1. By Squadron Leader Zahid Mir CS&IT Department , Superior University
PHY-BE Zener Diode By
Squadron Leader Zahid Mir
CS&IT Department , Superior University

2. Zener Diode
Reverse-biased heavily doped pn-junction diode which operated in the breakdown region.

3. Junction Breakdown
By increasing reverse bias voltage, a point is reaches where the junction breakdown and reverse current rises sharply to a value limited only by the external resistance connected in series with the junction.
This breakdown voltage depends on the width of the depletion region, which depends upon doping level.

4. Types of Breakdown in Zener Diode
There are two types of reverse breakdown in zener diode;
Avalanche Breakdown
It occurs in both rectifier diode and zener diode at a sufficiently high reverse voltage.
Zener Breakdown
It occurs in a zener diode at low reverse voltages.

5. Avalanche Breakdown
It occurs in junctions which have wide depletion layers due to light doping level.
Minority carriers (accelerated by this field) collide with semiconductor atoms in depletion region.
On collision with valance electrons, covalent bonds are broken and electron-hole pairs are generated.
These newly-generated charge carriers also accelerated by electric field resulting in more collisions and hence further production of charge carriers.
This leads to an avalanche (flood) of charge carriers.
Note:- Junction have very low resistance in breakdown region.

6. Zener Breakdown
It occurs in junctions which have narrow depletion layers due to heavily doped.
Breakdown voltage setup a very strong electric field across this narrow layer.
This field rupture or break the covalent bonds; thereby creating electron-hole pairs.
A slight further increase in reverse voltage is capable of producing large number of current carriers.
Note:- Junction have very low resistance in breakdown region.

7. Zener Biasing Diode should be;
Reverse Biased
Have voltage across it greater than Vz
Be in a circuit where current is less than Izmax
Note:- If a zener diode is forward-biased, it operates in the same way as a rectifier diode does.

8. V / I Characteristics
Vz = Zener breakdown voltage Izmin = Minimum current to sustain zener breakdown Izmax = Maximum zener current limited by maximum power dissipation For an ideal zener diode, the voltage does not change once it goes into breakdown. It means that Vz remains constant even when Iz increases considerably.

9. V / I Characteristics
As the reverse voltage (VR) is increased, the reverse current (IR) remains extremely small upto the “knee” of the curve.
The reverse current is also called the zener current, IZ.
At this point, the breakdown effect begins; the internal zener resistance, also known as zener impedance (ZZ), begins to decrease as the reverse current increase rapidly.
Note that from the bottom of the knee, the zener breakdown voltage (VZ) remains nearly constant.

10. Zener Impedance
A change in zener current (ΔIZ) produces a small change in zener voltage (ΔVZ). This is represented as zener impedance (ZZ), as ZZ = ΔVZ ΔIZ