SEMICONDUCTOR

ELECTRONICS
Any device whose action is based on
controlled flow of electrons through it is
called an electronic device.
CLASSIFICATION OF SOLIDS ON THE BASIS OF
ELECTRIC PROPERTIES
On the basis of resistivity solids are classified
into 3 categories.
1. METALS
They possess low resistivity.
(High conductivity)
2. SEMICONDUCTORS
Resistivity or conductivity intermediate between
metals and insulators .

INSULATORS
have high resistivity and low
conductivity
Semiconductors are classified as

1.ELEMENTAL SEMICONDUCTORS
eg.Silicon and Germanium

2. COMPOUND SEMICONDUCTORS
Eg.GaAs ,CdS ,CdSe
 ENERGY BAND THEORY
ENERGY BAND THEORY IN SOLIDS
IN SOLIDS
From Bohr atomic model,the electrons in an isolated
atom has well defined energy levels.But in a crystal
due to interatomic interaction the electrons of the
outer shell has energy different from isolated
atoms.Each energy level splits into a number of
energy levels forming a continuous band called
energy band.An enormously large number of energy
levels closely spaced in a small energy range
constitute an energy band.
The energy band filled with valence
electrons is called Valence band.
The energy band above the
valence band is called
conduction band.The gap
between the top of valence band
and the bottom of conduction
band in which no allowed energy
levels for electrons exist is called
energy gap(Eg)
Consider the case of Si of Ge crystal containing N
atoms.For silicon the outermost orbit is third
orbit(n=3).while for Ge it is the fourth orbit (n=4).The
number of electrons in the outermost orbit is 4(2s and
2p electrons).So the total number is of electrons in the
outermost orbit is 4N. The maximum possible number
of electrons in the outer orbit is 8(2s+6p electrons).So
for the 4N valence electrons there are 8N available
energy states.These 8N discrete energy levels can
either form a continuous band or grouped in different
bands.
At the distance between the atoms in the crystal
lattices of Si and Ge ,the energy band of these 8N
states is split apart into two which are separated by
an energy gap Eg.The lower band which is
completely filled by the 4N valence electrons at
temperature of absolute zero is the valence
band.The other band consisting of 4N energy states
called the conduction band is completely empty at
absolute zero.
Both valence band and conduction band has
infinite number of closely spaced energy levels.
In each energy level it can accommodate
atmost 2 electrons by Paulis Exclusion principle.
 DISTINCTION BETWEEN METALS, INSULATORS AND
SEMICONDUCTORS BASED ON BAND THEORY
1. METALS
Two types of band structures are found in metals.
(1) A small energy gap exists between the
completely filled valence band and partially filled
conduction band. Eg: Li , Na
(2) CB and VB partially overlap. Eg:Be ,Mg
In metals electrons from valence band can easily
move to conduction band so that a large number of
electrons are available for conduction. So resistance
for such materials is very low and the conductivity is
very high.
INSULATOR

A large band gap exist. Ie,Eg >3 ev.

The conduction band is empty so that no
conduction is possible.
SEMICONDUCTORS
In this case a small energy gap ie, Eg
<3ev
exist. At T=0k ,the conduction band is
empty and the valence band is filled and
so it is an insulator .But at room
temperature some electrons from the
valence band aquire enough thermal
energy and move to conduction band
and act as conductors.
 INTRINSIC AND
INTRINSIC AND EXTRINSIC
EXTRINSIC
SEMICONDUCTORS
SEMICONDUCTORS
INTRINSIC SEMICONDUCTORS
Semiconductors in its pure form are
called intrinsic Semiconductors.
In Silicon and Germanium atoms 4
valence electrons are there and
they form covalent bond with
neighbouring atoms.This happens
at low temperature.
When temperature increases, more thermal energy
becomes available to the electrons and some
electrons may break away and becomes free
electrons contributing to conduction. This creates
a vacancy in the bond. The neighbourhood from
which the free electron ( charge –q) has come
leaves out a vacancy with an effective ( charge +q)
This vacancy with an effective positive electronic
charge is called a hole.
In intrinsic semiconductors the number of free
electrons is equal to the number of holes.

