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SEMICONDUCTORS
Semiconductors :
Semiconductors are the substances which are insulators at zero kelvin, but starts conducting as the
temperature is increased. The size of forbidden energy gap is much smaller than that for insulators. Due to
smaller forbidden gap electrons can easily shift from valence band to the conduction band.
Semiconductors are divided into two categories:
(i)
Intrinsic Semiconductors
(ii)
Extrinsic Semiconductors
Intrinsic Semiconductors:
Pure germanium and silicon crystals in their natural states are called Intrinsic semiconductors. In
germanium crystals atoms are arranged on the corners of a regular tetrahedron. Each of the four with valence
electron is shared a nearest atom to form covalent bond. No free electrons are available in pure
semiconductors. However, some bonds are broken due to thermal agitation and electron is released. The
vacant space left by electron is called hole. Only 1 bond is broken for 109 germanium atoms. So number of free
electrons available for conduction are very small. As the crystal is electrically neutral so number of free electrons
is equal to number of holes.
If electric potential difference is applied across the semiconductors the electrons will move opposite to
field and hole moves in the direction of field, thus forming current. But due to small number of electrons and
holes the magnitude of current is very small.
Doping:
The process of addition of impurities to increase the conductivity of Silicon or Germanium crystal is called
doping and the impurity atom added is called dopant.
Extrinsic Semiconductors:
The impurity atoms added are of two types:
Pentavalent impurity atoms i.e. atoms having 5 valence electrons such as Antimony or Arsenic. Such
atoms added will create excess of free electrons. This type of doped semiconductors is called n-type
semiconductor.
Trivalent impurity atoms i.e. atoms having 3 valence electrons, such as Indium or Gallium. Such atoms on
being added to germanium crystal make the crystal deficient in electrons and holes will be produced. This type
of doped semiconductors are called p-type semiconductors.
N – Type Semiconductors:
When an impurity atom with 5 valence electrons is added to germanium crystal, it replaces one of the
germanium atoms. Four of the five valence electrons form covalent bonds with neighboring atoms and fifth
electron becomes free to move in the crystal structure. Thus by adding impurity atom we are increasing the
number of electrons and hence conductivity increases. Since charge carriers are negatively charged electrons
they are called n-type semiconductors. In n-type semiconductor, some covalent bonds are also broken resulting
in formation of electrons and hole.
In N type semiconductors, electrons are thus majority carriers and holes are minority carriers. An N type
semiconductor has freely moving electrons and an equal number of stationary positively charged donor atoms.
The crystal as a whole is neutral. The fifth valence electron is in fact bound with the parent atom with a very
small energy of the order of 0.05 eV.
P – Type Semiconductors:
If we add trivalent impurity to germanium crystal it will replace germanium atom in the crystal structure.
The three valence electron will form three covalent bond with neighboring Ge atoms and fourth space is left
vacant which is called a hole. Thus, for every trivalent impurity added a hole is created. The Ge crystal so
formed is called p-type semiconductor as it contains free holes. Each hole is equivalent to positive charge. The
Ge crystal also contains few electrons which are present due to breakage of covalent bonds. For each bond
breakage an electron and a hole are released. Thus, p-type semiconductor contains holes in majority and
electrons are present in minority.
Doping:
While doping following points should be noted:
(i)
The dopant atom should take the position of semiconductor atom.
(ii)
The presence of dopant should not distort the crystal.
(iii) The size of dopant should be almost same as semiconductor atom
(iv) Dopant atoms should not be more than 1% of the crystal atoms.
Methods of Doping;
Heat the crystalline semiconductor in an atmosphere having dopant atoms so that they can diffuse into
semiconductor. Implant dopants by bombarding the semiconductor with their ions.
Formation of P-N junction diode:
It is formed by placing a P-type crystal in contact
with N type crystal and subjecting it to high pressure so
that it becomes single piece. The assembly so obtained
is called P-N junction or junction diode or crystal diode.
The surface of contact of P and N type crystal is
called junction. During the formation of junction diode,
holes from P region diffuse into N region and electrons
from N region diffuse into P region. In both cases, when
an electron meets a hole, they cancel the effect of each
other and as a result a thin layer at the junction becomes
free from any of charge carriers. This is called depletion
layer. The thickness of depletion layer is of the order of
10—6 m.
Due to movement of electrons from N type it gets
positive potential and similarly P type gets negative potential. Due to this, there is potential gradient in the
depletion layer, negative on P side and positive on N side. In other words, it appears as if some fictitious battery
is connected across the junction with its negative pole connected to p-type and positive pole to N type. This
potential difference is called potential barrier.
