Chapter Eight Special Diodes

First year class Sameer Abdul Kadhim

Chapter Eight
Special Diodes and There Application
8-1 PIN Diode
P
It is composed of three sections.
These are the usual P and N-regions but
sandwiched between them is an intrinsic
layer or I-layer of pure silicon.
Being intrinsic (or un-doped) layer, it
offers relatively high resistance. This
high-resistance region gives it two
advantages as compared to an ordinary P-N diode.
The advantages are:
1. Decrease in capacitance CT because capacitance is inversely proportional
to the separation of P-and N-regions. It allows the diode a faster response
time. Hence, PIN diodes are used at high frequencies (more than300 MHz);
2. Possibility of greater electric field between the P-and N-junctions. It
enhances the electron-hole pair generation thereby enabling PIN diode to
process even very weak input signals.

8-2 Schottky Diode

It is also called Schottky barrier diode or hot-carrier diode. It is
mainly used s a rectifier at signal frequencies exceeding 300 MHz. It has
more uniform junction region and is more rugged than PIN diode its main
rival.

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Chapter Eight Special Diodes
First year class Sameer Abdul Kadhim
(a) Construction
It is a metal-semiconductor junction diode with no depletion layer. It
uses a metal (like gold, silver, platinum, tungsten etc.) on the side of the
junction and usually an N-type doped silicon semiconductor on the other
side.

(b) Operation
When the diode is unbiased, electrons on the N-side have lower
energy levels than electrons in the metal. Hence, they cannot surmount the
junction barrier (called Schottky barrier) for going over to the metal.
When the diode is forward-biased, conduction electrons on N-side gain
enough energy to cross the junction and enter the metal. Since these
electrons plunge into the metal with very large energy, they are commonly
called ‘hot-carriers’. That is why this diode is often referred to as hot-
carrier diode.
(c) Applications
This diode possesses two unique features as compared to an ordinary
P-N junction diode:

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1. It is a unipolar device because it has electrons as majority carriers on
both sides of the junction. An ordinary P-N junction diode is a bipolar
device because it has both electrons and holes as majority carriers;
2. Since no holes are available in metal, there is no depletion layer or stored
charges to worry about. Hence, Schottky diode can switch OFF faster than a
bipolar diode
Because of these qualities, Schottky diode can easily rectify signals
of frequencies exceeding 300 MHz. As shown in Fig.(8-1), it can produce
an almost perfect half-wave rectified output.
The present maximum current rating of the device is about 100 A. It is
commonly used in switching power supplies that operate at frequencies of
20 GHz. Another big advantage of this diode is its low noise figure which is
extremely important in communication receivers and radar units etc.
It is also used in clipping and clamping circuits, computer gating, mixing
and detecting networks used is communication systems.

Fig.(8-1) Schottky diode used as HWR of frequencies exceeding 300

MHz.

8-3 Gunn Diode

It is a negative-resistance microwave device for oscillator
applications. As shown in Fig.(8-2)(a), it consists of a thin slice of N-type

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gallium arsenide sandwiched between two metal conductors. The central
section is N-gallium arsenide whereas the two outer sections are epitaxial
grown from GaAsn with increased doping and higher conductivity. As an
oscillator, its frequencies range from 5 GHz and 100 mW output up to 35
GHz and 1 mW output. Efficiencies of 3 to 5 percent are possible at
present. Fig. (8-2)(b) shows the picture of a Gunn diode.

8-4 Light Emitting Diode (LED)

(a) Theory
As the name indicates, it is a forward-biased P-N junction which
emits visible light when energized. Charge carrier recombination takes
place when electrons from the N-side cross the junction and recombine with
the holes on the P-side. Now, electrons are in the higher conduction band
on the N-side whereas holes are in the lower valence band on the P-side.
During recombination, some of the energy difference is given up in the
form of heat and light (i.e. photons). For Si and Ge junctions, greater
percentage of this energy is given up in the form of heat so that the amount
emitted as light is insignificant. But in the case of other semiconductor
materials like gallium arsenide (GaAs), gallium phosphide (GaP) and
gallium-arsenide-phosphide (GaAsP), a greater percentage of energy
released during recombination is given out in the form of light.

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Chapter Eight Special Diodes
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If the semiconductor material is translucent, light is emitted and the

junction becomes a light source i.e. a light-emitting diode (LED) as shown
schematically in Fig.(8-3) The color of the emitted light depends on the
type of material used as given in table below.

material color
GaAs Infrared radiation (invisible).
GaP Red or green light.
GaAsP Red or yellow (amber) light.

LEDs that emit blue light are also available but red is the most
common. LEDs emit no light when reverse-biased. In fact, operating
LEDs in reverse direction will quickly destroy them. Fig.(8-3)shows a
picture of LEDs that emits different colors of light.
(b) Construction
Broadly speaking, the LED structures can be divided into two categories:
1. Surface-emitting LEDs: These LEDs emit light in a direction
perpendicular to the PN junction plane.

