Semiconductors

Basic Electronics Engineering (Semiconductors)
Prepared By Mr. A. B. Shinde, Electronics Engg., PVPIT, Budhgaon 1
Unit- II: Semiconductors
We know that some solids are good conductors of electricity while others are insulators.
There is also an intermediate class known as semiconductors.
1. Conductivity of insulators, Metals and Semiconductors
1.1. Conductivity of insulators in terms of energy bands:
Insulators (e.g. wood, glass etc.) are those
substances which do not allow the passage of
electric current through them.
In terms of energy band, the valence band
is full while the conduction band is empty. Further,
the energy gap between valence and conduction
bands is very large ( 15 eV) as shown in figure.
Therefore, a very high electric field is required to
push the valence electrons to the conduction
band.
For this reason, the electrical conductivity of such materials is extremely small and may
be considered as zero. At room temperature, the valence electrons of the insulators do
not have enough energy to cross over to the conduction band. However, when the
temperature is raised, some of the valence electrons may acquire enough energy to
cross over to the conduction band. Hence, the resistance of an insulator decreases with
the increase in temperature.
1.2. Conductivity of metals (conductors) in terms of energy bands:
Metals or Conductors (e.g. copper,
aluminium) are those substances which easily
allow the passage of electric current through
them. It is because there are a large number of
free electrons available in a conductor. In terms of
energy band, the valence and conduction bands
overlap each other as shown in figure. Due to this
overlapping, a slight potential difference across a
conductor causes the free electrons to constitute
electric current. Thus, the electrical behaviour of
conductors can be satisfactorily explained by the
band energy theory of materials.

1.3. Conductivity of semiconductors in terms of energy bands:
Semiconductors (e.g. germanium, silicon
etc.) are those substances whose electrical
conductivity lies in between conductors and
insulators. In terms of energy band, the valence
band is almost filled and conduction band is
almost empty. Further, the energy gap between
valence and conduction bands is very small as (
1 eV) shown in figure. Therefore, comparatively
smaller electric field (smaller than insulators but
much greater than conductors) is required to push
the electrons from the valence band to the
conduction band.
In short, a semiconductor has:
(a) Almost full valence band
(b) Almost empty conduction band
(c) Small energy gap ( 1 eV) between valence and conduction bands.
2. Bonds in Semiconductors:
The atoms of every element are held together by the bonding action of valence
electrons. This bonding is due to the fact that it is the tendency of each atom to
complete its last orbit by acquiring 8 electrons in it. However, in most of the substances,
the last orbit is incomplete i.e. the last orbit does not have 8 electrons. This makes the
atom active to acquire 8 electrons in the last orbit. To do so, the atom may lose, gain or
share valence electrons with other atoms.
In semiconductors, bonds are formed by sharing of valence electrons. Such
bonds are called covalent bonds. In the formation of a covalent bond, each atom
contributes equal number of valence electrons and the contributed electrons are shared
by the atoms engaged in the formation of the bond.
Commonly Used Semiconductors: Germanium (Ge) & Silicon (Si):
There are many semiconductors available, but very few of them have a practical
application in electronics. The two most frequently used materials are Germanium (Ge)
and Silicon (Si). It is because the energy required to break their covalent bonds is very
small; being about 0.7 eV for germanium and about 1.1 eV for silicon.
2.1. Bonds in Germanium:
Germanium has become the model substance among the semiconductors;
because it can be purified relatively well and crystallised easily.
The atomic number of germanium is 32. Therefore, it has 32 protons and 32 electrons.
2 electrons are in the 1st
orbit, 8 electrons in the 2nd
, 18 electrons in the 3rd
and 4
electrons in the last orbit. It is clear that germanium atom has four valence electrons

