n-type conductivity. The doping of the porous silicon can be carried out by in-diffusion of the dopant by using a spin-on-dopant or spin-on-diffusant (like e.g. As345 Arsenic spin-on-diffusant from Filmtronics Inc. for n-type doping ) or a metal-organic precursor (such as trimethylarsine) with which the pores of the porous semiconductor are filled. The diffusion of the dopant into the porous semiconductor material is achieved by a proper heat treatment and can be carried out e.g. by rapid thermal annealing. Alternatively, doping of the porous semiconducture material can be achieved by doping from the gas phase by exposing the porous semicon- ductor material at elevated temperatures to a gaseous dopant precursor such as arsine. The concentration of dopant incorporated in said porous semiconductor material to provide p-type and n-type conductivity typically lies in the range from 10¹⁸cπr³ to 1 xlO²² cm"³ and is especially about 1 x 10²⁰ cm-³.

In addition to the in-diffusion of the dopant, also the in-diffusion of other species might be beneficial since the incorporation of atoms different from the semiconductor material in the porous semiconductor usually reduces the thermal conductivity (the atoms act as additional scattering cenbers for the phonons). This in particular concerns the in-diffusion of Ge into porous Si, as the thermal conductivity of porous silicon can be reduced by diffusing Ge into porous silicon; but also the in-diffusion of other atomic species such as Sn e.g. might improve the thermoelectric properties of the porous material.

The need for a maximum electrical conductivity requires that the surfaces of the porous semiconductor structure are provided with a passivating coating, such as a SiO2 coating with low Si/Siθ2 interface state density or a coating of polar molecules. Such Siθ2 coating with low Si/Siθ2 interface state density can e.g. be achieved by rapid thermal annealing in an oxygen containing atmosphere.

The pores of the porous semiconductor material can be filled with an electrically and thermally poorly conducting material like e.g. Siθ2. A Siθ2 filling of the pores can e.g. be achieved by infiltrating the pores of the porous semiconductor material with a spin-on-dopant and annealing the porous semiconductor material, or by depositing SiO2 from the gas phase. As a further alternative, the said porous material can comprise a porous metal suicide.

By way of example, the porous metal suicide can be selected from the group consisting of FeSi2, CrSi2 and MnSi₁.₇4.

Such a metal suicide can be made by depositing a metal such as Fe, Cr or Mn) with a suitable dopant into the pores of porous silicon (e.g. V for Cr to provide a p-type material or Al for Fe to provide an n-type material.

As stated above one important metal suicide proposed here for use as a thermoelectric material is porous FeSi2. In this connection reference can be made to E. Groβ et al. J. Mater. Res. 10 34 (1995) or A.Heinrich et al., Thin Solid Films, 381,287-295 (2001). The metal suicide CrSi2 is for example described by H. Hohl et al. J. Alloys Comp. 248, 70-76 (1997), Z. J. Pan Scripta Mat. 56 257

(2007) and by Z. J. Pan et al. Scripta Mat. 56 245-248 (2007) Higher manganese suicides such as MnSiI.74 are described in papers by (I.Aoyama et al, J. J. Appl. Phys. 44, 4275-4281 (2005), by E. Groβ et al., J. Mater. Res. 10 34 (1995) and by Q. R. Hou et al. in Appl. Phys. A 86, 385-389 (2007). Other metal suicides such as ReSi described by W. Pitschke et al. in J. Appl. Phys. 89, 6 (2001) and MgSi2 described by J. Tani and H. Kido J. in J. Appl. Phys. 46, 3309-3314 (2007) are also known.

Some of them are sufficiently good and in particular sufficiently inexpensive thermoelectric materials. Transforming the porous Si structure (e.g. by metal deposition and proper heat treatment) into a porous metal sili- cide structure, is a further avenue to increase the figure of merit of such metal suicides. The invention also relates in general to the use of porous conductive material as junction material in a thermoelectric apparatus as more specifically set forth in claims 15 to 26.

The invention will now be described in more detail with reference to the embodiments and to the accompanying drawings in which are shown:

Fig. 1 a diagram giving figures of merit for the thermoelectric materials BiSb, Bi2Te3, PbTe and SiGe over a range of temperatures, the diagram being taken from T. M Tritt: Science 283 (5403) 804 (1999),

Fig. 2 a diagram showing the dimensionless thermoelectric figure of merit ZT at 323°K as a function of the content of Ge for Si_1-X Ge_x (x < 0.10) samples with n approximately = 2 x 10²⁰Cm'³ as published by Yamashita N. Samatomi in the J. Appl.Phys. 88 (1) pages 245 to 251 (2000).

