CoiMbatore institute of technology
(An Atonomous Institution affiliated to Anna University,Chennai)
PN JUNCTION DIODE
-Dr.S.Vasantha,CIT
1. DEFINITION
2. OPERATION
3. FORWARD AND REVERSE
BIAS CHARACTERISTICS
DEFINITION
When a p-type semiconductor is
suitably joined to n-type
semiconductor, the contact surface is
called p-n juntion.
OPERATION
FORWARD BIASING:
DEFINITION:
When external d.c. voltage applied to the
junction is in such a direction that it cancels the
potential barrier, thus permitting current flow, it
is called forward biasing.
p n
+ -
V
With forward bias to p-n
junction, the following points
are worth noting:
1. The potential barrier is reduced and at
some forward voltage (0.1 to 0.3 V), it is
eliminated altogether.
2. The junction offers low resistance(called
forward resistance Rf ) to current flow.
3. Current flows in the circuit due to
the establishment of low resistance
path.
4. The magnitude of current depends
upon the applied forward voltage.
REVERSE BIASING:
DEFINITION:
When the external d.c. voltage applied
to the junction is in such a direction that
potential barrier is increased it is called
reverse biasing.
p n
- +
V
With reverse bias to p-n junction,
the following points are worth
noting:
1. The potential barrier is increased.
2. The junction offers very high resistance
(called reverse resistance Rr) to current
flow.
3. No current flows in the circuit due to the
establishment of resistance path.
FORWARD AND REVERSE BIAS
CHARACTERISTICS
FORWARD BIAS CHARACTERISTICS:
Under forward bias condition, the V-I
characteristics of a PN junction diode is shown
in figure
KNEE VOLTAGE
• It is the forward voltage at which the
current through the junction starts to
increase rapidly.
• The knee voltage for silicon diode is
0.7V and 0.3V for germanium diode.
REVERSE BIAS CHARACTERISTICS
Under reverse bias condition, the V-I
characteristics of a PN junction diode is
shown in figure
(mA)
(volts) (volts)
• The width of the depletion region
which is depleted of mobile charge
carriers increases.
• Theoretically no current should flow
in the external circuit.
• But in practise, a very small reverse
current of the order of a few
microampere (reverse saturation
current) flows under reverse bias
condition.
• The magnitude of the reverse saturation
current mainly depends upon junction
temperature because the major source
of minority carriers is thermally broken
covalent bonds.
AVALANCHE EFFECT
• For large applied reverse bias, the
free electrons from the N type
moving towards the positive terminal
of the battery acquire sufficient
energy to move with high velocity to
dislodge valance electron from
semiconductor atom in the crystal.
• These newly liberated electrons, in
atom to acquire sufficient energy to
dislodge other parent electrons.
• Thus, a large number of free
electrons are found which is
commonly called as avalanche of
free electrons.
BREAKDOWN VOLTAGE
DEFINITION:
• It is the maximum reverse voltage at
which p-n junction breaks down with
sudden rise in reverse current.
• Once the breakdown voltage is
reached, the high reverse current
may damage the junction.
• Therefore care should be taken that
reverse voltage across a p-n junction
is always less than the breakdown
voltage.