i.e., it is a tetravalent element. Right side figure shows how the various germanium
atoms are held through covalent bonds. As the atoms are arranged in an orderly
pattern, therefore, germanium has crystalline structure.
Structure of Ge atom Covalent bonds in Ge Crystal structure of Ge
2.2. Bonds in Silicon:
Silicon is an element in most of the common rocks. Actually, sand is silicon
dioxide. The silicon compounds are chemically reduced to silicon which is 100% pure
for use as a semiconductor. The atomic number of silicon is 14. Therefore, it has 14
protons and 14 electrons. 2 electrons are in the 1st
orbit, 8 electrons in the 2nd
orbit and
4 electrons in the last orbit. It is clear that silicon atom has four valence electrons i.e. it
is a tetravalent element. Right side figure shows how various silicon atoms are held
through covalent bonds. Like germanium, silicon atoms are also arranged in an orderly
manner. Therefore, silicon has crystalline structure.
Structure of Si atom Crystal structure of Si
2.3. Energy Band Description of Semiconductors:
Below figure show that, the energy band diagrams of germanium and silicon
respectively. It may be seen that forbidden energy gap is very small; being 1.1 eV for
silicon and 0.7 eV for germanium. Therefore, relatively small energy is needed by their
valence electrons to cross over to the conduction band. At room temperature, a piece of
germanium or silicon is neither a good conductor nor an insulator. For this reason, such
substances are called semiconductors.

2.4. Effect of Temperature on Semiconductors:
The electrical conductivity of a semiconductor changes appreciably with
temperature variations.
(i) At absolute zero: At absolute zero temperature, all the electrons are tightly held by
the semiconductor atoms. The inner orbit electrons are bound whereas the valence
electrons are engaged in covalent bonding. At this temperature, the covalent bonds are
very strong and there are no free electrons. Therefore, the semiconductor crystal
behaves as a perfect insulator
(ii) Above absolute zero: When the temperature is raised, some of the covalent bonds
in the semiconductor break due to the thermal energy supplied; hence, some electrons
will become free. The result is that a few free electrons exist in the semiconductor.
These free electrons can constitute a tiny electric current if potential difference is
applied across the semiconductor crystal. This shows that the resistance of a
semiconductor decreases with the rise in temperature.
3. Intrinsic & Extrinsic Semiconductors:
3.1. Intrinsic Semiconductor:
A semiconductor in an extremely pure form is known as an intrinsic
semiconductor.
In an intrinsic
semiconductor, even at room
temperature, hole-electron pairs
are created. When electric field
is applied across an intrinsic
semiconductor, the current
conduction takes place by two
processes, namely; by free
electrons and holes as shown
in figure.

The free electrons are produced due to the breaking up of some covalent bonds
by thermal energy. At the same time, holes are created in the covalent bonds. Under
the influence of electric field, conduction through the semiconductor is by both free
electrons and holes. Therefore, the total current inside the semiconductor is the sum of
currents due to free electrons and holes.
It may be noted that current in the external wires is fully electronic i.e. by
electrons. What about the holes? Holes being positively charged move towards the
negative terminal of supply. As the holes reach the negative terminal B, electrons enter
the semiconductor crystal near the terminal and combine with holes.
3.2. Extrinsic Semiconductor:
As intrinsic semiconductor has little current conduction capability at room
temperature. To be useful in electronic devices, the pure semiconductor must be
altered so as to significantly increase its conducting properties. This is achieved by
adding a small amount of suitable impurity to a semiconductor.
It is then called impurity or extrinsic semiconductor. The process of adding
impurities to a semiconductor is known as doping. The purpose of adding impurity is to
increase either the number of free electrons or holes in the semiconductor crystal.
If a pentavalent impurity (having 5 valence electrons) is added to the semiconductor, a
large number of free electrons are produced in the semiconductor. On the other hand,
addition of trivalent impurity (having 3 valence electrons) creates a large number of
holes in the semiconductor crystal.
Depending upon the type of impurity added extrinsic semiconductors are classified into:
(i) n-type semiconductor (ii) p-type semiconductor
3.2.1. n-type Semiconductor:
When a small amount of
pentavalent impurity is added to a pure
semiconductor, it is known as n-type
semiconductor.
The addition of pentavalent impurity
provides a large number of free electrons
in the semiconductor crystal. Typical
examples of pentavalent impurities are
arsenic (Atomic no. 33) and antimony
(Atomic no. 51). Such impurities which
produce n-type semiconductor are known
as donor impurities because they donate
or provide free electrons to the
semiconductor crystal.
Arsenic is pentavalent i.e. its atom has five valence electrons. An Arsenic atom
fits in the Germanium crystal in such a way that its four valence electrons form covalent
bonds with four Germanium atoms. The fifth valence electron of Arsenic atom finds no

