A PN junction diode allows current to flow in one direction. It is formed by joining a P-type semiconductor with an N-type semiconductor. Current can be made to flow by applying either forward or reverse bias. In forward bias, the P region is connected to the positive terminal and the N region to the negative terminal. In reverse bias, the connections are reversed. The depletion region where there are no free charges forms across the PN junction. Breakdown can occur due to Zener or avalanche effects at high reverse voltages. Special diodes include Zener diodes, which have a controlled reverse breakdown voltage, and photo diodes, which generate a current when exposed to light.

1. PN JUNCTION DIODE
Pn diode is a semiconducting device,that allow current to flow in one direction.it is a combination of two
different types of semiconductor
*pn diode is formed by joining acceptor impurities to donar impurities.
where,
p=donar impurity (+)
n=acceptor impurity(-)
*circuit symbol of pn diode:
*graphical representatio of pn diode:
* Electron & hole movement in pn diode:

2. # To make current to flow through pn diode, "BIASING" of
pn diode is done. there are two types of biasing
.forward biasing
.reverse biasing
#FORWARD Biasing religion is connected to +ve terminal of
eternal dc source & n religion is connected to -ve terminal.
# REVERSE BAISING:N region is connected to +ve terminal
of external dc source & p region is connected to -ve terminal.
# The region in which only mobile ions & no free charge ions
are present is called "DEPELTION LAYER"

3. #Tthe minimum reverse voltage at which pn junction breakdown at certain rise in
tempreature.Two types of breakdown are
.....ZENER BREAKDOWN
.....AVALANCHE BREAKDOWN
#ZENER BREAKDOWN : it occurs due to heavy doping of p&n side of diode. deplition region is
narrow in reverse biasing as,all the reverse voltage appears across the deplition layer.
.in this breakdown electric feild is very intense across the deplition layer.
.due to intnse e.f the covalent bond between electrons breaks &electron
becomes free electrons.
.a large number of such electrons constiuets a large reverse current this is
known as” BREAKDOWN DUE TO ZENER EEFECT"
#AVALANCHE BREAKDOWN:it occurs due to large reverse voltage applied to a
diode, bywhich velocity of minority carrier increases therefore kinetic energy also increases.
.then free electrons moves &passes this energy to the covalent bond
electrons,due to which they jump to conduction bond for
& become free for conduction .Now they
knock out some more electrons from bond this series is known as"CHAIN REACTION"also as
"AVALANCHE BREAKDOWN"

4. *it is formed by joining forward & reverse characterstics of pn diode :
*V(br) is the reverse breakdown voltage,the voltage at which breakdown occurs.
*the reverse current I(0) upto V(br) is practically constant &neglegibly small.
*diode current equation is :
I(0) = I(0) [ e v/nT -1]
where,
I(0)=reversesaturation current,
V(T)=T/11600 ;volt equivalent of tempreature.
T=temp in kelvin,

5. *effect of temp on pn diode is can be represented by:
*I(0) i.e reverse saturatin current &V(T) i.e temp in kelvin is tempreature
dependent due to which diode deoands on tempreature
*by this graph :

6. there are some special types of diode which are known as special purposediode.
.zener diode
.varactor diode
.PIN diode
.L**** ZENER DIODE :
*it is a sepecil type of pn junction diode semiconductor diode.its construction is similar topn diode where breakdown voltage is adjusted
between 3V to 200V.
*doping of diode is controlled ,for proper function of diode.
*circuit symbol of zener diode:
*forward biasing of diode:
*reverse biasing of diode: ed diode
** ZENER DIODE :
*it is a sepecil type of pn junction diode semiconductor diode.its construction is similar to pn diode where breakdown voltage is adjusted between 3V to 200V
*doping of diode is controlled ,for proper function of diode.
*circuit symbol of zanier diode is: *forward biasing of diode: *reverse biasing of diode: .photo diode

7.  PIN DIODE

8.  VARACTOR DIODE

9.  LED DIODE

10. PHOTO DIODE
A photo diode is always operated at reverse biased condition.
* circuit symbol of a photo diode
*DARK CURRENT: current flowing through a photo diode in the absence of
light. this current increases
with increase in temperature.
photo diode :
*a photo diode is a highly sensitive diode & has high speed of operation as
compared to led.