Fig. 3 the thermal conductivity of porous silicon layers on a porous silicon substrate as a function of temperature for a porosity p=71 % and for two different layer thicknesses d = 31 μm indicated by squares and d = 46 μm indicated by inverse triangles as published by G. Gesele et al in Appl.Phys. 30, 2911 to 2916 (1997),

Fig. 4 a schematic diagram showing a thermoelectric apparatus in the form of a thermoelectric power generating apparatus in accordance with the present invention, Fig. 5 a diagram similar to Fig. 4 but showing a thermoelectric cooling apparatus in accordance with the present invention,

Fig. 6 a first way of generating a thermoelectric apparatus similar to that of Figs. 4 and 5 by a bonding technique, and indeed prior to bonding,

Fig. 7 a thermoelectric apparatus in accordance with the present invention formed by bonding together the two parts of Fig. 6,

Fig. 8 a further way of forming a thermoelectric apparatus similar to that of Figs. 4 and 5 showing a first stage of a manufacturing process,

Fig. 9 a thermoelectric apparatus in accordance with the present invention formed by adding metal layers and a cover plate to the intermediate product of Fig. 8,

Figs. 1OA to 1OC further diagrams illustrating the manufacture of a further thermoelectric apparatus in accordance with the present invention as shown schematically in completed form in Fig. 1OC, with Figs. 1OA and 1OB showing intermediate products used to form the apparatus of Fig. 1OC,

Fig. 11 a schematic diagram showing a further thermoelectric apparatus in the form of a thermoelectric power generating apparatus in accordance with the present invention,

Fig. 12 a diagram similar to Fig. 11 but showing a thermoelectric cooling apparatus in accordance with the present invention, Fig. 13 a schematic diagram showing a further thermoelectric apparatus in the form of a thermoelectric power generating apparatus in accordance with the present invention,

Fig. 14 a diagram similar to Fig. 13 but showing a thermoelectric cooling apparatus in accordance with the present invention,

Fig. 15 a schematic diagram showing a further thermoelectric apparatus in the form of a thermoelectric power generating apparatus in accordance with the present invention, and

Fig. 16 a diagram similar to Fig. 15 but showing a thermoelectric cooling apparatus in accordance with the present invention.

As already mentioned above, Fig. 1 shows the dimensionless figure of merit ZT for different semiconductor materials from which it can be seen that Bi2Te3 is the best material for relatively low temperature application around 400⁰K and that BiSb can be used for temperatures below room temperature. It will be noted that in no case does the dimensionless figure of merit usually exceed the value 1.0, which limits the commercial attractiveness of thermoelectric applications.

Fig. 2 shows that the ideal percentage of germanium for low temperature applications is around 5 %, but then the alloy Si_xGe₁Oo-X still reaches a dimensionless figure of merit of about 0.17 when doped to a doping concentration n of approximately 2 x 10²⁰Cm-³.

Fig. 3 shows how the thermal conductivity of porous silicon is significantly less than the thermal conductivity of solid silicon (150 W/mK) at temperatures in the range 0 to 320 K. The porous materials which can be used for the present invention will now be described in more detail.

The porous silicon can be obtained by an electrochemical etching method. For this, a silicon substrate such as single-crystalline or polycrystalline Si can be used. Polycrystalline Si with large grains is probably best because it is relatively inexpensive. The silicon substrate is immersed in HF- containing etching solution and a voltage is applied between the substrate and a non-corrosive counter-electrode. Silicon is etched away, resulting in a porous structure, the morphology (in particular the porosity and structure size) of which depends on the doping type and concentration of the substrate, the applied bias, the temperature, the composition of the etching solution and other parameters. The thickness of the porous silicon layer forming on the substrate can be adjusted via the etching duration. Such a porous silicon structure can have a low thermal conductivity. It seems that meso-porous silicon with very small mean structure size and large porosity shows the lowest thermal conductivity. However, for a thermoelectric applications, we not only need a low thermal conductivity, but also a low electrical resistance. If the mean structure size is too small, then the electrical resistance becomes very large. Consequently one has to properly adjust mean structure size and doping concentration. Since porous silicon formation depends on the doping of the substrate one has to first choose a doping (type and concentration) that leads to a structure size that is neither too small (in order not to inhibit electrical current flow too much) nor too big (in order that thermal conductivity is still sufficiently reduced). In a second step one can then dope the porous silicon to the required level by deposition of a doping element into the porous silicon and subsequent annealing (e.g. using a spin-on-dopant plus rapid thermal annealing), or by doping from the gas phase. In order to get a high figure of merit, one needs an extremely high doping concentration of the order of 10²⁰Cm-³. At such a high doping concentration porous silicon with mean structure size of smaller than 10 nm may- have a low enough electrical resistance. An average structure size of 10-30 nm is probably the best. Such structure sizes can be obtained by using n- type substrates with a doping concentration of the order of 10¹⁸ cπr³ (see V. Lehmann et al. Mater. Sci. Eng. B 69-70 11-22 (2000)).

The surface of the structures are properly passivated by e.g. a Siθ2 layer with low Si/Siθ2 interface state density or by polar molecules. This leads to a compensation of the usually present surface charges and thus allows a sufficiently high electrical conductivity.