place in covalent bonds and is thus free as shown in above figure. Therefore, for each
Arsenic atom added, one free electron will be available in the germanium crystal.
3.2.1.1. n-type conductivity:
The current conduction in an n-type
semiconductor is predominantly by free
electrons i.e. negative charges and is called
n-type. Consider the figure shown; when
voltage is applied across the n-type
semiconductor, the free electrons in the
crystal will be directed towards the positive
terminal, constituting electric current.
As the current flow through the crystal is by free electrons which are carriers of
negative charge, therefore, this type of conductivity is called negative or n-type
conductivity.
3.2.2. p-type Semiconductor:
When a small amount of trivalent
impurity is added to a pure semiconductor, it
is called p-type semiconductor.
The addition of trivalent impurity
provides a large number of holes in the
semiconductor. Typical examples of trivalent
impurities are gallium (Atomic no. 31) and
indium (Atomic no. 49). Such impurities
which produce p-type semiconductor are
known as acceptor impurities because the
holes created can accept the electrons.
Gallium is trivalent i.e. its atom has three valence electrons. Each atom of
Gallium fits into the Germanium crystal but now only three covalent bonds can be
formed. It is because three valence electrons of Gallium atom can form only three
single covalent bonds with three Germanium atoms as shown in figure. In the fourth
covalent bond, only germanium atom contributes one valence electron while gallium
has no valence electron to contribute. In other words, fourth bond is incomplete; being
short of one electron. This missing electron is called a hole. Therefore, for each Gallium
atom added, one hole is created.
3.2.2.1. p-type conductivity:
The current conduction in p-type
semiconductor is predominantly by holes i.e.
positive charges and is called p-type.
Consider the figure shown; when voltage is
applied to the p-type semiconductor, the
holes are shifted from one covalent bond to
another covalent bond.

As the holes are positively charged, therefore, they are directed towards the
negative terminal, constituting what is known as hole current. It may be noted that in p-
type conductivity, the valence electrons move from one covalent bond to another unlike
the n-type where current conduction is by free electrons.
3.2.3. Majority and Minority Carriers:
An intrinsic of pure germanium can be
converted into a p-type semiconductor by the
addition of an acceptor impurity which adds a
large number of holes to it.
Hence, a p-type material contains
following charge carriers:
(a) Large number of positive holes;
(b) A very small number of electrons.
Obviously, in a p-type material, the number of holes is much more than that of
electrons. Hence, in such a material, holes constitute majority carriers and electrons
form minority carriers as shown in figure.
Similarly, in an n-type material, the
number of electrons is much larger than the
number of holes.
Hence, in such a material, electrons
are majority carriers whereas holes are
minority carriers as shown in figure.
4. Hall Effect in Semiconductors:
Definition: When a magnetic
field is applied to a current
carrying conductor in a
direction perpendicular to that
of the flow of current,
a potential difference or
transverse electric field is
created across a conductor.
This phenomenon is known as
Hall Effect.
Hall Effect was discovered by Edwin Hall in 1879. The voltage or electric field
produced due to the application of magnetic field is also referred to as Hall voltage or
Hall field.

