A p-n junction is formed where a single crystal of silicon or germanium is doped such that one half is p-type semiconductor and the other half is n-type semiconductor. When forward biased, the barrier potential decreases allowing majority charge carriers to flow across the junction, decreasing resistance. When reverse biased, the barrier potential increases preventing carrier flow and increasing resistance. The voltage-current characteristics of a p-n junction diode are nonlinear, with negligible current below the threshold voltage and exponential increase in current above it. In reverse bias, very little reverse saturation current flows until the breakdown voltage is exceeded.

2. p-n JUNCTION
It is a single crystal of Ge
or Si doped in such a
manner that one half
portion of its acts as p
type semiconductor and
the other half as n type
semiconductor.
The term junction
implies the boundary or
region of transition
between n type and p
type semiconductor
material.
Fig: Diode representations: PN-
junction model, schematic symbol,
physical part.

3. fig:-Diode under forward bias
Few characteristics of forward bias:-
> Barrier potential decreases.
> Movement of majority charge carriers
i.e.. holes from p-side to n-side towards
the junction.
>Effective resistance across the p-n
junction decreases.
Working of a p-n
Junction
1.Forward biasing
If the positive terminal
of the battery is
connected to the p-side
and the negative
terminal to the n-side ,
then the p-n junction is
said to be forward
biased.

4. fig:-Diode under reverse bias
Few characteristics of reverse bias:-
> Barrier potential across the junction
increases.
>Movement of majority charge carriers
i.e.. electrons away from the junction.
>Effective resistance across the p-n
junction increases.
2. Reverse biasing
If the positive terminal
of a battery is connected
to the n-side and the
negative terminal to the
p-side , then the p-n
junction is said to be
reversed biased.

5. V-I
characteristics of a
p-n junction diode:-
1.Forwad bias
characteristics:-
The p-n junction diode
is connected to a
potentiometer (or
rheostat) so that voltage
applied can be changed.
For different values of
voltages, the value of
current is noted. A graph
is plotted between V
and I. This voltage-
current graph is called
Forward characteristics.

6. Features of Graph:
1.The V-I graph is not
straight line i.e.. A
junction diode doesn’t
follow Ohm’s law.
2.Initially current
increases very slowly
almost negligibly , till the
voltage across the diode
crosses a certain value ,
called threshold-voltage
or cut-in voltage . The
value of cut –in voltage is
about 0.2 for a Ge diode
and 0.7 for a Si diode.
3.After the cut-in voltage,
the diode current
increases
rapidly(exponentially),
even for a very small
increase in the bias
voltage.
Figure: Increasing forward bias from (a) to (b) decreases depletion region thickness.

7. 2.Reverse bias
characteristics:-
The p-n junction diode
is connected to a
potentiometer(or
rheostat) so that the
voltage the diode can be
changed. For different
values of voltage, the
value of current is noted
. A graph is plotted
between V and I. This
voltage-current graph is
called Reverse
characteristics.

8. Features of Graph:-
1.When the diode is reverse
biased , the reverse bias
voltage, the reverse bias
voltage produce very less
current, about a few
microamperes which almost
remain constant with bias
.This small current is called
Reverse saturation current.
2.When the reverse voltage
across p-n junction reaches
a sufficiently high value, the
reverse current increases to
a large value. This voltage at
which breakdown of
junction takes place is
called Zener breakdown
voltage or peak-inverse
voltage of the diode.

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