Forward Biasing:
When P type is connected to positive pole of battery and N
type with negative pole it is called forward biasing.
When the forward bias is more than the barrier voltage
then majority carrier i.e. electrons in N type and holes in P type
moves towards junction and cross it. As the forward bias voltage
is increased, the flow of majority carriers increases. Because of
attraction between electrons and holes, they rush towards each
other and recombine at the junction. For each combination a
covalent bond in P region near positive terminal of battery breaks
and electron hole pair is used. Hole moves forward and electrons
moves to the positive of the battery. Thus, in external circuit
current passes due to flow of electrons.
When the forward bias voltage is less than the barrier voltage then the charge carriers experience higher
resistance and cannot cross this region. As forward bias is increased the current increases with increase in
forward bias.Beyond a certain forward bias the electrons passing from the junction gain sufficient kinetic energy
to expel valence electrons from atoms, resulting in large increase in current.
Reverse Biasing:
When P type is connected with negative terminal
and N type with positive terminal of battery then it is said to
be reverse biased.
In this case, majority carriers move away from the
junction. The depletion region width increases and it offers
high resistance to charge carriers. However, a few minority
charge carriers on being accelerated by reverse bias cross
the junction and result in small current in reverse direction.
This current is called Leakage current. When reverse
voltage is 25V the excessive high temperature destroys
the covalent bond strucure in germanium and reverse
current rises sharply. This voltage is called Breakdown Voltage.
Diode As a Rectifier:
Rectifier is a device which changes A.C. to D.C.
Principle:
It is based on the principle that junction diode offers low resistance path when forward biased and high
resistance when reversed biased.
Half wave Rectifier:
A single diode is
used as a half wave
rectifier. The a.c. to be
rectified is connected
across the primary coil
P
of
step
down
transformer.
The
terminal A of the
secondary coil S of
transformer
is
connected
to
the
junction diode and load
resistance RL as shown.
During first half
cycle,
diode
gets
forward biased, the
conventional
current
flows in the direction of
arrow heads. During
second half cycle diode
get reverse biased and
hence no output is
obtained across RL. A
small
current
will
although be present
due to minority carriers. Thus, we shall get the discontinuous and pulsating output across the load resistance.
Full Wave Rectifier:
In case of full wave rectifier, both halves of input a.c. are rectified. The P region of two
diodes are connected to two extreme ends of secondary. A load resistance RL is connected across the common
N region and central tapping of the transformer. Output is taken across the load resistance RL.
During the first half cycle of input a.c.let the upper end of secondary be at positive potential with respect to
the central tapping. The upper junction diode will be forward biased and diode will be reversed biased. Current
will pass through in direction as shown. Similarly, during negative half cycle the diode will be forward biased, the
current passes through diodes D2 and the direction remains the same as in the first case. As the direction of
current (in both the cases) in load resistance remains the same, we call it d.c.
Types of Diodes:
Solar Cell:
A solar cell is a diode used to convert light energy into electrical energy. A p-n junction with p or n region
made so thin that light energy entering is not absorbed appreciably before reaching the junction constitutes solar
cell. The thin region is the emitter and other region is the base.
The LED (Light Emitting Diode):
An LED is made of semiconductors gallium arsenide or indium
phosphate emits radiation in the visible region. In an LED that is forward
biased, the combination of electrons and holes at junction releases energy
that is emitted as light.
Photodiode:
Photodiode is so fabricated so that light is allowed to fall at the junction. If light of frequency v falls on the
junction and hv is greater than the energy required to move as electron from valence to the conduction band.
Zener diode:
These are specially designed junction diodes which can operate in the reverse breakdown voltage region
without being damaged are called Zener diode.
An important application
of zener diode is that it can be
used as voltage regulator. The
regulating action takes place
because of the fact that in
reverse breakdown region, a
very small change in voltage produces large change in current. This causes a sufficient increase in voltage drop
across the resistance to lower voltage back to normal. Similarly, when the voltage across the diode tends to
decrease, the current through diode goes down out of proportion so that voltage drop across the resistor is
much less and it raises voltage back to normal.
Transistor:
It is a three layer semiconductor device. It can be either pnp or npn. In p-n-p, n layer is sandwiched
between two p layers and in n-p-n, p layer is sandwiched between two n layers. The three layers are called
emitter, base and collector. The base of transistor is lightly doped and is thin in comparison with emitter and
collector. The emitter supplies the charge barriers and collector collects them. Input circuit is generally forward
biased and output circuit is reverse biased.