4.1. Hall Effect in n-type
semiconductor
If the magnetic field is applied to
an n-type semiconductor, both free
electrons and holes are pushed down
towards the bottom surface of the n-
type semiconductor. Since, the holes
are negligible in n-type semiconductor,
so free electrons are mostly
accumulated at the bottom surface of
the n-type semiconductor.
This produces a negative charge on the bottom surface with an equal amount of
positive charge on the upper surface. As a result, the potential difference is developed
between the upper and bottom surface of the n-type semiconductor.
4.1. Hall Effect in p-type semiconductor
If the magnetic field is applied to
a p-type semiconductor, the majority
carriers (holes) and the minority carriers
(free electrons) are pushed down
towards the bottom surface of the p-
type semiconductor. In the p-type
semiconductor, free electrons are
negligible. So holes are mostly
accumulated at the bottom surface of
the p-type semiconductor.
So in the p-type semiconductor, the bottom surface is positively charged and the
upper surface is negatively charged. As a result, the potential difference is developed
between the upper and bottom surface of the p-type semiconductor.
5. Mechanism of current flow:
In general, current flows through the metals due to free electrons; but in case of
semiconductors current flows due to both electrons as well as holes. Normally there are
two types of currents flowing through the semiconductor:
Drift Current &
Diffusion current
5.1. Drift Current: The drift current, is the current in semiconductor due to the motion of
charge carriers due to the force exerted on them by an electric field.
5.2. Diffusion Current: The Diffusion current is a current is due to the diffusion of
charge carriers (holes and/or electrons). Diffusion current can be in the same or
opposite direction of a drift current.

For example: The current near the depletion region of a p–n junction is dominated by
the diffusion current. Inside the depletion region, both diffusion current and drift current
are present. At equilibrium in a p–n junction, the forward diffusion current in the
depletion region is balanced with a reverse drift current, so that the net current is zero.
5.3. Diffusion current versus drift current:
Diffusion current Drift current
In diffusion current the flow is caused by
variation in the concentration.
In drift current the movement caused
by electric fields.
The magnitude of the diffusion current
depends on the slope of the carrier
concentration.
The magnitude depends on the
carrier concentration.
Direction of the diffusion current depends on
the slope of the carrier concentration.
Direction of the drift current is always
in the direction of the electric field.
Does not obey Ohm's law Obeys Ohm's law
5.4. Einstein Relation:
Mobility characterizes how quickly an electron or hole can move through
a semiconductor, when electric field is applied to it.
The process of electrons or holes moving from the higher concentration region to the
lower concentration region is called diffusion. The drift current density of electrons (or
holes) is directly proportional to the mobility of electrons (or holes) while the diffusion
current density of electrons (or holes) is directly proportional to the diffusion coefficient
of electrons (or holes).
The equation which relates the mobility µ of electrons (or holes) and the diffusion
coefficient of electrons Dn (or holes Dp) is known as Einstein Relationship.
The Einstein Relationship is expressed as:
Where,
Dp = Diffusion coefficient of holes
Dn = Diffusion coefficient of electrons
µp = Mobility of holes
µn = Mobility of electrons
VT is called voltage equivalent of temperature and it can be expressed as
VT = KT/q = T/11600
VT = 26 mV at 300 K