Where is called intrinsic carrier

concentration .
The total current I is the sum of electron current
I and the hole current I
I = I + I
An intrinsic semiconductor will behave as an
insulator at T=0K .It is the thermal energy at
higher temperatures (T > 0K ),which excites
some electrons from the valence band to the
conduction band. These thermally excited
electrons at T > 0 K ,partially occupy the
conduction band.
 ENERGY
Energy band BAND of
diagram DIAGRAM
intrinsicOF INTRINSIC
semiconductors
SEMICONDUCTORS

T=0K T>0K
 EXTRINSIC SEMICONDUCTORS
The conductivity of an intrinsic semiconductor
depends on its temperature, but at room
temperature the conductivity is very low. When
a small amount, a few parts per million (ppm) of
a suitable impurity is added to the pure
semiconductor, the conductivity is increased.
Semiconductors obtained by adding/doping the
pure semiconductor with a small amount of
impurity atoms having a valency different from
that of the host atoms are called extrinsic S C .
 DOPING
DOPING
The process of deliberate addition of an
impurity to a pure semiconductor so as to
increase its conductivity is called doping.
The impurity atoms added are called dopants.
There are 2 types of dopants.
1. Pentavalent :- have 5 valence electrons
Eg: As, Sb, P
2. Trivalent:- have 3 valence electrons.
Eg: In , B , Al
 n – type semiconductor
N-type SEMICONDUCTOR

This semiconductor is obtained by doping Si or Ge

with pentavalent impurities. When the impurity is
added it substitutes a tetravalent silicon atom and
it uses 4 of its 5 valence electrons in forming
covalent bonds with the neighbouring si atoms
while the 5th electron is loosely bound to the
impurity atom.
A very small amount of ionization energy,
ie ,0.01 eV for Ge and 0.05 eV for Si is required
to detach this e- .Then the dopant atom gets
converted into an ionized +ve core. This is in
contrast to the energy required to jump the
forbidden band (about 0.72 eV for Ge and 1.1 eV
for Si) at room temperature in the intrinsic
semiconductor.
Since the impurity atom donates an extra electron
for conduction, it is called a donor impurity.
Schematic
SCHEMATICdiagram
DIAGRAMof OF
n- type
N-TYPE
semiconductor
SEMICONDUCTOR
In a doped semiconductor the total number
of conduction electrons n is due to the
electrons contributed by the donors and those
generated intrinsically, while the total number of
holes n is only due to the holes from the
intrinsic source. But the rate of recombination of
holes would increase due to the increase in the
number of electrons. As a result the number of
holes would get reduced further.
So in an n- type semiconductor electrons are the
majority charge carriers and holes are the
minority charge carriers.
So for an n-type
n >> n
 P-type
P- type semiconductor
Semiconductor
P-type is obtained by doping Si or Ge with trivalent
impurities like B,Al, In .The impurity uses its 3
valence electrons in forming covalent bonds with 3
neighbouring Si atoms but doesn’t have an
electron to offer to the fourth Si atom. So the bond
between the fourth neighbour and the trivalent
atom has a vacancy or a hole.Then an electron
from the outer orbit of the neighbouring Si atom
will jump to fill this vacancy leaving a hole at its
own site.
Thus a hole is available for conduction. The
dopant atom can be treated as a core of negative
charge along with its associated hole. So this
impurity is called acceptor impurity.
In P-type holes are the majority charge carriers
and electrons are the minority charge carriers.
ie, in P-type
n >> n
SCHEMATICdiagram
Schematic DIAGRAMofOF P-TYPE
P-type
SEMICONDUCTOR
 ENERGY BAND DIAGRAM FOR n-TYPE
SEMICONDUCTOR

