In reverse biased P-N junction diode carries only a small current called reverse saturation current. However , if the reverse voltage across the device increases, eventually after certain voltage limit, breakdown occurs and a large current is observed.
This breakdown phenomenon can be due to two effects
- Zener effect
- Avalanche effect
Based on the effects , the breakdown can be named as Zener breakdown or Avalanche breakdown.
Zener breakdown (Between 3-8 v)
The depletion region in a P-N junction is devoid of any carriers. However, a high electric field in this region will be sufficient enough to detach remaining electrons from the covalent bond. Once freed, the electrons are accelerated by the field and swept to n side of the junction. This effect occurs at strength of about 10^6 V/cm.
In order to create such high field, a narrow depletion region is required which translates to high doping levels on both sides of the junction. This type of breakdown occurs at a reverse voltages in the order of 3-8 V.
Junctions with moderate/low doping levels won’t exhibit Zener breakdown. But as the reverse bias voltage across such devices increases, avalanche effect takes place. Each carrier entering the depletion region experiences a very high electric field and hence a large acceleration as denoted by the equation below
These accelerated electrons will have enough force to break the other electrons from their covalent bonds. This phenomenon can lead to avalanche i.e each electron freed by the impact may itself speed up in the electric field and can detach few more electrons from their respective covalent bonds. Now these 2 electrons may collide further atoms and displace few more carriers as depicted in the picture below.
Temperature effects on these breakdowns.
- As the temperature increases, the breakdown voltage decreases in case of Zener diodes( Zener effect shows negative temperature coefficient). For example, if the Zener effect was visible at V=4v, then this would be visible at 3V if the temperature is increased.
- As the temperature increases, the breakdown voltage increases in case of Avalanche diodes.(Avalanche effecct shows positive temperature coefficient).