6. Semiconductor Materials:
Semiconductor materials are nominally small band gap insulators. The defining
property of a semiconductor material is that it can be doped with impurities that alter its
electronic properties.
Most commonly used semiconductor materials are crystalline inorganic solids.
Silicon and Germanium are the popular semiconductors and are called as elemental
semiconductors. There are another important type of semiconductors; III-V compound
semiconductors. Similarly there is one more semiconductor compound called as II-VI
compound.
6.1. II-VI compound:
II – VI semiconductor compounds are composed of metals from 2nd
or 12th
group
and non metals from 16th
group. II – VI compounds generally exhibits large band gaps;
hence they are popular in short wavelength applications.
Material Formula
Band gap
(eV)
Description
Cadmium
selenide
CdSe 1.74
Nanoparticles used as quantum dots. Possible use
in optoelectronics. Tested for high-efficiency solar
cells.
Cadmium
sulfide
CdS 2.42 Used in photo resistors and solar cells.
Cadmium
telluride
CdTe 1.49
Used in thin film solar cells and other cadmium
telluride photovoltaic’s. Used in electro-optic
modulators.
Zinc
selenide
ZnSe 2.7 Used for blue lasers and LEDs.
Zinc sulfide ZnS 3.54/3.91
Band gap 3.54 eV (cubic), 3.91 (hexagonal).
Common scintillator/phosphor when suitably
doped.
Zinc
telluride
ZnTe 2.25
Used in solar cells, components of microwave
generators, blue LEDs and lasers.
Zinc oxide ZnO 3.37
Used for preparing transparent conductive
coatings. Resistant to radiation damage. Possible
use in LEDs and laser diodes.

6.2. III-V compounds
III – V semiconductor compounds can be of the following three types:
1. III – V Binary compounds
2. III – V Ternary compounds
3. III – V Quaternary compounds
6.2.1. III – V Binary compounds
III – V Binary semiconductor compounds are made from element of III group and
V group. The fundamental properties of III – V binary compounds are:
Average atomic number Band gap energy
Refractive index Effective mass
Dielectric constant
Material Formula
Band
gap (eV)
Description
Boron
nitride
BN 6.36 Useful for ultraviolet LEDs
Boron
arsenide
BAs 1.14
Resistant to radiation damage, possible
applications in betavoltaics.
Gallium
phosphide
GaP 2.26
Used in early low to medium brightness cheap
red/orange/green LEDs.
Gallium
arsenide
GaAs 1.43
Used for near-IR LEDs, fast electronics and high-
efficiency solar cells.
6.2.2. III – V Ternary compounds
When we add one extra element form group III or group V to the III – V binary
compound, it becomes III – V ternary compound. The added element is distributed
randomly in the crystal lattice.
Material Formula
Band
gap (eV)
Description
Aluminium
gallium
arsenide
AlxGa1−xAs 1.42
Used for infrared laser diodes. Used as a barrier layer
in GaAs devices.
Indium
gallium
arsenide
InxGa1−xAs 0.36
Used in infrared sensors, photodiodes, laser diodes,
optical fiber communication detectors, and short-
wavelength infrared cameras.
gallium
phosphide
InxGa1−xP 1.35
Used for HEMT and HBT structures and high-
efficiency multi-junction solar cells. Ga0.5In0.5P is
almost lattice-matched to GaAs, with AlGaIn used for
quantum wells for red lasers.

III – V Quaternary compounds
Similar to the ternary compounds, we can obtain III – V Quaternary compounds
by using four different elements form III group and V group. By controlling the
composition of quaternary alloy, it is possible to control both its band gap energy and
lattice parameters.
Material Formula
Band
gap (eV)
Description
Copper zinc
tin sulfide,
CZTS
Cu2ZnSnS4 1.49
Cu2ZnSnS4 is derived from CIGS, replacing the
Indium/Gallium with earth abundant Zinc/Tin.
Copper zinc
antimony
sulfide, CZAS
Cu1.18Zn0.40Sb
1.90S7.2
2.2
Copper zinc antimony sulfide is derived from
copper antimony sulfide (CAS), a famatinite class
of compound.
Aluminium
gallium indium
phosphide
AlGaInP Used for waveengths between 560–650 nm
7. The p-n junction diode:
When a p-type semiconductor is suitably joined to n-type semiconductor, the
contact surface is called p-n junction or p-n junction diode.
7.1. Properties of p-n Junction
The moment p & n semiconductor materials are
attached together to form p-n junction, the free
electrons near the junction diffuses across the junction
with holes. The result is that n region loses free
electrons as they diffuse into the junction; this creates a
layer of positive charges (ions) near the junction. The p
region also loses holes as the electrons and holes
combine; this creates a layer of negative charges (ions)
near the junction.
These two layers of positive and negative
charges form the depletion region (or depletion layer).
The depletion layer is formed very quickly and is very
thin compared to the n region and the p region. This
depletion region acts as a barrier to the further
movement of free electrons across the junction.
The positive and negative charges set up an
electric field. There exists a potential difference across
the depletion layer and is called barrier potential (V0).
For Silicon, V0 = 0.7 V; Germanium, V0 = 0.3 V

