ELECTRONIC DEVICES

SEMICONDUCTORS:
Solid semiconductors are substances which have their
electrical conductivities lying between that of good conductors
and insulators.

Intrinsic semiconductor:
Semiconductors without any impurity are called intrinsic
semiconductors. For example Germanium (Ge) and silicon (Si).
All temperatures near absolute zero, pure Ge and Si behaves like
perfect insulators. But their conductivities increases with
increase in temperature. For Germanium, the binding energy of
an electron in the covalent bond is 0.7 eV.
In conductors (metals) free electrons act as charge carriers. In
insulators there are no free electrons, while on intrinsic
semiconductors both electrons and holes act as charge carriers.
When the temperature is increased, the resistance of a conductor
increases but the resistance of semiconductor decreases.

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Extrinsic Semiconductor:
The conductivity of a pure Ge, Si crystal can be considerably
increased by adding small quantities of impurities. The
addition of controlled amount of impurity to a semiconductor
is called doping and a doped Ge or Si crystal is called an
extrinsic semiconductor. The quantity of impurity is of the
order of one impurity atom in 100 million atoms of Ge or Si.
There are two types of extrinsic semiconductors

(i) n  type (ii) p  type

(i) n  type semiconductor :

Elements like Aresenic (As), Antimony (Sb), Phosphorus (P)
are pentavalent i.e. they have five valance electrons while Ge
or Si are tetravalent.
As the conductivity of Ge or Si doped with pentavalent
element is due to free electrons, the semiconductor is called
ntype semiconductor. Electrons are major carrier and holes
are called minority carriers.

(ii) P  type Semiconductor:

If a trivalent impurity like Indium, aluminium, boron is
added to Ge or Si then three covalent bonds are formed with 3
Ge atoms. But the fourth valance electron of Ge can not form
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 a covalent bond with indium. The absence of an electron is
called a hole.
Each hole is regarded as a positive charge at that point.

PN JUNCTION:
E
P N
 +   
   
+
  

Hole + Electron
Depletion layer
P N

Forward Biasing:
  
  
  
P N mA

+ 

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In this case, when the applied p.d. is greater than the barrier p.d.
the diode starts conducting. The direction of hole current is same
as that of the conventional current.

Reverse Biasing:

+ + +   
+ + +   
+ + +   
P N

 +

I
Forward Bias
Width of depletion
region increased and it V
I 0
acts like an insulator.
This current flow due to
majority carriers is zero.

PN JUNCTION DIODE AS RECTIFIER:

(a) PN junction Diode

D as Full wave Rectifier:

P S RL V0

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 Input
T/2 3T/2
voltage
0 T

Output
3T/2
voltage
0 T/2 T

During the positive half cycle of the input voltage the diode is
forward biased hence if conducts and current flows through RL.
In the negative half cycle of the input voltage the diode is
reverse biased and hence no current flows through RL.

Thus V0 is unidirectional. Hence it is called as halfwave

rectifier.

(b) PN junction Diode as Full wave Rectifier:

D1
A

C RL

D2
B

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 Input
T/2 3T/2
voltage
0 T 2T

Output

voltage
0 T/2 T 3T/2 2T

In the positive half cycle, A is positive w.r.t. C and B has an

equal negative voltage. In this case D1 is forward biased hence it
conducts but D2 is reverse biased and it does not conduct.
In the negative half cycle of input voltage; B is positive w.r.t. C
and A has an equal negative voltage. In this case D2 is forward
biased hence it becomes conducting and D1 becomes
nonconducting.
Since the current flows through RL in both halves of the input
cycle and since the AC input voltage is falling converted into
DC voltage, the arrange is called a full wave rectifier.

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TRANSISTOR:

p n p n p n
E B C E B C

C C

B B

E E
pnp
npn

Action in Transistor:

E B C
n p n

mA IE A IC mA
Ib

VB VCC

The emitter base junction is forward biased. Some of the

electrons combine with the hole in the base and constitute the
base current Ib. The electron current flowing from emitter to
base forms the emitter current IE.
The collector base junction is reverse biased. Hence the
electrons are attracted towards the collector, which is maintained

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at a higher positive potential. The motion of these electrons
gives rise to the collector current IC.

According to Kirchoff’s law

IE = IC + IB

Amplifying action of a Transistor:

RL

Ri n C VCC
p B
n
Vs E V0
Vi

VBB

In common current emitter transistor amplifier,

IC
dc  DC current gain = when any input signal is not
IB

applied
I C
ac  AC current gain = when Vs is supplied.
I B

R L I C RL
Similarly, voltage gain = = 
R i I B Ri

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So, power gain = voltage gain  current gain
RL R
=   ()   2 L
Ri Ri

In common Base transistor amplifier,

DC current gain  = IC
IE

I C
AC current gain ac =
I E

VC I C R L
AC voltage gain = 
Vi I E R i

Ro
 ac 
Ri

R0
Where is called resistance gain
Ri

VC  I C Ro R
AC power gain = = ac  ac  ac
2
 o
Vi I E Ri Ri

Logic gates:
A digital circuit with one or more input signals but only one
output signal is known as logic gate. There are three basic logic
gates:
(i) OR gate (ii) AND Gate (III) NOT Gate

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OR gate
The functional statement for ‘OR’ gate is the output (Y) of ‘OR’
gate will be 1 when the input A or B or both are 1.
The OR gate in terms of Boolean expression is
A B Y
This statement can also be given in the form of a table know as
truth table, which is given below

A
Y
B

Truth table of two input OR gate.

A B C
0 0 0
0 1 1
1 0 1
1 1 1

AND GATE
The functional statement for AND, gate is the output Y of
AND gate is 1, if all the inputs simultaneously have the
state 1. The AND gate in terms of Boolean expression is
Y = A.B

The above function can be stated in the form of the

following truth table
A
Y
B

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 Truth table of AND gate
A B C
0 0 0
0 1 0
1 0 0
1 1 1
There is no supply current and hence no drop across R only
when both A and B are at +5V. Only in that case the output
Y goes to supply a voltage of +5V.
A
Y
B
R
V(1)=5V

NOT GATE
The output of the NOT gate is not same as the input, we
can say that it performs a negation operation on the input.
The truth table is given below
A Y

Truth table of NOT gate

A Y
0 0
1 0

NOR GATE
If we put an NOT Gate at the output of OR Gate gives a
NOT of OR gate or NOR gate. The Boolean equation for

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 NOR gate is Y  A  B . Truth table for NOR gate can be
written as follows.
A A+B A+B
Y
B

The NOR Truth Table

A B Y  A B
0 0 1
0 1 0
1 0 0
1 1 0
NOR Gate is a universal gate because we can obtain all the
possible gates by using this NOR gate as basic building
block.

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