Transistor Actions:
N-P-N Transistor (Common Base):
To understand the action of
common base transistor, the base
emitter junction is forward biased
and collector base junction is
reverse biased. The electrons being
majority carriers in the emitter are
repelled by the negative potential of
the emitter junction towards the
base. The base has holes as
majority carriers and some holes
and electrons recombine in the base
region and the base is lightly doped. Due to this, the probability of the electron-hole combination in the base
region is very small (about 5%). The remaining electrons enter the collector region due to positive potential at
the collector terminal. For each electron entering the collector terminal an electron from ve terminal of emitter
base battery enters the emitter junction maintaining the number of electrons in emitter junction. Thus in NPN
transistor, current inside as well as outside is carried by electrons. For transistor,
IE = IC + IB
P-N-P Transistor (Common Base):
In this case also the emitter
base junction is forward biased
and base collector junction is
reversed biased with batteries VEB
and VCB respectively. The holes
being in majority in P type emitter
are repelled towards the N type
base region by positive potential
of emitter base battery. In the
base region some of the holes
recombine with electrons in the N
type semiconductor. Remaining holes move into the collector region due to ve terminal of the collector base
battery. For each hole that reaches collector terminal an electron leaves the negative pole of the battery V CB and
neutralize it. At the same time, an electron from some covalent bond in emitter enters into the positive terminal
of battery VEB, creating a hole in the emitter. Thus current inside the semiconductor is carried by holes and
outside the semiconductor it is carried by electrons. In this case also
IE = IC + IB
Concept of Amplifier:
Amplifier is a device which produces enlarged version of the input signal. It is used for increasing the
amplitude of variation of alternating voltage or current or power.
Common Base Amplifier:
The signal to be amplified is
connected between emitter and base
while a high load resistance is
connected between collector and base.
The input circuit is forward biased while
the output circuit is reverse biased. The
input circuit being forward biased is a
low resistance circuit and output circuit
being reverse biased is a high
resistance circuit. When no a.c. signal is
fed to the input circuit voltage drop
across RL due to collector is ICRL. If VCB
is the voltage of collector-base battery, then potential difference, VC between the collector and the base would
be given by,
VC = VCB  IC RL
Suppose now an a.c. signal is fed to the input circuit. As the emitter is negative w.r.t the base, positive half
cycle of input wave opposes and hence decreases the forward bias. Thus, IE and IC both decreases. The value
of VC increases as IC decreases i.e. the collector becomes more positive.
During negative half cycle of a.c., the forward bias increases. Due to increase in forward bias, IE increases
and consequently IC also increases. VC would decrease i.e. collector becomes less positive or more negative.
Current Gain:
It is the ratio of collector current to the emitter current, i.e.
I
 C
IB
A.C. Current Gain:
It is defined as the ratio of change in collector current to the change in emitter current (IE),
I
 ac  C
I E
is constant.
Voltage Gain:
It is defined as the change in output voltage to the change in input voltage.
VC
I C R0
VoltageGain 


V I
I E R1
Voltage Gain   ac  resistance gain
Power Gain:
It is defined as the change in output power to the change in input power.
 V C I C
Power Gain 

V I  I E
  2  Resistance Gain
Common Emitter Amplifier:
Here also the input circuit is
forward biased and the output
circuit is reverse biased. When no
a.c. signal is applied the potential
differene VC
between the
collector and emitter is given by,
VC = VCE  IC RL
When an a.c. signal is fed
to the input circuit, the forward
bias increases during positive half
cycle of the input. This results in
increase in IC and consequent decrease in VC , thus during positive half cycle of the input, the collector
becomes less positive.
During negative half cycle of the input, forward bias decreases, therefore, the value of IE and IC also
decreases and VC would increase making the collector more positive. In common emitter amplifier, thus there is
180ºout of phase amplification.
Current Gain:
It is defined as the ratio of collector current to the base current.
I
IC
  C 
Ib
I E  IC
Dividing the numerator and denominator by IE,
IC
IE

 

I  IC
1 
IE
A.C. Current Gain:
It is defined as the change in collector current to the change in base current,
I
 ac  C
I b
Voltage Gain:
It is defined as the change in output to the change in input voltage,
V C
I C  R 0
VoltageGain 

V I
I b  R i
  a .c .  Resistance Gain
Power Gain:
It is defined as the change in output power to the change in input power.
V
I
Power Gain  C  C
Vi I b
  2  Resistance Gain
 >  therefore, extremely high power gain is possible in common emitter configuration.
Transistor as an Oscillator:
Oscillator converts direct current into alternating current and produces high frequency undamped
oscillations.
