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Zener Diode

Dr. Pravin Prajapati

Outlines

 Introduction of Zener Diode

 Construction of Zener Diode

 Working of Zener Diode

 Application of Zener Diode

 Numerical of Zener Diode

Construction of Zener

Zener diodes are designed to operate in reverse breakdown. Two types of reverse breakdown in a zener diode are avalanche and zener. The avalanche break down occurs in both rectifier and zener diodes at a sufficiently high reverse voltage. Zener breakdown occurs in a zener diode at low reverse voltages.

A zener diode is heavily doped to reduced the breakdown voltage. This causes a very thin depletion region. The zener diodes breakdown characteristics are determined by the doping process Zeners are commercially available with voltage breakdowns of 1.8 V to 200 V.

Working of Zener

A zener diode is much like a normal diode. The exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve illustrates the operating range for a zener. Note that it’s forward characteristics are just like a normal diode.

ZENER BREAKDOWN

• Zener and avalanche effects are responsible

for such a dramatic increase in the value of

current at the breakdown voltage.

• If the impurity concentration is very high, then

the width of depletion region is very less.

Less width of depletion region will cause high

intensity of electric field to develop in the

depletion region at low voltages.

• Lets take an example to understand things

clearly.

• Let say the width of depletion region

is 200 Å (very small). If a reverse

bias voltage of just 4 V is applied to

the diode, then the electric field

intensity in the depletion region will

be

4 = 2 x 10^8 V/m

200 x 10-

AVALANCHE BEAKDOWN

• Zener effect predominates on diodes whose breakdown voltage is below 6 V. The breakdown voltage can be obtained at a large value by reducing the concentration of impurity atom.
• We know that very little amount of current flows in the reverse biased diode. This current is due to the flow of minority charge carriers i.e., electrons in the p type semiconductor and holes in the n type semiconductor.

• The width of depletion region is large when the impurity concentration is le. ss.
• When a reverse bias voltage is applied across the terminals of the diode, the electrons from the p type material and holes from the n-type materials accelerates through the depletion region.
• This results in collision of intrinsic particles (electrons and holes) with the bound electrons in the depletion region. With the increase in reverse bias voltage the acceleration of electrons and holes also increases.
• Now the intrinsic particles collides with bound electrons with enough energy to break its covalent bond and create an electron-hole pair. This is shown in the figure.

.

• (^) The collision of electrons with the atom creates an electron-hole pair.
• (^) This newly created electron also gets accelerated due to electric field and breaks many more covalent bond to further create more electron-hole pair.
• (^) This process keeps on repeating and it is called carrier multiplication. The newly created electrons and holes contribute to the rise in reverse current. The process of carrier multiplication occurs very quickly and in very large numbers that there is

Thus the breakdown is

apparently an avalanche of charge called

carriers. avalanche breakdown.

DIFFERENCE BETWEEN ZENER AND AVALANCHE BREAKDOWN Zener Breakdown 1.This occurs at junctions which being heavily doped have narrow depletion layers

2. This breakdown voltage sets a very strong electric field across this narrow layer.

3.Here electric field is very strong to rupture the covalent bonds thereby generating electron-hole pairs. So even a small increase in reverse voltage is capable of producing Large number of current carriers.

4.Zener diode exhibits negative temp: coefficient. Ie. breakdown voltage decreases as temperature increases.

Avalanche breakdown

1. This occurs at junctions which being lightly doped have wide depletion layers. 2.Here electric field is not strong enough to produce Zener breakdown. 3.Her minority carriers collide with semi conductor atoms in the depletion region, which breaks the covalent bonds and electron-hole pairs are generated. Newly generated charge carriers are accelerated by the electric field which results in more collision and generates avalanche of charge carriers. This results in avalanche breakdown. 4.Avalanche diodes exhibits positive temp: coefficient. i.e breakdown voltage increases with increase in temperature.

Ideal Model & Ideal Characteristic Curve of Zener Diode

Practical Model & Ideal Characteristic

Curve of Zener Diode

Zener Limiting

Zener diodes can used in ac applications to limit voltage swings to desired levels.

VZ : zener voltage V d : Diode voltage V d = 0.

Numerical of Zener Diode