7.2. Working of p-n junction diode:
Forward biasing:
When external d. c. voltage is applied to the
junction, it will cancel the potential barrier,
permitting the current flow is called as forward
biasing.
To apply forward bias, connect positive terminal of the battery to p-type and
negative terminal to n-type as shown in above figure. Once the potential barrier is
eliminated by the forward voltage (0.7 V for Si and 0.3 V for Ge) junction resistance
becomes almost zero and a low resistance path is established for the entire circuit.
Therefore, current flows in the circuit. This is called forward current.
Reverse biasing:
When the external d. c. voltage is applied to
the junction which will increase the potential
barrier is called as reverse biasing.
To apply reverse bias, connect negative
terminal of the battery to p-type and positive
terminal to n-type as shown in figure. The
increased potential barrier prevents the flow of
charge carriers across the junction.
Thus, a high resistance path is established for the entire circuit and hence the
current does not flow.
V-I Characteristics of p-n junction diode:
To plot the Voltage-
Ampere (V-I) characteristics
curve of p-n junction diode,
the circuit arrangement is
made as shown in figure.
Volt meter (V) is placed
across and current meter
(mA) is placed in series with
p-n junction diode.

The V-I characteristics curve of p-n junction diode is as shown in figure. Forward
characteristics are there in I – quadrant. When the forward voltage is increased, the
current increase slowly. As soon as the externally applied voltage exceeds the barrier
voltage, heavy current starts to flow. Reverse characteristics are shown in III –
quadrant. When reverse voltage is increased, it will further increase the barrier voltage.
At one instance, the barrier breaks and heavy current (reverse current) starts to flow.
Breakdown Voltage: It is the minimum reverse voltage at which p-n junction breaks
down with sudden rise in reverse current.
Knee Voltage: It is the forward voltage at which the current through the junction starts
to increase rapidly.
7.3. Diode equivalent circuit:
An equivalent circuit of a device is a combination of electric elements, which
when connected in a circuit, acts exactly as does the device when connected in the
same circuit.
Approximate Equivalent circuit:
When the forward voltage VF is applied across a diode, it will not conduct till the
potential barrier V0 at the junction is overcome. When the forward voltage exceeds the
potential barrier voltage, the diode starts conducting as shown in figure.
Simplified Equivalent circuit:
For most applications, the internal resistance rf of the crystal diode can be ignored in
comparison to other elements in the equivalent circuit. The equivalent circuit then
reduces to the one shown in figure.
Ideal diode model:
An ideal diode is one which behaves as a perfect conductor when forward biased and
as a perfect insulator when reverse biased. Obviously, in such a hypothetical situation,
forward resistance rf = 0 and potential barrier V0 is considered negligible.

7.5. Diode Current equation:
Diode current equation expresses the relationship between the current flowing
through the diode as a function of the voltage applied across it.
Mathematically it is given as:
Where,
I = current flowing through the diode
I0 = dark saturation current
q = charge on the electron
V = Voltage applied across the diode
η = constant, (for Ge, η = 1 & for Si, η = 2)
is the Boltzmann constant (26mV at room temp.)
T = absolute temperature in Kelvin
KT = 26 mV at room temperature
I0 is the Dark Saturation Current. It indicates the leakage current density flowing through
the diode in the absence of light.
η is the constant. The value of η is typically considered to be 1 for germanium diodes
and 2 for silicon diodes.
In forward biased condition, there will a large amount of current flow through the diode.
Thus the diode current equation becomes,
On the other hand, if the diode is reverse biased, then the exponential term in above
equation becomes negligible. Thus we have