After the post-etch doping (p-type or n-type of the order of 10²⁰cnr³), the porous silicon can be filled with a material of low thermal conductivity material (like SiOa), optionally followed by a short etch or planarization step. Electrical contact to the porous structure can then be established e.g. by depositing a metal layer. The porous silicon structure can then be used as part of a thermoelectric apparatus.

As can be seen in Fig. 2, the thermoelectric efficiency of Si_xGe_1-x is superior to that of Si. Adding 5% of Ge increases the figure of merit by one order of magnitude. Therefore it is advantageous to produce porous SiGe instead of porous Si structures. In particular (at least for small amounts of Ge) porous Si_xGe_1-X structures can be obtained by an electrochemical process very similar to the one employed for porous silicon formation (see for example M. Schoisswohl et al. Phys. Rev. B 52 11898-11903 (1995)). The process for producing porous SiGe is similar to that described for silicon. Single crystalline or polycrystalline SiGe is used as a substrate for the electrochemical etching process resulting in a porous SiGe layer. In a second step the porous SiGe will be doped to about 10²⁰cπr³ by deposition and annealing of a dopant element. The pores can then be filled with a low-thermal-conductive material and electrical contacts made to the porous layer. The porous SiGe structure can then be used as part of a thermoelectric device.

For porous metal suicide layers use is made of the porous silicon layers obtained by the aforementioned electrochemical etching process. Then a metal (e.g. Fe, Cr or Mn) plus a suitable dopant species (e.g. V in case of Cr) is deposited into the pores or onto the porous layer (e.g. by an electrochemical or electroless deposition method). The porous silicon with the deposited metal is then heated in such a way that metal and porous silicon transform into a porous metal suicide, (e.g. FeSi2, CrSi2 or MnSiI.74). The porous silicide structure can then be used as part of a thermoelectric device.

What is needed for a semiconductor thermoelectric device , be it for cooling or power generation , is a series of p-n junctions, i.e. junctions of p- doped material with n-doped material. One type of junctions (e.g. the n-p junctions) is thermally connected to the hot side, the other type (in this case the p-n junctions) is thermally connected to the cold side of the device. Typically, all blocks of n-type and p-type material are electrically connected in series.

Typical thermoelectric modules may, for example, have of the order of 100 blocks in a single device. Thus, schematically, a thermoelectric power gen- era ting device having three pairs of adjacent p-type and n-type semiconductor material portions or blocks may take the form shown in Fig. 4.

Turning now to Fig. 4, there can be seen a thermoelectric apparatus 10 in the form of a thermoelectric power generating apparatus comprising at least one pair 11 of adjacent portions 12, 14 of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions 16, 18 (first surface regions) and 20, 22 (second surface regions). The first surface regions 16, 18 of each said pair are coupled either directly (not shown), or indirectly via a metallic conductor 24, or indirectly via a doped electrically conductive semiconductor (also not shown). The second surface regions 20, 22 are electrically connectable to an external circuit 26 having a load resistor 25 as a power consumer. This is clearly only a simple example, normally the item 25 will be some other type of electrical load. The p-type and n-type semiconductor materials 12, 14 comprise porous semiconductor material such as the three basic types described above.

In the example the adjacent portions of p-type and n-type semiconductor material 12, 14 have side regions 28, 30 between said surface regions 16, 18 (first surface regions) and 20, 22 (second surface regions). The semiconductor material portions 12, 14 are either spaced apart at said side regions 28, 30 as shown, or are separated by resistive or insulating material at said side regions (not shown but as discussed earlier).

Each pair 11 of p-type and n-type semiconductor material portions 12, 14 is electrically connected in series at said first surface regions, wherein said first surface regions 16, 18. In this example the connection is effected by the metallic contacts 24. These are made of a type of metal solder and can, for example, be made of an antimony-tin alloy or any other metallic material conventionally used with p-type and n-type semiconductor material of the composition described above. The first surface regions 16, 18 form a heat source side 27 and the second surface regions 20, 22 form a heat sink side 29.

More specifically Fig.4 shows a plurality of pairs 11 of spaced apart p-type and n-type semiconductor material portions 12, 14 and the first surface regions 16, 18 of each pair 11 of semiconductor portions 12, 14 are connected together by respective first metallic layers 24. The second surface regions 20, 22 of adjacent pairs 1 1 of first and second semiconductor material portions of different conductivity types are connected together either by direct connections (not shown in Fig. 4 but in Fig. 9), or indirectly by respective second metallic layers 32, or indirectly by electrically conducting semiconductor material (again not shown), whereby said plurality of pairs 11 of semiconductor material portions are connected electrically in series. The first metallic layers 24 are provided at the heat source side 27 and the second metallic layers 32 are provided at said heat sink side 29 The first and last ones of said second metallic layers 32 of said pairs 11 of semiconductor material portions included in said series circuit are con- nectable to the external circuit 26. The plurality of pairs 11 of semiconductor material portions are connected thermally in parallel between the heat source side 27 and the heat sink side 29.

