JP4873448B2 - Rectifier diode - Google Patents

Description

  The present invention relates to a rectifier diode, and more particularly to a rectifier diode having a semiconductor polarization junction.

The ratio of electric energy to the total energy consumption (electricity generation rate) is increasing year by year. Currently, the rate of electrification in Japan has reached 40% and is expected to increase further in the future. In order to effectively use limited resources, it is necessary to develop a technology for converting and controlling electric energy with higher efficiency.
The semiconductor field that handles this power conversion and control is called power electronics. Among them, a rectifier diode using a semiconductor plays a central role. The reverse breakdown voltage of the rectifier diode and the on-resistance in the forward direction greatly affect the conversion / control efficiency. Therefore, there is a need for a rectifier diode that has a higher breakdown voltage and a lower on-resistance. By improving these characteristics, loss in power conversion and control can be reduced. However, since there is generally a trade-off relationship between breakdown voltage and on-resistance, if one characteristic is improved, the other characteristic tends to deteriorate.

  There are roughly two types of methods for overcoming this trade-off relationship and improving the characteristics of the rectifier diode. First, the most fundamental solution is a method of making a rectifier diode by replacing Si, which has been generally used so far, with a wide band gap semiconductor. A wide band gap semiconductor is a semiconductor whose band gap energy is larger than that of Si (1.1 eV). At present, the most attention is given to SiC (up to 3.0 eV), group III nitride semiconductor (up to 6.2 eV) and II-VI group oxide semiconductors (˜7.8 eV). Since the breakdown voltage increases as the band gap energy increases, the use of these semiconductors reduces the on-resistance to a few hundredths compared to a rectifier diode using Si while having the same reverse breakdown voltage. Expected.

  The other is a method using a new technique called super-junction, as represented by MR-JBS (Multi RESURF Junction Barrier Schottky Rectifier). A superjunction is an improved version of a conventional pn junction, in which a p-type region and an n-type region are formed in a semiconductor, and the total charge amount of these two regions is designed to be approximately equal, thereby improving the effect of carrier compensation. This is a technique for generating a constant electric field distribution in the depletion layer. Using this super-junction, it has been found that performance exceeding the material limit of Si previously considered can be realized.

  However, since fabrication of this superjunction requires precise semiconductor process technology or growth technology, it is currently put to practical use only with semiconductor elements using Si. For example, in MR-JBS, a method of once forming a deep groove (trench structure) in an n-type Si substrate by etching and then embedding the groove with p-type Si by crystal growth is used. At this time, the groove to be formed is required to have a high aspect ratio (ratio of groove width to depth). Also, in order to make an ideal superjunction, it is necessary to keep the total doping amount of positive charge and negative charge completely equal. In the super junction structure, when this relationship is broken, the usable element breakdown voltage is limited. However, this is impossible in principle, and in practice, as disclosed in, for example, Patent Document 1, the superjunction is controlled with an error of the total amount of positive and negative charges within several percent. Form.

  As described above, in order to improve the trade-off between on-resistance and breakdown voltage, it has been described that there are a method using a wide band gap semiconductor and a method using a super junction. That is, an ideal rectifier diode for power electronics can be realized if these two methods can be used simultaneously to produce an element having a superjunction in a wide band gap semiconductor. However, as described above, super-junction fabrication requires highly accurate semiconductor process technology and growth technology, and it has been extremely difficult to apply this to semiconductors other than Si.

  The basic structures of the various semiconductor devices invented so far are p-type and n-type conductivity control by doping and band structure control by heterojunction. Is realized. Of course, a Si semiconductor element having a super junction is also manufactured by these techniques.

  On the other hand, it is known that the conductivity type can be controlled using the polarization of the semiconductor. Polarization is a phenomenon in which fixed charges are generated at the interface of the heterojunction, and includes spontaneous polarization that occurs even when the crystal is unstrained and piezoelectric polarization that occurs due to strain. Electrons or holes are generated in the vicinity of the heterojunction interface by being attracted by this fixed charge.

