P-N Junction Diode

• When a P-type semiconductor is joined to N-

type semiconductor, junction is formed which
is called P-N Junction, and the device so
formed is called P-N junction diode.
Depletion layer and potential barrier
• Due to the combination of two P and N type
semiconductor there is exchange ( or
diffusion) of electron and hole take place and
get neutralize so, free charge carrier depleted
at connection region. This region is called
depletion layer. ( thickness of depletion layer
is around few microns)
• The positive and negative charge near the
depletion layer establish some electric field
directed from N-region to P-region. This field
produces a potential difference which prevent
further diffusion of charge carriers. This p.d. is
called barrier p.d.
 Biasing of P-N Junction Diode
• The process of applying the potential
difference across P-N Junction Diode is called
biasing of the diode.
There are two type of biasing:
i) Forward biasing
ii) Reverse biasing
Forward biasing
• A diode is said to be in forward biased if p-side
is connected to positive terminal and N-side is
connected to negative terminal of battery ( or
source of potential).
In this biasing, the negative terminal of a
battery repels the free electrons in the N-
region towards the Junction & the positive
terminal on P-region pushes the holes towards
the junction. The free electrons & holes
combine and current flows through the
diode.
 Features of forward biasing
• The width of the depletion layer decreases
• The barrier potential is reduced
• The diode acts as closed circuit
• The diode offers very low resistance (called
Forward resistance)
• The high current flows through the diode due to
majority charge carriers.
Reverse biasing
• A diode is said to be in forward biased if p-
side is connected to negative terminal and
N-side is connected to positive terminal of
battery ( or source of potential).
• In this biasing, the electrons from N-
region are attracted towards the positive
terminal & holes in the P-region
are attracted towards the negative
terminal of the battery.
• The departing electrons leaves more positive
ions near junction & departing holes leaves
more negative ions & width of depletion layer
increased. So only small current flow due
to minority charge carriers.

Features of forward biasing

• The width of the depletion layer increases.
• The barrier potential is increased.
• The diode acts as open circuit
• The value of reverse current is negligibly small
called Leakage current.
• The diode offers very high resistance (called
Reverse resistance) to allow current.
• The less current flows through the diode due
to minority charge carriers
Characteristics of a Junction diode
• The graphical relationship between circuit
current & voltage across the junction diode is
called Characteristics of the junction diode. It
is also known as I-V characteristics of the
Junction Diode.
Forward biased characteristics
• The circuit diagram for forward characteristics
of p-n junction diode and graphical variation
of current with applied voltage is as shown in
figure.
When we increases the external voltage on
diode with the help of rheostat there is no
current until the external potential exceed the
barrier potential of diode, after exceeding the
barrier potential current in circuit start to
increase rapidly as shown in graph.
The forward voltage at which the current
through the diode starts to increase rapidly or
sharply is called Knee voltage (Vk)
VK =0.7V for Silicon & 0.3V for Germanium.
Reverse Biased Characteristics
• The circuit diagram for reverse characteristics of
p-n junction diode and graphical variation of
current with applied voltage is as shown in figure.
• When we increases the external voltage on diode
with the help of rheostat majority charge carriers
are blocked & only a small current flows through
the diode due to minority charge carriers.
If the reverse bias voltage is gradually
increased, at one point the current in circuit
suddenly raise. This voltage where the reverse
current suddenly increases sharply is called
reverse breakdown voltage (VB )
Breakdown voltage depends on the density of
Doping impurities & thickness of depletion
region.
 Application of diodes
• It is used as a switch in logic circuits in
computer.
• It is used as ac signal diode in communication
circuits
• It is used as a rectifier or power diodes in dc
power supplies.
• It is used as Zener diode in voltage regulation.
 Rectifier and rectification
• An electronic device which converts ac
(alternating current) into dc (direct current) is
called rectifier.
• The process of conversion of ac into dc is
called rectification.
• When P-N junction diode is forward biased, it
offers low resistance and current flow through
it. But when reverse biased, it offers high
resistance and no current flows through it.
 Half wave rectifier
• A device which rectifies only one half cycle of ac
into dc is called Half-Wave Rectifier. A junction
diode can be used as a half-wave rectifier.
A circuit diagram of half-wave rectifier as shown
in figure which consists of a Transformer, a diode
(D) & a load resistance (RL) . The primary coil of
a transformer is connected to ac mains supply &
secondary coil is connected to the Load
resistance (RL) through the diode (D).
During Positive half cycle of input Voltage Vin, the
diode D is forward biased and the current flows
in the circuit & output voltage Vout is dropped
across load RL.
During Negative Half cycle of input Voltage Vin, the
diode D is Reverse biased and no current flow
through load RL. In this way, only unidirectional
current flow through load RL and it act as the half
wave rectifier.
• The input and output of half wave rectifier is
as shown in figure
 Full Wave rectifier
• A device which converts full cycle of ac into dc
is called Full wave rectifier.
There are two type of full wave rectifier
i) Center tapped rectifier
ii) Bridge rectifier
 Center tapped rectifier
• A device which converts full cycle of ac into dc
is called Full wave rectifier. A centre tapped
rectifier consists of a transformer, two diodes
(D1 & D2) & a Load resistor (RL). The two
diodes D1 & D2 are connected to the centre
tapped secondary coil of a transformer
through RL. The diodes are connected in such
a way that they conduct during alternative
half cycles of input or supply voltage.
• During positive half cycle of input voltage Vin ,
the diode D1 is forward biased & diode D2 is
reversed biased. Hence Diode D1 conducts &
D2 doesn't conduct. Current flows through
the way of ADCA.
During negative half cycle of input voltage Vin,
the diode D2 is forward biased & D1 is reverse
biased. Current flows through the way of
BDCB. In both half cycle, unidirectional
current flows through R
 Thus one Half-wave is rectified by D1 & next
half-wave is rectified by D2. Thus combination
of diodes D1 & D2 works as Full-Wave Rectifier.
Hence, the ac input voltage is converted into
pulsating dc voltage.
The input and output of centre trapped full
wave rectifier is as shown in figure
 Bridge rectifier
• A Full-Wave Rectifier in the form of Bridge is
called Bridge Rectifier. It converts full cycle of
ac into dc. It consists of four diodes D1, D2, D3
& D4 & a load resistor RL. The diodes
are connected in such a way that two diodes
conduct during Positive half-cycle & other
two diodes conduct during Negative Half
Cycle.
• During positive half cycle, the terminal A is
positive & C is negative. The diodes D1 & D3
becomes forward biased and hence conduct
current whereas the diodes D2 & D4 are non
conducting since they are reversed biased .
Hence current flows along ABRLDCA
producing drop across RL
• During positive half cycle, the terminal C is
positive & A is negative. The diodes D2 & D4
becomes forward biased and hence conduct
current whereas the diodes D1 & D3 are non
conducting since they are reversed biased .
Hence current flows along CBRLDAC
producing drop across RL
In this way the circuit acts as a full wave
rectifier. Input and output characteristics of a
bridge rectifier are as shown in figure
 Filter circuit
• The circuit which is connected between
rectifier & load to convert pulsating dc into
steady dc is called filter circuit. It consists of
Passive elements like Capacitor, Inductor or
their combination.
 Break down of diode
There are two types of breakdown in diode