In n-type the 5th electron is weakly attracted by

the donor impurity. A very small energy, nearly
0.01eV is required to free this electron from the
impurity. So the energy level of this electron is
drawn slightly below the bottom of the conduction
band E and is known as donor energy level E
 ENERGY BAND DIAGRAM OF P-TYPE
SEMICONDUCTOR
In P-type the acceptor energy level E is slightly
above the top E of the valence band. With a very
small supply of energy of 0.01 eV an electron from
the valence band can jump to the level E and
can ionise the acceptor negatively. At room
temperature most of the acceptor atoms get
ionized leaving holes in the valence band.
P N JUNCTION
DIODES
•p-n junction
•It is the basic building block of semiconductor
devices like diodes,transistors etc.
•p-n junction formation
•To a thin p- type silicon semiconductor wafer, a
small quantity of pentavalent impurity is added so
that part of the p-Si wafer is converted to n-type.
Two important processes occur during the formation of
a pn junction. DIFFUSION and DRIFT
DIFFUSION
In an n- type, the concentration of electrons (number
of electrons per unit volume) is more than holes.
Similarly in p type the concentration of holes is more
than electrons.
During the formation of a pn- junction and due to the
concentration gradient across p and n sides, holes
diffuse from p to n side(p – n ) and electrons diffuse
from n to p side ( n- p ). This motion of charge carriers
gives rise to diffusion current across the junction.
When an electron diffuses from n- p it leaves behind an
ionised donor (positive charge) on n- side. It is immobile.
As the electrons continue to diffuse from n-p , a layer of
positive charge or( a positive space charge region ) is
developed on n-side. Similarly when a hole diffuses from
p-n ,it leaves behind an ionised acceptor ( negative
charge) which is immobile. As the holes continue to
diffuse, a layer of negative charge (negative space-
charge region) on the p- side of the junction is
developed. These space-charge regions on either side of
the junction together is called depletion region. ( as this
region is depleted of any free charge carriers).The
thickness of this region is about 1/10 th of a micrometer.
• DRIFT
Due to the positive space charge region on n side of the
junction and negative space charge region on the p side of
the junction, an electric field directed from positive
charge towards negative charge develops.Due to this field
an electron on p side of the junction moves to n side and
a hole on the n side of the junction moves to p side.This
motion of the charge carriers due to electric field is called
drift. Thus a drift current which is opposite to the
diffusion current starts.
•Initially diffusion current is large and drift current
is small. As diffusion continues ,the space charge
regions extend, thus increasing the electric field
strength and hence drift current. This process
continues till diffusion current equals the drift
current. Thus a p n junction is formed. In a p n
junction under equilibrium there is no net
current.
BARRIER POTENTIAL ( V )
The loss of electrons from the n region and gain of
electrons by the p region causes a difference of
potential across the junction of two regions. The
polarity of the potential is such as to oppose
further flow of carriers so that a condition of
equilibrium exists. Since this potential tends to
prevent the movement of electrons from n region
into the p region it is called a barrier potential.
Barrier Potential under no
bias
 SEMICONDUCTOR DIODE
•It is basically a p n junction with metallic contacts
provided at the ends for the application of an
external voltage. It is a two terminal device. The
symbolic representation is
 P N JUNCTION DIODE UNDER FORWARD
BIASING.

•When an externall voltage

vol V is applied across a
semiconductor diode is such that p side is connected
to the positive terminal of the battery and n side to
the negative terminal is said to be forward biased.
The applied voltage mostly drop across the depletion
region and the voltage drop across p and n side is
negligible.( since resistance of the depletion region is
high compared to p and n region. ).
•The direction of the applied voltage V is opposite
to that of the barrier potential V . So the
depletion layer width decreases and the barrier
height is reduced. Under forward biasing the
effective barrier height is (V - V)
•Due to the applied voltage electrons from n side
cross the depletion region and reach the p side
where they are minority charge carriers.
•Similarly holes from p side cross the depletion
region and reach n side where they are minority
charge carriers. This process under forward bias is
called minority carrier injection.