Compared to carriers formed by doping, carriers due to this polarization have different characteristics. First, there is a feature that it is easy to control the total amount of fixed charges distributed in the semiconductor. For example, when considering a heterojunction of two types of semiconductors, the total amount of fixed charges generated therein is determined by the type of semiconductor to be bonded. That is, changing the steepness of the junction or inserting another semiconductor layer at the interface does not affect the total amount of fixed charges. In addition, when two kinds of semiconductors are alternately stacked, polarization charges having the same magnitude and opposite signs appear alternately at each hetero interface.
Second, a high concentration of fixed charges, which is impossible with the doping technique, can be generated in a spatially concentrated manner. For example, in a group III nitride semiconductor heterojunction, polarization charge can be obtained with a surface density of about 10 ¹³ cm ⁻² . At this time, assuming that the steepness of the interface is 1 nm, the fixed charge density is about 10 ²⁰ cm ⁻³ , which is extremely high. This makes it possible to realize a spatial distribution of carriers that is difficult only with the doping technique.

  Third, in principle, the activation energy of carriers can be ignored. The activation energy here is thermal energy for generating carriers. Wide band gap semiconductors have a problem in that the activation energy tends to be larger than the thermal energy at room temperature in controlling the conductivity by doping. However, this problem does not occur if polarization is used. That is, there is a possibility that the conductivity can be controlled even in a semiconductor in which p-type and n-type control is difficult by doping.

Fourth, the carriers generated at this time are spatially concentrated as described above, and the influence of ionized impurity scattering is low, so that the mobility can be high. When carriers are generated by doping, for example, the electron mobility of bulk GaN is about 200 cm ² / Vs, but mobility of 1000 cm ² / Vs or more can be easily obtained by using a two-dimensional electron gas by polarization. .

  As described above, carriers generated by polarization have various features. Therefore, in addition to the p-type or n-type control technology by doping and the control of the band lineup by heterojunction, a semiconductor element that has never been realized can be realized by actively utilizing the above-described polarization phenomenon.

  This phenomenon of polarization is seen in many semiconductors. However, a semiconductor element based on the above viewpoint has not received much attention so far. One reason is that a semiconductor having a cubic crystal structure such as GaAs has a small polarization. On the other hand, it is known that a group III nitride semiconductor having a hexagonal crystal structure in which high-quality single crystals have been obtained in recent years produces extremely large polarization.

Group III nitride semiconductors are group III-V compound semiconductors that have begun to attract attention as optical devices since the 1990s. The chemical formula is represented by _{B x Al y Ga z In 1} -x-y-z N. In a light emitting device using a group III nitride semiconductor, this large polarization has been regarded as a drawback because it leads to a decrease in light emission efficiency due to the quantum confinement shktar effect. To date, there is a high electron mobility transistor using a group III nitride semiconductor as a semiconductor element that actively uses polarization. This utilizes positive fixed charges generated by polarization. However, a rectifier diode that utilizes both positive and negative fixed charges and positively uses electrons and holes generated thereby has not been reported so far.
JP 2001-111041 A JP-A-8-148699 S. Kunoriet al, Proc.ISPSD'02, p.33 (2002)

  The problem to be solved by the present invention is to provide a rectifier diode that positively utilizes both positive and negative fixed charges due to polarization and utilizes a polarization junction formed thereby.