1. Avalanche Breakdown
The avalanche breakdown occurs when a high reverse voltage is applied across the diode. As we
increase the applied reverse voltage, the electric field across the junction increases. This electric
field exerts a force on the electrons at the junction and frees them from covalent bonds. These
free electrons start moving with high velocity across the junction and collide with the other
atoms, thus creating more free electrons. This results in a rapid increase in net current.

2. Zener Breakdown
The phenomenon of the Zener breakdown occurs in the very thin depletion region. If the p-n
junction diode is heavily doped, the width of the depletion region becomes very thin. The thin
depletion region has more numbers of free electrons. The reverse bias applies across the PN
junction develops the electric field intensity across the depletion region. The strength of the
electric field intensity becomes very high.

The electric field intensity increases the kinetic energy of the free charge carriers. Thereby the
carriers start jumping from one region to another. These energetic charge carriers collide with the
atoms of the p-type and n-type material and produce the electron-hole pairs.

The reverse current starts flowing in the junction because of which depletion region entirely
vanishes. This process is known as the Zener breakdown.

.
 Zener Diodes
Zener diode is a heavily doped p-n junction that is specifically designed to work in the reverse
biased condition..
Features of Zener diode
Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant
for a wide range of currents. Due to this feature, it is used as a voltage regulator in d.c. circuit.
IV curve of Zener diode.

From the I-V characteristics curve above, we can see that the Zener diode has a region in
its reverse bias characteristics of almost a constant negative voltage regardless of the value of the
current flowing through the diode.
This voltage remains almost constant even with large changes in current providing the Zener
diodes current remains between the breakdown current IZ(min) and its maximum current
rating IZ(max).
Zener diode as voltage regulator
Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for
a wide range of currents. Thus we will connect the zener diode parallel to the load such that the
applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds
the knee voltage, the voltage across the load will be constant.

When Vin < Vz, then no current will flow through the zener diode. In this case all input

voltage will appear in the load resistance, ie Vin = VL.

When Vin>Vz, then the Zener breakdown occurs and further increase in voltage will increase
only the current but the output voltage remains constant.
If I be the current drawn from the supply, applying Kirchhoff’s current law at a junction,

I=lz + IL
If Rz be the zener diode resistance then
Vz = Vo
or, IzRz - IL RL
Again applying Kirchhoff’s voltage law to the mesh containing resistance Rs, Zener diode and
supply voltage Vin, then

IRs + Vz = Vin

Vz = Vin-IRs

Vo = Vin-IRs
Hence the voltage is regulated
Logic gates
Logic gates are the basic building blocks of any digital system. It is an electronic circuit having
one or more than one input and only one output. The relationship between the input and the
output is based on a certain logic. Based on this, logic gates are named as AND gate, OR gate,
NOT gate etc.
AND gate

An AND gate requires two or more inputs and produces only one
output. The AND gate produces an output of logic 1 state when each
of the inputs are at logic 1 state and also produces an output of logic 0
state even if any of its inputs are at logic 0 state

OR gate
An OR gate requires two or more inputs and produces only one output. The OR gate produces an
output of logic 1 state even if any of its inputs is in logic 1 state and also produces an output of
logic 0 state if any of its inputs is in logic 0 state.
NOT GATE
The NOT gate is also called an inverter, simply because it changes the input to its opposite. The
NOT gate is having only one input and one corresponding output.
NOR gate
NOR gate is a combination of an OR gate and a NOT gate.

NAND GATE
NAND gate is a combination of an AND gate and a NOT gate.
NAND gate also called universal gate because it can be convert into all gates.