1. Under no bias
2. Low battery voltage
3. High battery voltage
 V I CHARACTERISTICS OF A FORWARD BIASED
DIODE
The circuit arrangement for studying the V-I
characteristics
• The V-I characteristics of a diode is drawn for
studying the variation of current as a function of
applied voltage.
• In forward biasing, current first increases very
slowly till the voltage across the diode crosses a
certain value. After a particular voltage, the diode
current increases exponentially even for a small
increase in the voltage. This voltage is called
threshold voltage or cut-in voltage or knee
voltage.
•It is approximately 0.2 V for Germanium diode and
0.7 V for Silicon diode.
•Dynamic resistance in forward biasing
• lt is the ratio of small change in voltage V to a
small change in current I
•P N JUNCTION DIODE UNDER REVERSE BIASING
•When an external voltage V is applied across the
diode such that n side is positive and p side is
negative, it is said to be reverse biased.
•The applied voltage mostly drop across the
depletion region. The direction of the applied
voltage V is same as that of the barrier
potential V .
• As a result, barrier height increases and the depletion
region widens due to change in electric field.
• The effective barrier height under reverse bias is
V + V . This suppresses the flow of electrons from n to
p and holes from p to n .Thus diffusion current
decreases enormously.
• Due to the electric field at the junction electrons on
the p side and holes on the n side ( minority carriers)
, when come close to the junction, they will be swept
to its majority zone giving rise to a drift current in
the order of microamperes.
The diode reverse current is not much dependent
on applied voltage. Even a small voltage is
sufficient to sweep a minority carrier from one part
of the junction to other part. So the reverse
current is not limited by the magnitude of voltage,
but is limited due to the concentration of minority
carriers on either side of the junction.
BARRIER
POTENTIAL
UNDER REVERSE
BIAS
 V I CHARACTERISTICS OF A REVERSE
BIASED PN JUNCTION DIODE
•The circuit arrangement for studying the V I
characteristics
•For the diode in reverse biasing, the current is very
small ( ) and always remains constant with
change in bias. It is called reverse saturation
current. The current under reverse bias is voltage
independent upto a critical bias voltage called
breakdown voltage ( V ). When V = V , the
diode reverse current increases sharply.
APPLICATION OF PN
JUNCTION DIODE-
RECTIFIER
RECTIFICATION

The process of converting an

alternating current into direct
current is called rectification .
TWO TYPES OF RECTIFIERS

 HALF WAVE RECTIFIER

 FULL WAVE RECTIFIER
HALF WAVE RECTIFIER
No need to draw
this diagram in the
notebook.
 WORKING OF HALF WAVE RECTIFIER

 Junction diode allows current to pass only when it

is forward biased. So if an alternating voltage is
applied across a diode the current flows only in
that part of the cycle when the diode is forward
biased. This unidirectional property of a diode is
made use of when it is working as a rectifier.
When an ac is supplied to the primary of
the transformer , the secondary applies an

alternating voltage across terminals A and B

During the positive half cycle of ac , end A
is positive and the diode is forward biased
so that a current I flows through R .
Output voltage across R is of the same
waveform as the positive half of the wave
input.
 During the negative half cycle, end A becomes
negative and the diode is reverse biased.

 Hence no current flows across R . In the next

positive half cycle, again we get the output
voltage. So the output obtained is restricted
only to one direction and is said to be rectified.
Since the rectified output is only for half of the
input ac wave, it is called a half wave rectifier .
GRAPHICAL REPRESENTATION
FULL WAVE RECTIFIER
No need to
draw this
diagram in
the notebook
 WORKING

