Diode
1.Diode
 |
| Fig:-Diode |
The semi-conductor device formed simply by joining on n-type and p-type
material together is called diode.
Ideal Diode
It is a two terminal device having
the circuit symbol as shown in fig(a) and i-v characteristics curve as shown in
figure(b).
Biasing of diode
No Biasing applied(VD=0v)
In absence of an applied bias, the net flow of electron or holes(charges)
in one direction is zero.
Hence, no current flow if no biasing voltage is applied.
 |
| VD=0 |
Reverse Biasing(VD<0v)
Due to reverse biasing the width of depletation region is increased
resulting the majority carrirer flow to zero.
The number of minority carriers, however entering the region will not
change, resulting in minority-carrier flow vectors of the same magnitude
indicated in Figure below with no applied voltage.
The current that exist under reverse-bias condition is called the reverse
saturation current and is represented by IS.
Forward biasing
A forward bias condition is established by appying the positive potential
p-type material and the negative potential to n-type material as shown in the
figure.
The application of forward bias vol tage reduces the width of depletation
region and hence enhances the flow of majority charge carriers. And the result
is increase in forward current. As the applird bias increases in magnitude,the
depletion region will continue to decrease until a flood of electrons can pass
through the junction, resulting in exponential rise in current in forward-bias
region.
Diode Models
Ideal Diode Model
The ideal model of a diode is a simple switch. When diode is forward biased
it acts like a closed(ON) switch and when it is reversed biased it acts as
open(OFF) switch.
VF=Forward bias voltage
VR=Reverse bias voltage
IF=Forward bias current
IR=reverse bias current
Practical Diode Model
Practically we need to overcome the barrier potential by application of
forward bias voltage.
Hence practical model adds the barrier potential to the ideal diode model.
When the diode is forward-biased, it is equivalent to a closed switch in
series with a small equivalent voltage source equal to barrier potential(0.7v
for silicon and 0.3v for germanium) with positive side towards the anode. When
the diode is reverse-biased, it is equivalent to open switch.
Complete Diode Model
The complete diode model consist of the barrier potential, the small
forward dynamic resistance(rd') and the large reverse resistance(rR').The
reverse resistance is taken into account because it provides path for the
reverse current which is included in this model.
When the diode is forward biased, it acts as closed switch in series with
the barrier potential voltage and the small forward dynamic resistance((rd').
When the diode is reverse-biased, it acts as open switch in parallel with
the large internal resistance((rR').
I-V characteristics curve of a diode
 |
| Fig:-I-V characteristics of diode |
The general characteristics of semi-conductor diode can be defined by the
shuckley's equation for forward and reverse biased regions.
ID=Is(eVD/nVT-1)
ID ~IseVD/nVT------------------------------------------------------------------------------------------(1)
Where, ID=current in diode
Is-reverse
saturation current
VD=applied
forward-bias viltage
VT=KT/q, is
thermal voltage
K=Boltzmann's
constant=1.38*10-23
T=Absolute Temperature
in Kelvin
q=magnitude of
electronic charge=1.6*10-19
1<=n(eta)<=2depending
a wide variety of factors.
Differntiating equation(1), we have
rd(dynamic resistance of diode)=vd/id=nVT/ID
ID1 =IseVD1/nVT
ID2 =IseVD2/nVT
ID2/ID1=eVD2-VD1/nVT
Taking natural log on both the sides, we get
VD2-VD1/nVT=In(ID2/ID1)
VD2-VD1=nVTIn(ID2/ID1)
Zener Diodes
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| Fig-:Zener diode |
The Zener diode is a silicon p-n junction device that differs from
rectifier diodes because it is designed for operation in the reverse-breakdown
region.If a Zener diode is forward biased, it operates as a rectifier
diode.
There are two types of breakdown in zener diode:
(i)Zener breakdown
 |
| Fig:-I-V characteristics Zener diode |
It occurs in a zener diodes at
low reverse voltages.A zener diode is very heavily doped to reduce
breakdown voltage. This causes a very thin depletation region.As a result, an
intense electric field exist within the depletation region.Near zener breakdown
voltage(VZ),the field is intense enough to pull electrons from their
valance bonds and create current.This phenomenon is called breakdown.
(ii)Avalanche Breakdown
The multiplication of conduction electrons due to knocking of valence
electrons by free minority electrons under the action of reverse bias voltage
is called Avalanche Breakdown.
Breakdown characteristics
As the reverse voltage VR is increased the reverse
current IR remains extremly small upto the knee of the curve.
Current at this point of curve is Zener Knee current IZK.From this
point zener breakdown start and current rises rapidly upto IZM,
Zener maximum current. The Zener breakdown voltage(VZ) remains
essentially constant altough it increases slightly as the zener current IZ
increases.
Zener Regulation
The ability to keep the reverse voltage across its terminals essentially
constant is the key feature of zener diode, which enables for its property of
voltage regulation.
A zener diode operating in breakdown region acts as voltage regulator over
a specified range of reverse-current values(IZK to IZM).
Below knee current IZK voltage regulation is lost and above IZM(zener
maximum current) diode may be damaged due to excessive power dissipation.
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