The present invention relates to a rectifier diode used for superjunction formed by polarization, and the problem is solved by providing the following rectifier diode.
(1) It has a laminated structure in which two or more kinds of semiconductors are laminated to form at least two semiconductor heterojunctions, and the positive and negative fixations generated by polarization at the heterojunction interface. In a rectifier diode having a polarization junction in which a carrier of a first conductivity type and a carrier of a second conductivity type are generated simultaneously by electric charge,
A first electrode having Schottky characteristics with respect to the carrier of the first conductivity type at one side end of the laminated structure;
A rectifier diode comprising a second electrode having ohmic characteristics with respect to the carrier of the first conductivity type at the other side end.
(2) The rectifier diode, wherein the first electrode has an ohmic characteristic with respect to the carrier of the second conductivity type.
(3) A positive and negative fixing generated by polarization at the interface of the heterojunction, having a laminated structure in which two or more semiconductors are laminated to form at least two semiconductor heterojunctions. In a rectifier diode having a polarization junction in which a carrier of a first conductivity type and a carrier of a second conductivity type are generated simultaneously by electric charge,
A first electrode having a Schottky characteristic with respect to the carrier of the first conductivity type and an ohmic characteristic with respect to the carrier of the second conductivity type and a first electrode at one side end of the stacked structure. A third electrode electrically connected to the other electrode;
A rectifier diode comprising a second electrode having ohmic characteristics with respect to the carrier of the first conductivity type at the other side end.
(4) A rectifier diode, wherein a part of the semiconductor adjacent to the other side end of the stacked structure is highly doped.
(5) The rectifier diode in which the two or more types of semiconductors are III-V compound semiconductors having different compositions.
(6) The group III-V compound semiconductor is a III-nitride semiconductor, chemical formulas rectifier diode represented by _{B x Al y Ga z In 1} -x-y-z N.
(In the formula, x, y and z have numerical values satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, and x + y + z ≦ 1.)
(7) The rectifier diode, wherein the group III nitride semiconductor is stacked in a c-axis direction.
(8) The rectifier diode, wherein the two or more types of semiconductors are II-VI group oxide semiconductors having different compositions.
(9) The rectifier diode, wherein the two or more types of semiconductors are SiC compound semiconductors having different crystal structures.
(10) The rectifier diode is characterized in that the rectifier diode is integrated on a semiconductor substrate.

According to the present invention, the on-resistance can be reduced by the amount of carriers generated by polarization without sacrificing the reverse breakdown voltage, thereby improving the trade-off between the reverse breakdown voltage and the on-resistance of the rectifier diode. Can do.
In addition, the on-resistance of the rectifier diode is further reduced by using the high mobility of at least the first conductivity type carrier or the second conductivity type carrier generated by positive or negative fixed charges due to polarization, and high frequency characteristics. Can also be improved.

Furthermore, in the rectifier diode described in (2) to (3), the ohmic electrode for the second conductivity type is formed to improve the expansion and contraction of the depletion layer in the transient response. Can be improved.
Further, in the rectifier diode described in (4) above, the contact resistance can be reduced by doping the polarization junction near the second electrode, thereby further improving the on-resistance of the rectifier diode. I can do it.
Furthermore, in the rectifier diode described in (10) above, the volume of the electronic circuit can be reduced by integrating the rectifier diode according to the present invention on the same substrate.

FIG. 1 is a schematic diagram of a band lineup when a GaN layer 1 and an Al _y Ga _1-y N layer 2 are stacked in the c-axis direction. Thus, fixed charges due to positive and negative polarizations at the Al _y Ga _1-y N (000-1) / GaN (0001) and Al _y Ga _1-y N (0001) / GaN (000-1) interfaces. Thus, electrons and holes can be generated respectively. In this specification, a pn junction of a semiconductor in which electrons and holes are alternately generated using polarization is defined as “polarization junction”.

The polarization junction can be used as an ideal super junction for the following two reasons. First, it is possible to equalize the positive and negative fixed charge amounts with high accuracy by suppressing the fixed charges other than the polarization in the semiconductor to be lower than the fixed charges due to the polarization. For example, assuming that the magnitude of polarization charge ρ _p = 1 × 10 ¹³ cm ⁻² , the fixed charge amount other than polarization ρ _back = 1 × 10 ¹⁶ cm ^−3, and the thickness of each layer of the polarization junction is 100 nm, And the error in the magnitude of the negative fixed charge is only 2%. Second, by using polarization junction, it is possible to greatly increase the aspect ratio of the width and length of the n-type region and the p-type region. This is because it is not necessary to use etching or ion implantation to form the polarization junction.

The characteristics of a normal rectifier diode to which the present invention is not applied and a rectifier diode having a polarization junction to which the present invention is applied are compared by device simulation. FIG. 2 is a schematic diagram of a rectifier diode to which the present invention is not applied. The anode electrode 5 has a Schottky characteristic for electrons, and the cathode electrode 6 has an ohmic characteristic for holes. Thereby, rectification is obtained. The characteristics of the n ⁻ GaN layer 3 were examined by changing the fixed charge concentration ρ _back and its length d _drift .