 The input ac signal is fed to the primary of the

transformer. The two ends A and B of the
secondary are connected to the p sides of
diodes D and D .The secondary is tapped at
the center which is connected to the n sides of
two diodes through a load resistance R .
 At any instant , voltage at A( input of D ) and
at B( input of D ) with respect to the centre tap
 will
be out of phase with each other.
 During the positive half cycle of the ac input,

 end A is +ve and at the same time

end B is –ve.
 So D conducts and D will not conduct . So an
output voltage is obtained across R due to D
.
 During -ve half cycle, end A becomes -ve and
end B becomes +ve.
Hence D conducts while D does not.
Due to D an output is obtained across R
.This continues and hence during both the
half cycles of ac input , an output is
obtained. This process is called
full wave rectification.
GRAPHICAL REPRESENTATION
 FILTER CIRCUIT

 The output obtained from a rectifier is

unidirectional but pulsating.
 Such a signal can be considered as the sum of
dc signal superimposed with ac signals of
different frequencies.
 To obtain a pure dc output, filter circuits are
used which filter out the ac components.
 CAPACITOR FILTER

 The
output of a rectifier is connected in parallel to a
capacitor with a load resistance R
 Capacitor has reactance X = 1/ C .
 So
it offer a low impedance path to high frequency
ac and infinite resistance to low frequency dc.
 Thus
ac components are filtered through the
capacitor and a smooth dc voltage appear across
R
No need to
draw this in the
notebook
 ROLE OF CAPACITOR IN FILTER
CIRCUIT

 When the voltage across the capacitor rises it

gets charged. Through the load resistance R ,it
will get discharged and the voltage across it
begins to fall.
 The rate of fall of voltage is inversely proportional
to CR, the time constant of the circuit. ie,
 Rate of fall of voltage 1/CR
 Soto reduce the rate of fall, time constant
should be large or C should be large. So filter
circuits use large capacitors.
 When rate of fall decreases , a steady dc
output is obtained , since the capacitor remains
charged at the peak value.
 SPECIAL
PURPOSE PN
JUNCTION
DIODES
OPTOELECTRONIC JUNCTION DEVICES

The semiconductor devices in which charge

carriers are generated by the process of
photo excitation are called optoelectronic
junction devices .
Some of them are ■ 1. PHOTO DIODE
■ 2. LED
■ 3. SOLAR CELL
• PHOTO DIODE

It is used for detecting optical

signals.
( photo detector) .

A photo diode is fabricated with a

transparent window to allow light
to fall on the diode.

It is operated under reverse biasing.

.
When a photo diode is illuminated with light
having photons with energy h greater than E
of the semiconductor falls on it ,electron-hole
pairs are generated due to the absorption of
photons.
Due to the electric field of the junction formed by
reverse biasing, electrons and holes are
separated before they recombine ,so that the
electrons reach n side and holes reach p side
giving rise to an emf. When an external load is
connected current flows.
The magnitude of photo current
depends on the intensity of incident
light. It is easier to observe the change
in current with change in light intensity
if a reverse bias is applied. Thus photo
diode can be used as a photo detector.
V- I CHARACTERISTICS OF A PHOTO DIODE
LIGHT EMITTING DIODE
LED
 CONSTRUCTION OF AN LED