7.4. Diode as Switch:
Whenever a specified voltage is exceeded, the diode resistance gets increased,
making the diode reverse biased and it acts as an open switch. Whenever the voltage
applied is below the reference voltage, the diode resistance gets decreased, making the
diode forward biased and it acts as a closed switch.
The following circuit explains the diode acting as a switch.
A switching diode has a PN junction in which P-region is lightly doped and N-
region is heavily doped. The above circuit symbolizes that the diode gets ON when
positive voltage forward biases the diode and it gets OFF when negative voltage
reverse biases the diode.
7.5. Testing of Diode:
Anode-Cathode Diode
Resistance Test
Place the positive probe of
ohmmeter on the anode of the
diode and the negative probe on
the cathode of the diode, as
shown in figure. In this setup, the
diode should read a moderately
low resistance, maybe a few tens
of ohms. For example, you may
read 10 Ω – 20 Ω.
Cathode-Anode Diode Resistance Test
Take the ohmmeter place the positive probe of the multimeter on the cathode of the
diode and the negative probe on the anode. In this setup now, the diode should read a
much higher resistance, over few KΩ - 1 MΩ.

Open Diode
If the diode reads high resistance in both directions, this is a sign that the diode is open.
A diode should not measure very high resistance in the forward biased direction.
Shorted Diode
If the diode reads low resistances in both directions, this is a sign that the diode is short
circuited. A diode should not measure low resistance in the reverse biased direction.
The diode should be replaced in the circuit.
8. Other Diodes:
A number of specific types of diodes are manufactured for specific applications. Some
of the more common special-purpose diodes are:
a. Zener diode
b. Light-emitting diode (LED)
c. Photo-diode
d. Tunnel diode
e. Varactor Diode
f. PIN Diode
g. Schottky diode
h.
8. 1. Zener Diode:
A properly doped crystal diode which has a
sharp breakdown voltage is known as a zener
diode.
A zener diode is a special type of diode
that is designed to operate in the reverse
breakdown region.
A zener diode is heavily doped to reduce
the reverse breakdown voltage. This causes a
very thin depletion layer. As a result, a zener
diode has a sharp reverse breakdown voltage
VZ. This is clear from the reverse characteristic
of zener diode are shown in figure. Note that
the reverse characteristic drops in an almost
vertical manner at reverse voltage VZ.
From the curve two things are clear,
when VZ value is reached:
(i) The diode current increases rapidly.
(ii) The reverse voltage VZ across the diode
remains almost constant.
Symbol of Zener Diode
Characteristics of Zener Diode

Equivalent Circuit of Zener Diode
8. 2. Light-Emitting Diode (LED):
A light-emitting diode (LED) is a diode that gives off visible light when forward
biased.
Light-emitting diodes are made by using elements like
gallium, phosphorus and arsenic.
When a LED is manufactured using gallium
arsenide, it will produce a red light. If the LED is made with
gallium phosphide, it will produce a green light.
When light-emitting diode (LED) is forward biased
as shown below figure, the electrons from the n-type
material cross the p-n junction and recombines with holes
in the p-type material. When recombination takes place,
the recombining electrons release energy in the form of
heat and light.
In germanium and silicon diodes,
almost the entire energy is given up in
the form of heat and emitted light is
insignificant. However, in materials like
gallium arsenide, the number of
photons of light energy is sufficient to
produce quite intense visible light.
Symbol of LED