The above described thermoelectric power generating apparatus of Fig.4 can also be easily realized in the form of a thermoelectric cooling apparatus as shown in Fig. 5. It will be noted that the apparatus of Fig. 5 is closely similar to that of Fig. 4 which is why the same reference numerals are used for parts having the same design or function so that the description of these parts as given earlier also applies to Fig.5 and only the differences will be explained in the following. The first difference is that the hot side of Fig. 4 is now the cold side of Fig. 5, because heat is extracted there and the cold side of Fig. 4 is now the hot side of Fig. 5 because heat is dissipated there. The heat extraction side of the thermoelectric cooling apparatus of Fig. 5, the cold side, is now designated by the reference numeral 40 whereas the heat dissipation side, the hot side, is designated by the reference numeral 42. The load resistance 25 of the thermoelectric power generating apparatus of Fig. 4 has been replaced in Fig. 5 with a DC power supply 44 which connects to the terminals A and B in the form of metal contacts to the first and last semiconductor material portions of the module 10, in the same way as in Fig.5.

A thermally conductive, electrically isolating material 36, 38 is preferably - but not necessarily - provided at at least one of said heat source side 27 and said heat sink side 29 or at at least one of said heat extraction side 40 and said heat dissipation side 42 in Fig. 5. The thermally conductive, electrically isolating material is, for example, AI2O3.

Thus, in both the embodiment of Fig. 4 and that of Fig. 5 the plurality of pairs 11 of spaced apart p-type and n-type semiconductor material portions 12, 14 are connected electrically in series in an array, as shown here in the form of a linear array to form a thermoelectric power generating module or a cooler module. The semiconductor material portions 12, 14 can also be arranged in second, third or further rows behind or in front of the row shown to form a module in the form of a rectangular array. In principle there is no restriction on the number of pairs 11 of semiconducting material portions 12, 14 in each row or on the number of rows.

Also a plurality of modules such as 10 in Fig. 4 or Fig. 5 can be connected together to form an array of modules. The thermoelectric apparatus in accordance with Fig. 4 or Fig.5 is actually quite complicated to manufacture and similar structures, which are equally useful can be manufactured more easily as will now be described with reference to Figs. 6 and 7 and Figs. 8 and 9. In both these embodiments the same reference numerals have been used as in Fig. 4 and the description already given applies for parts having the same design or function and only the differences will be discussed here.

Figs. 11 to 16, essentially show the same schematic diagrams as introduced in Figs. 4 and 5. In these figures the same reference numerals are used as in Figs. 4 and 5 and the description given for the items identified by these numerals will be understood to apply equally to Figs. 11 to 16 unless something is stated to the contrary.

Turning now to Fig. 1 1, there can be seen a thermoelectric apparatus 10 in the form of a thermoelectric power generating apparatus comprising at least one pair 11 of adjacent portions 12, 14 of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions 16, 18 (first surface regions) and 20, 22 (second surface regions). The second surface regions 20, 22 of each said pair are coupled either directly (not shown), or indirectly via a metallic conductor 32, or indirectly via a doped electrically conductive semiconductor (also not shown). The first surface regions 16, 18 are electrically connectable to an external circuit 26 having a load resistor 25 as a power consumer. This is clearly only a simple example, normally the item 25 will be some other type of electrical load. The p-type and n-type semiconductor materials 12, 14 comprise porous semiconductor material such as the three basic types described above. The above described thermoelectric power generating apparatus of Fig.11 can also be easily realized in the form of a thermoelectric cooling apparatus as shown in Fig. 12. It will be noted that the apparatus of Fig. 12 is closely similar to that of Fig. 11 which is why the same reference numerals are used for parts having the same design or function so that the description of these parts as given earlier also applies to Fig.12 and only the differences will be explained in the following. The first difference is that the hot side of Fig. 11 is now the cold side of Fig. 12, because heat is extracted there and the cold side of Fig. 11 is now the hot side of Fig. 12 because heat is dissipated there. The heat extraction side of the thermoelectric cooling apparatus of Fig. 12, the cold side, is now designated by the reference numeral 40 whereas the heat dissipation side, the hot side, is designated by the reference numeral 42. The load resistance 25 of the thermoelectric power generating apparatus of Fig. 11 has been replaced in Fig. 12 with a DC power supply 44 which connects to the terminals A and B in the form of metal contacts to the first and last semiconductor material portions of the module 10, in the same way as in Fig. 11.

Thus, in both the embodiment of Fig. 11 and that of Fig. 12 the plurality of pairs 11 of spaced apart p-type and n-type semiconductor material portions 12, 14 are connected electrically in series in an array, as shown here in the form of a linear array to form a thermoelectric power generating module or a "cooler" module. The semiconductor material portions 12, 14 can also be arranged in second, third or further rows behind or in front of the row shown to form a module in the form of a rectangular array. In principle there is no restriction on the number of pairs 11 of semiconducting material portions 12, 14 in each row or on the number of rows.