FIG. 3 is a schematic diagram of the structure of a rectifier diode to which the present invention is applied. The polarization junction region 7 was formed by a laminated structure of an n ^- Al _0.09 Ga _0.91 N layer having an Al composition of 9% and an n ^- GaN layer. The thickness of each layer was 100 nm. The magnitude ρ _p of the polarization charge at each hetero interface was set to 5 × 10 ¹² cm ⁻² from the theoretical value. The anode electrode 9 has a Schottky characteristic for electrons, and the cathode electrode 10 has an ohmic characteristic for holes. Further, the n-type polarization junction region 8 in which the negative charge due to the polarization is canceled is formed by doping a donor impurity in the polarization junction region near the cathode electrode 10. This n-type polarization junction region serves to reduce the contact resistance at the cathode electrode and to prevent punch-through during reverse bias. Changes in the characteristics were investigated by changing the fixed charge concentration ρ _{back of the} polarization junction region 7 other than the polarization charge and its length d _drift .

The relationship between reverse breakdown voltage and on-resistance in the rectifier diodes of FIGS. 2 and 3 was examined. FIG. 4 shows the simulation result. The withstand voltage was obtained by increasing the avalanche current and the tunnel current at the cathode electrode. First, when looking at a rectifier diode to which the present invention is not applied, it can be seen that there is a limit of a withstand voltage that can be realized depending on the magnitude of ρ _back . The limit of the pressure resistance tends to increase as ρ _back is smaller. The same tendency is also observed in the rectifier diode to which the present invention is applied, and the limit of the withstand voltage at each ρ _back is the same as that of the rectifier diode to which the present invention is not applied. This indicates that the positive and negative polarization charges compensate each other, and the remaining ρ _back determines the breakdown voltage.

  On the other hand, it can be seen from FIG. 4 that the on-resistance is greatly reduced by applying the present invention. That is, according to the present invention, the on-resistance can be reduced by the amount of carriers generated by polarization without sacrificing the withstand voltage, thereby greatly improving the trade-off between on-resistance and withstand voltage. Moreover, it turns out that the effect of this invention improves, so that it becomes high withstand pressure | voltage. For example, in the high breakdown voltage region of 10 kV or more, it can be expected that the on-resistance is improved to 1/100 or less by applying the present invention.

In general, the performance of a superjunction improves as the error between the positive and negative fixed charge amounts in a semiconductor decreases. The error in magnitude of the positive and negative fixed charges in FIG. 3 is 40%, 4% and 0.4% at ρ _back = 1 × 10 ¹⁷ , 1 × 10 ¹⁶ and 1 × 10 ¹⁵ cm ⁻³ , respectively. is there. Even when the error in the size of the fixed charge is as large as 40%, the effect of the present invention can be obtained. However, there is a limit to the withstand voltage depending on the size of ρ _back as described above. Further, the on-resistance tends to decrease as ρ _p increases. Therefore, it is desirable for application of the present invention that ρ _back is small and ρ _p is large.

  Next, rectifier diodes are mostly used at high frequencies. In order to improve the high-frequency characteristics of the rectifier diode, it is necessary to increase the response speed of the expansion and contraction of the depletion layer. For this purpose, it is necessary to efficiently inject electrons and holes into the depletion layer. Therefore, the anode electrode 9 in FIG. 3 desirably has ohmic characteristics with respect to holes. Some group III nitride semiconductors use metals such as Ni and Pt as electrodes having Schottky characteristics for electrons and ohmic characteristics for holes. However, it is generally difficult to freely adjust the height of the Schottky barrier for electrons while maintaining good ohmic characteristics for holes. In such a case, two types of electrodes may be used as the anode electrode as shown in FIG.

  The present invention can also be used in power device integrated circuits. Conventional power devices using Si require a large area to reduce on-resistance, and are difficult to integrate. On the other hand, by replacing Si with a wide band gap semiconductor, the element size can be reduced without degrading the characteristics. The present invention can further improve the trade-off between the on-resistance and the reverse withstand voltage, and has a structure suitable for manufacturing a lateral element. Therefore, the rectifier diode according to the present invention can be integrated on the same substrate and integrated with other semiconductor elements such as a high electron mobility transistor.