The base oscillatory circuit consists of an
inductance and capacitance called tank circuit. Due to
resistance of circuit, a part of energy is dissipated,
therefore, amplitude of oscillations goes on decreasing
with time and damped oscillations are produced.
In order to maintain these oscillations, energy is
supplied to circuit at the right moment and in the right
direction using a feedback arrangement. The feedback
arrangement consists of primary P and secondary with
variable capacitor C of suitable range. The secondary
coil of inductance L. The inductance L and capacitance C
constitute tank circuit.
Working:
When key is inserted collector current in primary
increases. The flow of electrons will be in upward direction. Induced current is produced in secondary in
opposite direction. The lower plate of the capacitor will become negatively charged and upper plate positively
charged. The forward bias in emitter base circuit increases. This further increases the collector current. When
collector current attains maximum value, induced emf supporting the forward bias decreases. Thus collector
current decreases and emf will be induced in such a way that it opposes the forward bias. Thus, collector
current reaches a certain minimum value. In this way the oscillations in tank circuit will be maintained.
DIGITAL ELECTRONICS
In these electronics circuits, the current or voltages will have only two values, High (1) and Low (0). In
digital circuits, the electrical pulses of two levels only are used as signal voltages.
Logic Gates:
A gate is a digital circuit which is used to perform certain specific function. The three basic logic gates are:
a.
OR gate
b.
AND gate
c.
NOT gate
All other logic gates can be formed by combination of these three gates.
Truth Table:
It is table that indicate all possible combinations of input signals and their output.
Boolean Algebra:
This is the algebra which can be applied to logic gates based on Binary number system.
OR Gate:
It is a two input single output gate. The output is one if
any of the two inputs or both the inputs are one.
The truth table and symbol of OR gate are:
A
B
Y
0
0
0
0
1
1
1
0
1
1
1
1
The circuit diagram for OR gate is:
The diodes used are considered to be ideal diodes ie. during forward bias they offer zero resistance and
during reverse bias they offer infinite resistance.
Case I: When A = 0, B = 0: Here both the diodes are in off state. There is no current that flows through R, thus
output voltage is Y=0.
Case II: When A = 0, B = 1: In this case D1 is in off state and D2 is forward biased. The current flows through
D2 and sets up a potential difference of 5V across it, so Y = 1.
Case III: When A = 1, B = 0: in this case diode D1 is forward biased and D2 is reverse biased. Diode D1
conducts and Y=1.
Case IV: When A=1, B=1: Here both the diodes are forward biased and hence conduct perfectly. A potential
difference of 5V appears across resistance. Thus, Y = 1.
AND Gate:
It is also a two input single output gate. The
output is one iff both the inputs are one.
(a)
Suppose A=0 and B=0: The potentials at A
and B are forward biased and offers no resistance.
The diode D1 conducts and net potential difference
appears across R and Y=0.
(b) Suppose A = 0 and B = 1: In this case also A
is
forward biased and B is in off state. The diode D1
conducts and net potential difference appears across
R
and Y = 0.
(c)
When A = 1 and B = 0: In this case also A is in
off state and B is forward biased. The diode D2
conducts and Y = 0
(d) When A = 1 and B = 1: Here both diodes are in off state, hence no potential drop occurs across R and Y
= 1.
NOT Gate:
It is a single input single output gate. The truth table and symbol is
A
Y A
0
1
1
0
It is realised with the help of a transistor. Consider an
pnp transistor to be used as NOT gate.
If A = 0, the emitter base junction is reverse biased and no
current flows through it. Correspondingly current through
RC is also equal to zero. The potential Y = 1.
On the other hand, if A is 5V i.e. A =1, the emitter
base junction is forward biased. Potential drop occurs R and
Y=0
NAND Gate:
It is AND gate followed by a NOT Gate.
It is two input single output gate. The truth table and symbol are,
Y X
A
B
X
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
NOR Gate:
It is OR gate followed by a NOT gate. It is a two input single output gate. The truth table and symbol are,
Y X
A
B
X
0
0
0
1
0
1
1
0
1
0
1
0
1
1
1
0
Exclusive OR (XOR) Gate:
It is also two input, single output gate. The output is one iff one of the inputs is one. The truth table and
symbol are;
A
B
0
0
0
1
1
0
1
1
Exclusive NOR Gate:
It is an exclusive
The truth table is,
A
0
0
1
1
B
0
1
0
1
Y  AB  AB
0
1
1
0
OR gate followed by a NOT gate. Output is one either both the inputs are one or zero.
Y  AB  A B
1
0
0
1