Turning now to Fig. 13, there can be seen a thermoelectric apparatus 10 in the form of a thermoelectric power generating apparatus comprising at least one pair 11 of adjacent portions 12, 14 of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions 16, 18 (first surface regions) and 20, 22 (second surface regions). The second surface regions 20, 22 of each said pair are coupled either directly (not shown) , or indirectly via a metallic conductor 32, or indirectly via a doped electrically conductive semiconductor (also not shown). An external circuit 26 having a load resistor 25 as a power consumer, is electrically connectable between the first surface regions 16, 18 of side A and the second surface regions 20, 22 of side B of the apparatus 10. This is clearly only a simple example, normally the item 25 will be some other type of electrical load. The p-type and n-type semiconductor materials 12, 14 comprise porous semiconductor material such as the three basic types described above.

The above described thermoelectric power generating apparatus of Fig.13 can also be easily realized in the form of a thermoelectric cooling apparatus as shown in Fig. 14. It will be noted that the apparatus of Fig. 14 is closely similar to that of Fig. 13 which is why the same reference numerals are used for parts having the same design or function so that the description of these parts as given earlier also applies to Fig.14 and only the differences will be explained in the following. The first difference is that the hot side of Fig. 13 is now the cold side of Fig. 14, because heat is extracted there and the cold side of Fig. 13 is now the hot side of Fig. 14 because heat is dissipated there. The heat extraction side of the thermoelectric cooling apparatus of Fig. 14, the cold side, is now designated by the reference numeral 40 whereas the heat dissipation side, the hot side, is designated by the reference numeral 42. The load resistance 25 of the thermoelectric power generating apparatus of Fig. 13 has been replaced in Fig. 14 with a DC power supply 44 which connects to the terminals A and B in the form of metal contacts to the respective semiconductor material portions of the module 10, in the same way as in Fig. 13.

Thus, in both the embodiment of Fig. 13 and that of Fig. 14 the plurality of pairs 11 of spaced apart p-type and n-type semiconductor material portions 12, 14 are connected electrically in series in an array, as shown here in the form of a linear array to form a thermoelectric power generating module or a "cooler" module. The semiconductor material portions 12, 14 can also be arranged in second, third or further rows behind or in front of the row shown to form a module in the form of a rectangular array. In principle there is no restriction on the number of pairs 11 of semiconducting material portions 12, 14 in each row or on the number of rows.

Turning now to Fig. 15, there can be seen a thermoelectric apparatus 10 in the form of a thermoelectric power generating apparatus comprising at least one pair 11 of adjacent portions 12, 14 of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions 16, 18 (first surface regions) and 20, 22 (second surface regions). The second surface regions 20, 22 of each said pair are coupled either directly (not shown), or indirectly via a metallic conductor 32, or indirectly via a doped electrically conductive semiconductor (also not shown). An external circuit 26 having a load resistor 25 as a power consumer, is electrically connectable between the second surface regions 20, 22 of side A and the first surface regions 16, 18 of side B of the apparatus 10. This is clearly only a simple example, normally the item 25 will be some other type of electrical load. The p-type and n-type semiconductor materials 12, 14 comprise porous semiconductor material such as the three basic types described above. The above described thermoelectric power generating apparatus of Fig.15 can also be easily realized in the form of a thermoelectric cooling apparatus as shown in Fig. 16. It will be noted that the apparatus of Fig. 16 is closely similar to that of Fig. 15 which is why the same reference numerals are used for parts having the same design or function so that the description of these parts as given earlier also applies to Fig.16 and only the differences will be explained in the following. The first difference is that the hot side of Fig. 15 is now the cold side of Fig. 16, because heat is extracted there and the cold side of Fig. 15 is now the hot side of Fig. 16 because heat is dissipated there. The heat extraction side of the thermoelectric cooling apparatus of Fig. 16, the cold side, is now designated by the reference numeral 40 whereas the heat dissipation side, the hot side, is designated by the reference numeral 42. The load resistance 25 of the thermoelectric power generating apparatus of Fig. 15 has been replaced in Fig. 16 with a DC power supply 44 which connects to the terminals A and B in the form of metal contacts to the respective semiconductor material portions of the module 10, in the same way as in Fig. 15.

Thus, in both the embodiment of Fig. 15 and that of Fig. 16 the plurality of pairs 11 of spaced apart p-type and n-type semiconductor material portions 12, 14 are connected electrically in series in an array, as shown here in the form of a linear array to form a thermoelectric power generating module or a "cooler" module. The semiconductor material portions 12, 14 can also be arranged in second, third or further rows behind or in front of the row shown to form a module in the form of a rectangular array. In principle there is no restriction on the number of pairs 11 of semiconducting material portions 12, 14 in each row or on the number of rows.