• It is a heavily doped p-n junction under fo rward

biasing which emit spontaneous radiations.
• It is fabricated with a transparent cover so that
emitted light can come out.
•
 WORKING
• When the diode is forward biased, electrons move from
n- p and holes moves from p-n ( where they are
minoritycarriers.)
• At the junction boundary, the concentration of minority
carriers increases compared to the equilibrium
concentration.
• These excess minority carriers recombine with majority
carriers near the junction.
• On recombination energy is released in the form of
photons.
• Photons with energy equal to or less than the band gap
of the semiconductor are released.
• When the forward current of the diode is small,
the intensity of light emitted is small.
• As the forward current increases, intensity
increases and reaches a maximum.
• Further increase in the forward current results in
decrease of light intensity and may damage the
LED. So LEDs are biased such that the light
emitting efficiency is maximum.
 V-I CHARACTERISTICS
• The V-I characteristics of a LED is similar to that
of a silicon junction diode. But the threshold
voltages are much higher and slightly different
for each colour.
• Since the reverse breakdown voltages are the
very low, care should be taken that high
reverse voltages do not appear across them.
 TYPES OF LEDS AND MATERIALS USED
• The semiconductors used for making visible LED’s
must atleast have band gap 1.8 eV,since visible
light energy range is from 3 eV to 1.8 eV .
• The compound semiconductor Gallium Arsenide
Phosphide (GaAs P) is used for making red LED.
• ( E = 1.9 eV ).
• Infrared LED ‘s are made by Galliium Arsenide
( GaAs) whose E = 1.4 eV. This LED is used in
Burglar alarms, remote controls, optical
communication etc.
 ADVANTAGES OF LED LAMPS OVER
INCANDESCENT LAMPS
• (1) Low operational voltage and less power.
• (2) Fast action and no warm up time required.
• (3) The band width of emitted light is 100 A to
500 A or it is nearly monochromatic.
• (4) Long life and ruggedness.
• (5) Fast on- off switching capability.
INCANDESCENT LED
 FACTORS THAT DETERMINE THE
WORKING OF LED
• Colour of light emitted by an LED depends on
its bandgap energy .The energy of photons
emitted is equal to or slightly less than the
band gap energy.
• The intensity of light emitted by an LED is
determined by the forward current conducted
by the junction.
• A given LED has a safe value for the forward
current it can carry.lt is around 5mA for simple
LED’S and can go upto 30 mA for LED’S which
emit high intense light.
• Usually a series resistor is connected to the LED
so that forward current is limited within the safe
value.
SOLAR CELL
 CONSTRUCTION OF A SOLAR CELL
• A solar cell is basically a pn junction which
generates emf when solar radiation falls on the p-n
junction. Here no external bias is applied. The
junction area is kept larger for solar radiation to be
incident to produce more power output.
• A p-silicon wafer of about 300 micrometer is taken
over which a thin layer ( 0.3 m) of n-Si is grown on
one side by diffusion process. The other side of p-Si
is coated with a metal ( back contact).
• On the top of n-Si layer , metal finger electrode is
deposited. This acts as a front contact.
• Since this metallic grid occupies very small area,
light can be incident on the cell from the top.
 WORKING OF A SOLAR CELL
• The generation of emf by a solar cell when
light falls on it is due to three basic processes.
• (1) GENERATION: of electron- hole pairs due to
light with energy h greater than E close to the
junction.
• (2) SEPARATION: of electrons and holes due to
electric field of the depletion region. Electrons are
swept to n-side and holes are swept to p-side.
• (3) COLLECTION : electrons reaching n- side are
collected by the front contact and holes reaching p-
side are collected by the back contact making p-
side positive and n-side negative giving rise to a
photo voltage .

• When an external load is connected, a photo

current I flows through the load.
 V-I CHARACTERISTICS OF A SOLAR CELL

• The V-I characteristics of a solar cell is drawn on

the fourth quadrant . This is because a solar cell
does not draw current but supplies the same to the
load.
 MATERIALS USED FOR MAKING SOLAR CELL
Semiconductors with band gap close to 1.5 eV are
preferable materials for solar cell fabrication.
Eg. Si ( E = 1.1 eV )
Ga As (E = 1.43 eV )
Cd Te (E = 1.45 eV )
Cu In Se = 1.04 eV )
 CRITERIA FOR THE SELECTION OF MATERIALS FOR
SOLAR CELL FABRICATION
• (1) Band gap ( 1- 1.8 eV)
• (2) High optical absorption ( 10 / cm )
• (3) High electrical conductivity
• (4) Availability of raw materials
• (5) cost
 USES

Solar cells are used to power electronic devices in

satellites and space vehicles and also as power supply to
certain calculators. Sunlight is not always required for a
Solar cell. Any light with photon energies greater than
the band gap will do.