In this specification, the effect of polarization junction has been described by taking a group III nitride semiconductor as an example, but the present invention is applicable to all semiconductors in which polarization occurs. For example, the present invention can be applied to a heterojunction of a group II-VI oxide semiconductor typified by ZnO because fixed charges are generated due to large polarization. A heterojunction such as ZnO / Zn _m Mg _1-m O is an example. The II-VI group oxide semiconductor is a semiconductor having a very large band gap, and has great potential as a power device.

  Furthermore, SiC heterojunctions of different polytypes are also known to generate large polarization, and the present invention can be applied. Heterojunctions such as 4H—SiC / 3C—SiC and 6H—SiC / 3C—SiC are examples. SiC is also a wide band gap semiconductor, and since it is made of the same group 4 element as Si, it has the advantage that many existing technologies established with Si can be used.

Schematic diagram of the band lineup when two types of Group III nitride semiconductors are stacked Schematic diagram of rectifier diode structure not applying the present invention Schematic diagram of the structure of a rectifier diode to which the present invention is applied Simulation results of on-resistance and breakdown voltage in rectifier diodes Schematic diagram of a rectifier diode using two types of metal for the anode electrode

Explanation of symbols

1 i-GaN layer _{2 i-Al y Ga 1-} y N layer 3 ⁿ - GaN layer 4 ⁿ + GaN layer 5 anode electrode 6 cathode electrode 7 polarized junction region 8 n-type polarization junction region 9 the anode electrode 10 cathode electrode 11 Polarization junction region 12 Anode electrode 13 Anode electrode 14 using a metal different from anode electrode 12 Cathode electrode

Claims (10)

Two or more kinds of semiconductors have a laminated structure in which three or more layers are laminated so as to form a heterojunction of at least two semiconductors, and positive and negative fixed charges generated by polarization at the interface of the heterojunction, In a rectifier diode having a polarization junction adapted to simultaneously generate a carrier of a first conductivity type and a carrier of a second conductivity type,
A first electrode having Schottky characteristics with respect to the carrier of the first conductivity type at one side end of the laminated structure;
A rectifier diode comprising a second electrode having ohmic characteristics with respect to the carrier of the first conductivity type at the other side end.   2. The rectifier diode according to claim 1, wherein the first electrode has an ohmic characteristic with respect to the carrier of the second conductivity type. Two or more kinds of semiconductors have a laminated structure in which three or more layers are laminated so as to form a heterojunction of at least two semiconductors, and positive and negative fixed charges generated by polarization at the interface of the heterojunction, In a rectifier diode having a polarization junction adapted to simultaneously generate a carrier of a first conductivity type and a carrier of a second conductivity type,
A first electrode having a Schottky characteristic with respect to the carrier of the first conductivity type and an ohmic characteristic with respect to the carrier of the second conductivity type and a first electrode at one side end of the stacked structure. A third electrode electrically connected to the other electrode;
A rectifier diode comprising a second electrode having ohmic characteristics with respect to the carrier of the first conductivity type at the other side end.   4. The rectifier diode according to claim 1, wherein a part of the semiconductor adjacent to the other side end of the multilayer structure is highly doped. 5.   The rectifier diode according to any one of claims 1 to 4, wherein the two or more types of semiconductors are III-V compound semiconductors having different compositions. The group III-V compound semiconductor is a III-nitride semiconductor, chemical formulas _{B x Al y Ga z In 1} -x-y-z rectifier diode according to claim 5 which is represented by N.
(In the formula, x, y and z have numerical values satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, and x + y + z ≦ 1.)   The rectifier diode according to claim 6, wherein the group III nitride semiconductor is stacked in a c-axis direction.   The rectifier diode according to any one of claims 1 to 4, wherein the two or more types of semiconductors are II-VI group oxide semiconductors having different compositions.   The rectifier diode according to any one of claims 1 to 4, wherein the two or more types of semiconductors are SiC compound semiconductors having different crystal structures. The rectifier diode according to any one of claims 1 to 9, wherein the rectifier diode is integrated on a semiconductor substrate.

Images