Also a plurality of modules such as 10 in Figs 11 to 16 can be connected together to form an array of modules. The thermoelectric apparatus in accordance with Figs. 11 to 16 might be quite complicated to manufacture and similar structures, which are equally useful can probably be manufactured more easily as have been described with reference to Figs. 6 and 7 and Figs. 8 and 9. In both these embodiments the same reference numerals have been used as in Figs. 11 to 16 and the description already given applies for parts having the same design or function and only the differences will be discussed here.

Turning now to Figs, 6 and 7 these illustrate a method of manufacturing a thermoelectric apparatus 10 in which first portions of a porous n-type material 14 are formed on a first substrate 90, with first surface regions 18 of said first portions being disposed at said first substrate 90 and second surface regions 22 of said first portions being free surface regions. A metallic contact 24 to each of said first portions of semiconducting material 14 is formed at or adjacent each said first surface region 18 of said second portions on said substrate 90. The metallic contacts 24 are separated from each other.

Second portions of a porous p-type material 12 are formed on a second substrate 92 with, second surface regions 20 of said second portions being disposed at said second substrate 92 and first surface regions 16 of said second portions being free surface regions. Again a metallic contact 30 is formed at or adjacent each said second surface region 20 of said p-type portions, said metallic contacts 30 being separated from each other. The structures on the two substrates are designed to fit inside each other in interdigitated fashion so that the first and second substrates 90, 92 with said first and second portions 14, 12 thereon can be subsequently bonded together so that said first surface regions 16 of the first portions 12 bond to metal contacts 24 at the first substrate 90 and said second surface re- gions 22 of the second portions 14 bond to metal contacts 30 at the second substrate 92. The resulting structure after bonding is shown in Fig. 7 and is basically electrically equivalent to that shown in either of Fig. 4 or Fig. 5 or Figs. 11 to 16.

Thus, in this embodiment, there are n-type blocks 14 on one substrate and p-type blocks 12 on a second substrate and the two substrates are then bonded together.

The substrates 90 and 92 could for example be undoped Si substrates of low conductivity on which further Si material is grown and subsequently made porous and doped to form the n-type and p-type regions 14, 12. Alternatively, a larger block of an Si substrate can be made porous over part of its depth and then etched to form the general shape of Fig. 6 with the regions 12 and 14 then being doped. Also the substrates 90 and 92 could be removed and replaced with say AI2O3 layers or plates. Indeed, if this route is followed, the surfaces 16 and 18 and 20 and 22 could be fully metalized prior to adding the AI2O3 layers or plates to result in the structure of Fig. 4 or Fig. 5 or Fig. 11 to 16.

In an alternative method which will be described with reference to Figs. 8 and 9 first portions 12 of a porous p-type material having first and second surface regions 16, 20 are formed on a first substrate 90, with said second surface regions 20 of said portions being disposed or formed at said first substrate 90 and said first surface regions 16 of said first portions being free surface regions. Second portions 14 of a porous p-type material also having first and second surface regions 18, 22 are formed on the first substrate 90 between the portions 12, with the second surface regions 22 of the second portions 14 being disposed at the first substrate 90 and the first surface regions 18 of said second portions 14 being free surface re- gions. A metallic contact 24 is formed at or adjacent each said first surface region 16 of said first portions and at or adjacent each said first surface region 18 of the respectively adjacent second portions 14. In this way the same metal contacts 24 can be formed for each pair 11 of semiconductor material portions 12, 14 as in the embodiment of Fig. 4 or Fig. 5 or Fig. 11 to 16.

A thermally conductive electrically isolating layer 36 of AI2O3 (or a semiconducting but electrically isolating layer such as substrate 92) can then be bonded to the upper side of the structure in Fig. 8, using the metal contacts 24 as a type of solder to result in the structure of Fig. 9. I.e. a second substrate 36 is subsequently bonded to the first surface regions 16, 18 of said first and second portions 12, 14. It can readily be appreciated from Fig. 9 that the structure of Fig. 4 or 5 or Figs. 11 to 16 could be realized by removing the substrate 90, by avoiding or removing the p-type material 12' between the individual columns 12 and 14, (which could also be n-type material 14', by metalizing the columns 12 and 14 at the second surface regions 20, 22 so that the second surface regions of adjacent column pairs are connected together as in Figs. 4, 5 and Figs. 11 to 16 and then optionally adding a further substrate, such as 38 in Figs. 4, 5 and Figs. 11 to 16, if deemed necessary.