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2. Edge-emitting LEDs : These LEDs emit light in a direction parallel to
the PN junction plane.
Fig. (8-3) shows the construction of a surface-emitting LED. As seen
from this figure, an N-type layer is grown on a substrate and a P-type layer
is deposited on it by diffusion. Since carrier recombination takes place in
the P-layer, it is kept upper most. The metal anode connections are made at
the outer edges of the P-layer so as to allow more central surface area for
the light to escape. LEDs are manufactured with domed lenses in order to
lessen the reabsorption problem.

Fig.(8-3) picture of LEDs and its internal structure

A metal (gold) film is applied to the bottom of the substrate for
reflecting as much light as possible to the surface of the device and also to
provide cathode connection. LEDs are always encased in order to protect
their delicate wires. Being made of semiconductor material, it is rugged and
has a life of more than 10,000 hours.
(c) Working
The forward voltage across an LED is considerably greater than for a
silicon PN junction diode. Typically the maximum forward voltage for

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LED is between 1.2 V and 3.2 V depending on the device. Reverse
breakdown voltage for an LED is of the order of 3 V to 10 V. Fig. (8-4) (a)
shows a simple circuit to illustrate the working of an LED. The LED emits
light in response to a sufficient forward current. The amount of power
output translated into light is directly proportional to the forward current as
shown in Fig. (8-4)(b). It is evident from this figure that greater the forward
current, the greater the light output.

Fig.(8-4) LED
operation

(d) Applications
To choose emitting diodes for a particular application, one or more of
the following points have to be considered : wavelength of light emitted,
input power required, output power, efficiency, turn-on and turn-off time,
mounting arrangement, light intensity and brightness etc. Since LEDs
operate at voltage levels from 1.5 V to 3.3 V, they are highly compatible
with solid-state circuitry.
Their uses include the following:
1. LEDs are used in burglar-alarm systems;
2. For solid-state video displays which are rapidly replacing cathode-ray
tubes (CRT);

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3. In image sensing circuits used for ‘picture phone’;
4. In the field of optical fiber communication systems where high-radiance
GaAs diodes are matched into the silica-fiber optical cable;
5. In data links and remote controllers;
6. In arrays of different types for displaying alphanumeric (letters and
numbers) or supplying input power to lasers or for entering information into
optical computer memories;
7. For numeric displays in hand-held or pocket calculators.

As shown in Fig.(8-5)(a) a seven-segment display consists of seven

rectangular LEDs which can form the digits 0 to 9. The seven LED
segments are labeled ‘a’ to ‘g’. Each of these segments is controlled
through one of the display LEDs. Seven-segment displays come in two
types, common-cathode and common-anode type. In the common-cathode
type, all the cathodes of the diodes are tied together as shown in Fig.(8-
5)(b).

This makes it possible to light any segment by forward-biasing that

particular LED. For example, to light number 5, segments a, f, g, c and d

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must be forward-biased. Since the cathodes are tied to ground, only 5 volt
is to be applied to the anode of these segments to light them.
The common-anode seven-segment display has all its anodes tied
together to +5 volt and ground is used to light the individual segments. Fig.
(8-5)(c) shows a picture of a seven-segment
display.

Multicolored LEDs
LEDs are available which gives out
light in either two or three colors. There are
also blinking LEDs. A two-color LED is a
three-terminal device as shown in Fig. (8-6)
the longest lead is the cathode and the
remaining two leads are the anodes. When
leads a1 and k are forward-biased, the LED
emits red light and when leads a2 and k are
forward-biased, LED emits green light.
The tricolor LED looks similar to the ordinary LED but emits red,
green or yellow light depending on operating conditions. It has two leads
and each of these acts as both anode and cathode. When DC current flows
through it in one direction, LED emits red light but when current flows in
the opposite direction, LED emits green light. However, with AC current,
yellow light is given out. The blinking LED is a combination of an
oscillator and a LED in one package. Since it has an anode and a cathode
lead, it looks like an ordinary LED. The blinking frequency is usually 3 Hz
when the diode forward bias is 5 V. It conducts about 20 mA of current
when ON and 0.9 mA when OFF.

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Chapter Eight Special Diodes
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8-5 Photodiode
It is a two-terminal junction device which is operated by first reverse-
biasing the junction and then illuminating it. A reverse-biased P-N junction
has a small amount of reverse saturation current I0 due to thermally-
generated electron-hole pairs. In silicon, I0 is the range of nano-amperes.
The number of these minority carriers depends on the intensity of light
incident on the junction. When the diode is in glass package, light can
reach the junction and thus change the reverse current.
The basic biasing arrangement, construction and symbols of a
photodiode are shown in Fig. (8-7)(b) As seen, a lens has been used in the
cap of the unit to focus maximum light on the reverse-biased junction. The
active diameter of these devices is about 2.5 mm but they are mounted in
standard TO-5 packages with a window to allow maximum incident light.

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Chapter Eight Special Diodes
First year class Sameer Abdul Kadhim
The characteristics of Fig.(8-8) show that for a given reverse voltage, I0
increases with increase in the level of illumination. The dark current refers
to the current that flows when no light is incident. By changing the
illumination level, reverse current can be changed. In this way, reverse
resistance of the diode can be changed by a factor of nearly 20. A
photodiode can turn its current ON and OFF in nanoseconds. Hence, it is
one of the
fastest photo
detectors. It is
used where it
is required to
switch light
ON and OFF
at a maximum
rate.

