The linear approximation to the I-V characteristic of a zener diode in the reverse bias and breakdown regions is as follows.
Zener Diode
The slope of the line at Q is 1/rZ
rZ is called the incremental resistance of the zener diode
This is exaggerated for clarity in the figure. In practice rZ is small (a few ohms) and the breakdown voltage is approximately constant irrespective of the reverse current.
Zener Diode
Zener breakdown occurs when the electric field in the depletion layer increases to the point where it can break covalent bonds and generate electron-hole pairs.
Electrons generated in this way are swept by the electric field into the n side.
Holes generated in this way are swept by the electric field into the p side.
Zener Current
These electrons and holes constitute a reverse current through the junction.
Once the zener effect starts a large number of carriers can be generated with negligible increase in the junction voltage.
In the breakdown region the reverse current is thus determined by the external circuit,the reverse voltage across the diode remains close to the rated breakdown voltage.
Zener Diode
The other breakdown mechanism is avalanche breakdown.
This occurs when minority carrier in the depletion layer gain sufficient kinetic energy to break covalent bonds in atoms when they collide.

Zener Diode
Avalanche breakdown.
Carriers liberated may have or gain sufficient energy to cause other carriers to be generated.
This process continues in the fashion of an ‘avalanche’
Many carriers can be created to support any reverse current determined by the external current.
Zener Diode
The device is operated in reverse bias.
Thus we reverse the sign notation that we normally use for diode voltages and currents, as shown on the next slide
Zener Diode
The device is operated in reverse bias.
Thus we reverse the sign notation that we normally use for diode voltages and currents, as shown on the next slide
Zener Diode
The device is operated in reverse bias.
Thus we reverse the sign notation that we normally use for diode voltages and currents, as shown on the next slide
Zener Diode
With this notation, in the zener region:
         Vz = Vzo + rzIz

Zener Diode
The Zener diode is used to provide a stable reference voltage in the face of a varying supply voltage.
Zener Reference Voltage Circuit
Circuit diagrams and model:
Circuit diagrams (Same circuit)



Model

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