Thus, this embodiment utilizes selective doping of both p-type and n-type regions on a single substrate such that n-type and p-type blocks are connected pairwise. Afterwards additional electrical connections, for example of metal, as well as an optional cover plate are attached. In an electric power generator the current then again flows through the external circuit between the contacts A and B. In an alternative embodiment shown in Fig. 1OC the thermoelectric apparatus 50 comprises a stack of semiconductor material portions 52, 54 of alternating p-type and n-type conductivity. The semiconducting material portions of opposite conductivity types at the ends 56, 58 of the stack are contacted for connection to an external circuit such as 26 in Fig. 4 or Figs. 11, 13, 15 or 46 in Fig. 5 or Figs. 12, 14, 16. Thermally conductive spacers of semiconductor material 60, 62 are provided between each two adjacent semiconductor material portions 52, 54 of different conductivity types, with every second spacer 62 contacting a heat source 64 or heat extraction medium at one side of said stack and the alternating spacers 60 contacting a heat sink 66 or heat dissipating medium at an opposite side of said stack. Again the p-type and n-type semiconductor material portions comprise porous semiconductor material such as any of the three above described basic kinds. It will be noted that the semiconductor spacers 60, 62 are of high conductivity. They can for example be doped silicon substrates with the density of dopants being, for example 2 x 10²⁰ cm"³. This is also a convenient way of fabricating the structure. More specifically, porous n-type and p-type silicon can be formed on respective substrates 60 and 62 as shown in Figs. 1OA and 1OB and such substrates with porous material 52, 54 can then be bonded together to form the core of the stack shown in Fig. 1OC. For this metal contacts 67, such as the antimony-tin alloy mentioned earlier, can be applied to the free surfaces 68, 70 of the substrates and/or to the free substances 72, 74 of the porous semiconductor portions 52, 54 and then serve as an adhesive, a type of solder for low temperature bonding of the stack. The metal layers 67 are however not essential to the functioning of the apparatus and could be omitted if the stack is formed in a different way. If desired thermally conducting layers 78, 80 of metal, or of semiconductor material or of insulating material can be applied to the projecting ends 76, 77 of the substrates 60, 62 in Fig. 1OC. The layers 78, 80 could for example also be Of Al₂O₃ as in the examples of Figs. 4, 5 and Figs. 11 to 16.

Also it is not necessary for the ends of the substrates 60, 62 to project so that the layers 78, 80 are spaced from the sides of the stack. Such spac- ings could be filled with insulating material.

Thus, in this embodiment p-type and n-type substrates (or pieces thereof) are used as building blocks for the assembly of a thermoelectric device. The (non-porous) substrates are themselves used for the thermal connection to the hot or cold sides. It is also possible to use p-type and n-type regions on opposite sides of the same substrate.

It should be noted that none of Figs. 4 to 16 are drawn to scale. In practice, in particular with regard to the embodiments of Figs. 4 to 9 and Figs. 11 to 16 the porous regions will tend to be much shorter than as shown and will typically have a height less than their width or breadth. The columns of material 12, 14 will generally be of square or rectangular cross section.

Claims

Patent Claims

1. A thermoelectric apparatus (10) comprising at least two adjacent portions (12, 14) of p-type and n-type semiconductor material each portion having respective, generally oppositely disposed, first and second surface regions (16, 18 (first surface regions) and 20, 22 (second surface regions)), said portions (12, 14) of p-type and n-type semiconductor material being electrically connected in series at said first (16, 18) and second (20,22) surface regions either directly, or indirectly via a metallic conductor (24), or indirectly via a doped electrically conductive semiconductor and said electrical serial connection of said portions of p-type and n-type semiconductor material being electrically connectable to an external circuit (26 or 46), characterised in that the p-type and n-type semiconductor material (12, 14) comprises porous semiconductor material.

2. A thermoelectric apparatus in accordance with claim 1 wherein two of said portions (12, 14) of p-type and n-type semiconductor material are electrically connectable to an external circuit and portions (12) of p-type semiconductor material that are not connectable to an external circuit are connected at said first surface region (16) to one portion (16) of n-type semiconductor material and at said second surface region (20) to another portion (16) of n-type semiconductor material, and portions (14) of n-type semiconductor material that are not connectable to an external circuit are connected at said first surface region (18) to one portion (12) of p-type semiconductor material and at said second surface region (22) to another portion (12) of p-type semiconductor material, such that alternating portions of n-type and p-type material are electrically connected in series and said serial connection of said n-type and p-type semiconductor portions iselectrically connectable to an external circuit.

3. A thermoelectric apparatus in accordance with claim 1 or claim 2 in the form of a thermoelectric power generating apparatus, wherein a plurality of p-type and n-type semiconductor material portions are provided, wherein said first surface regions form a heat source side and said second surface regions form a heat sink side such that said of portions of p-type and n-type semiconductor material are thermally connected in parallel, wherein said portions of p-type and n-type semiconductor material are electrically connected in series either by direct connections, or indirectly by metallic layers, or indirectly by electrically conducting semiconductor material and are electrically connectable to the external circuit to supply the power generated to the external circuit.

4. A thermoelectric apparatus in accordance with claim 1 or claim 2 in the form of a thermoelectric cooling apparatus, wherein a plurality of p-type and n-type semiconductor material portions are provided, and wherein said first surface regions form a heat extraction side and said second surface regions form a heat dissipation side such that said of portions of p-type and n-type semiconductor material are thermally connected in parallel, wherein said portions of p-type and n-type semiconductor material are electrically connected in series either by direct connections, or indirectly by metallic layers, or indirectly by electrically conducting semiconductor material and are electrically connectable to said external circuit to extract heat from said heat extraction side.