The graph shows the graph
between radiated light and the
forward current of the LED. It is
clear from the graph that the
intensity of radiated light is directly
proportional to the forward current
of LED.
8. 3. Photo Diode:
A photo-diode is a reverse-biased silicon or germanium p-n junction in which
reverse current increases when the junction is exposed to light.
The reverse current in a photo-diode is directly proportional to the intensity of
light falling on its p-n junction. This means that greater the intensity of light falling on the
p-n junction of photo-diode, the greater will be the reverse current.
It consists of a p-n junction mounted on an insulated substrate and sealed inside a
metal case. A glass window is mounted on top of the case to allow light to enter and
strike the p-n junction. The two leads extending from the case are labelled anode and
cathode. The cathode is typically identified by a tab extending from the side of the case.
8. 4. Tunnel Diode:
A tunnel diode is a p-n junction that exhibits negative resistance between two
values of forward voltage (i.e., between peak-point voltage and valley-point voltage).
The tunnel diode is basically a p-n junction with heavy doping of p-type and n-
type semiconductor materials. A tunnel diode is doped approximately 1000 times as
heavily as a conventional diode. This heavy doping, result in a large number of majority
carriers. In comparison with conventional diode, the depletion layer of a tunnel diode is
100 times narrower.
Tunneling effect: Because of the large number of
carriers, there is much drift activity in p and n sections.
This causes many valence electrons to have their energy
levels raised closer to the conduction region. Therefore, it
takes only a very small applied forward voltage to cause
conduction.
Symbol of Photo Diode
Symbol of Tunnel Diode

The movement of valence electrons from the valence energy band to the
conduction band with little or no
applied forward voltage is called
tunneling.
8. 5. Varactor Diode:
A junction diode which acts as a variable capacitor under changing reverse bias
is known as a varactor diode.
When a p-n junction is formed, depletion layer is created in the junction area.
Since there are no charge carriers within the depletion zone, the zone acts as an
insulator. The p-type material with holes (+ve charge) as majority carriers and n-type
material with electrons (−ve charge) as majority carriers act as charged plates. Thus the
diode may be considered as a capacitor with n-region and p-region forming oppositely
charged plates and with depletion zone between them acting as a dielectric.
8. 6. PIN Diode:
PIN diode is composed of three sections.
Intrinsic semiconductor (I-layer) is sandwiched
between P and N type material, as shown in figure.
Being intrinsic (or undoped) 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 Cpn because capacitance is inversely proportional to the
Symbol of Varactor Diode
V-I Characteristics of Tunnel diode

separation of P-and N-regions. It allows the diode a faster response time. Hence, PIN
diodes are used at high frequencies (more than 300 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.
8.7. Schottky Diode:
It is also called Schottky barrier diode or hot-
carrier diode. It is mainly used as a rectifier at signal
frequencies exceeding 300 MHz.
(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 pushed into the metal
with very large energy, they are commonly called ‘hot-carriers’ hence 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:
1. It is a unipolar device because it has electrons as majority carriers on both sides of
the junction;
2. Since no holes are available in metal, there is no depletion layer or stored charges.
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. 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.
Symbol

8.8. Solar Cell:
Solar cells are photodiodes with very large surface areas.
Compared to usual photodiodes, the large surface area in
photodiode of a solar cell yields
– A device that is more sensitive to incoming light.
– A device that yields more power (larger current/volts).
• Solar cells yield more power.
• A single solar cell may provide up to 0.5V that can supply 0.1A when exposed to
bright light.
Solar Cell Basic Operation—Power Sources
 Each solar cell produces an open-
circuit voltage from around 0.45 to
0.5 V and may generate as much
as 0.1 A in bright light.
 Similar to batteries, solar cells can
be combined in series or parallel.
 Adding cells in series, yields
output voltage that is the sum of
the individual cell voltages.
 Adding solar cells in parallel,
yields an increased output current.
Solar Cell Basic Operation—Battery Charger
 Nine solar cells placed in series can
be used to recharge two 1.5 V NiCd
cells.
 The diode is added to the circuit to
prevent the NiCd cells from
discharging through the solar cell
during times of darkness.
 It is important not to exceed the
safe charging rate of NiCd cells. To
slow the charge rate, a resistor can
be placed in series with the
batteries.
Symbol

Semiconductors

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