Applications of a photodiode include:

1. Detection, both visible and invisible.
2. Demodulation in optical communication.
3. Photo sensor for light level measurement.
4. Photo sensor for high speed switch ( opto-coupler )
5. IR remote control receiver ( TV , receiver, ….)

8-6 Photo Voltaic or Solar Cell

Such cells operate on the principle of photovoltaic action i.e.
conversion of light energy into electrical energy. This action occurs in all
semiconductors which are constructed to absorb energy.

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(a) Construction
As shown in Fig.(8-9)(a), a basic solar cell consists of P-type and N-type
semiconductor material (usually, silicon or selenium) forming a P-N
junction. The bottom surface of the cell (which is always away from light)
covered with a continuous conductive contact to which a wire lead is
attached. The upper surface has a maximum area exposed to light with a
small contact either along the edge or around the perimeter. The surface
layer of P-type material is extremely thin (0.5 mm) so that light can
penetrate to the junction.

Although silicon is commonly used for fabricating solar cells,

another construction consists of P-type selenium covered with a layer of N-
type cadmium oxide to form P-N junction as shown in Fig.(8-9) (b). Two
alternative circuit symbols are shown in Fig. (8-9)(c). Power solar cells are
also fabricated in flat strips to form efficient coverage of available surface
area. Incidentally, the maximum efficiency of a solar cell in converting
sunlight into electrical energy is nearly 15 per cent at the present.
(b) Theory
When the P-N junction of a solar cell is illuminated, electron-hole
pairs are generated in much the same way, as in photovoltaic cell. An
electric field is established near the P-N junction by the positive and
negative ions created due to the production of electron-hole pairs which
leads to the development of potential across the junction. Since the number

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of electron-hole pairs far exceeds the number needed for thermal
equilibrium, many of thee electrons are pulled across the junction by the
force of the electric field.

Those that cross the junction contribute to the current in the cell and
through the external load. The terminal voltage of the cell is directly
proportional to the intensity of the incident light. The voltage may be as
high as 0.6 V depending on the external load. Usually a large number of
cells are arranged in an array in order to obtained higher voltages and
currents as shown in Fig. (8-10)(a)

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Solar cells act like a battery when connected in series or parallel.

Fig. (8-10)(b) shows two groups of 10 series cells connected in parallel
with each other. If each cell provides 0.5 V at 150 mA, the overall value of
the solar bank is 5 V at 150 mA. The two parallel solar banks provide 5 V
at 300 mA. This solar power source supplies the load and also charges the
Ni-Cd battery. The battery provides power in the absence of light. A
blocking diode D is used to isolate the solar cells from the Ni-Cd battery
otherwise in the absence of light, the battery will discharge through the
cells thereby damaging them.
A solar cell operates with fair efficiency, has unlimited life, can be
easily mass-produced and has a high power capacity per weight. It is
because of these qualities that it has become an important source of power
for earth satellites.

8-7 Laser Diode

Like LEDs, laser diodes are typical PN junction devices used under a
forward-bias. The word LASER is an acronym for Light Amplification by
Stimulated Emission of Radiation. The use of laser is (becoming
increasing common) in medical equipment used in surgery and in consumer

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products like compact disk (CD) players, laser printers, hologram scanners
etc.
(a) Construction
Broadly speaking, the laser diode structure can be divided into two
categories:
1. Surface-emitting laser diodes: These laser diodes emit light in a
direction perpendicular to the PN junction plane.
2. Edge-emitting laser diodes: These laser diodes emit light in a direction
parallel to the PN junction plane.
Fig.(8-11) (a) shows the structure of an edge-emitting laser diode.
This type of structure is called Fabry-Perot type laser. As seen from the
figure, a P-N junction is formed by two layers of doped gallium arsenide
(GaAs). The length of the PN junction bears a precise relationship with the
wavelength of the light to be emitted. As seen, there is a highly reflective
surface at one end of the junction and a partially reflective surface at the
other end. External leads provide the anode and cathode connections.

(b) Theory
When the P-N junction is forward-biased by an external voltage
source, electrons move across the junction and usual recombination occurs
in the depletion region which results in the production of photons. As
forward current is increased, more photons are produced which drift at
random in the depletion region. Some of these photons strike the reflective

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surface perpendicularly. These reflected photons enter the depletion region,
strike other atoms and release more photons. All these photons move back
and forth between the two reflective surfaces as shown in Fig. (8-11) (b) .
The photon activity becomes so intense that at some point, a strong beam of
laser light comes out of the partially reflective surface of the diode.
(c) Application
Laser diode used in variety of application ranging medical equipment
used in surgery to consumer products like optical disk equipment, laser
printer, hologram scanners etc. laser diodes emitting visible light are used
as pointers. Those emitting visible infrared light are used to measure range
(or distance). The laser diodes are also widely used in parallel processing of
information and in parallel interconnection between computers.

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