5. A thermoelectric apparatus in accordance with claim 3 or claim 4, wherein said plurality of portions of said p-type and n-type semiconductor material are connected electrically in series in an array, for example in a linear array or a rectangular array, to form a thermoelectric power generating module or a cooler module.

6. A thermoelectric power generating apparatus or cooler apparatus in accordance with any one of the preceding claims 1 to 5, wherein a thermally conductive, electrically isolating material, for example AI2O3, is provided at at least one of said heat source side and said heat sink side or at at least one of said heat extraction side and said heat dissipation side.

7. A thermoelectric apparatus in accordance with any one of the preceding claims 1 to 6, wherein said porous semiconductor material portions comprise a porous composition of silicon and germanium Si_xGe_1-X, with Ge preferably being present in an amount between 1% and 10% (atomic percent) especially around 5% (atomic percent).

8. A thermoelectric apparatus in accordance with any one of the preceding claims 1 to 6, wherein said porous semiconductor material portions comprise porous silicon.

9. A thermoelectric apparatus in accordance with any one of the preceding claims 1 to 8 wherein said porous semiconductor material is doped with at least one of B, Al, Ga, In for p-type conductivity and with at least one of P, As, Sb, Bi for n-type conductivity, and wherein the concentration of dopant incorporated in said semiconductor material to provide p-type and n-type conductivity lies in the range from 10¹⁸cπr³ to 1 xlO²² crrr³ and is especially about 1 x 10²⁰ cm-³.

10. A thermoelectric apparatus in accordance with any one of the claims 1 to 9, wherein the doping of the porous silicon is carried out by infiltrating the porous semiconductor material with a metal-organic liquid or a spin-on-dopant, followed by a suitable heat treatment, or by doping the porous semiconductor material from the gas phase.

11. A thermoelectric apparatus in accordance with claim 1 to 10, wherein the porous semiconductor materials has an average structure size in the range from lnm to lOOnm, preferably in the range from 5nm to 50nm.

12. A thermoelectric apparatus in accordance with any one of the preceding claims 1 to 6, wherein said porous semiconductor material comprises a porous metal silicide.

13. A thermoelectric apparatus in accordance with claim 12 wherein said porous metal silicide is selected from the group consisting of FeSi₂, CrSi2 and MnSii.74.

14. A thermoelectric apparatus in accordance with claim 12 or claim 13 and made by depositing a metal such as Fe, Cr or Mn and a suitable dopant (e.g. V for Cr to provide a p-type material or Al for Fe to provide an n-type material into the pores of a porous semiconductor like porous silicon.

15. Use of porous semiconductor material as active material in a thermoelectric apparatus.

16. Use in accordance with claim 15, wherein said porous semiconductor material comprises portions of p-type and n-type porous semiconductor material.

17. Use in accordance with claims 15 or 16, wherein said porous semiconductor material comprises portions of porous silicon germanium Si_xGe_1-X material with Ge preferably being present in an amount less than 10 % (atomic percent) especially 5% (atomic percent).

18. Use in accordance with claim 15 or 16, wherein said porous semiconductor material comprises portions of porous silicon.

19. Use in accordance with any of the preceding claims 15 to 18, wherein said porous semiconductor material has been obtained by an electrochemical etching process involving an etching solution containing hydrofluoric acid, like stain etching or metal assisted etching for example.

20. Use in accordance with claim 15, wherein said porous semiconductor material comprises a porous metal suicide or a porous metal germanide or a porous metal suicide /germanide.

21. Use in accordance with claim 15, wherein said porous semiconductor material comprises a porous metal suicide.

22. Use in accordance with claim 21, wherein said porous metal suicide is selected from the group consisting of FeSi2, CrSi2 and MnSh.74.

23. Use in accordance with claim 20 to 22 and made by depositing a metal such as Fe, Cr or Mn plus a suitable dopant (e.g. V for Cr to provide a p-type material or Al for Fe to provide an n-type material) plus possibly small amounts of other impurities like Ge for example into the pores of porous silicon and heating the resulting material combination.

24. Use in accordance with any of the preceding claims 15 to 23, wherein the average structure size of said porous semiconductor materials has an average structure size in the range from lnm to lOOnm, preferably in the range from IOnm to 50nm.

25. Use in accordance with any of the preceding claims 15 to 24, wherein said porous semiconductor material is provided with a pas- sivating coating such as a Siθ2 with low semiconductor/ Siθ2 interface state density coating or a coating with a organic material.

26. Use in accordance with any of the preceding claims 15 to 25, wherein the pores of said porous semiconductor material are filled with a thermally and electrically poorly conducting material, like SiO₂.