PN Junction Breakdown : Electrical break down of any material (say metal, conductor, semiconductor or even insulator) can occur due to two different phenomena. Those two phenomena are 1) Zener breakdown and 2) Avalanche breakdown
These two phenomena are quite like a natural occurrence. It even applies to our daily life while lightning. We all know air is an insulator under normal conditions. But when lightning occurs (an extremely high voltage), it charges the air molecules nearby and charges get transferred via air medium. Now that’s a kind of electrical break down of an insulator. A similar kind of situation arises in zener and avalanche breakdown as well.
Avalanche breakdown occurs in a pn junction diode which is moderately doped and has a thick junction (means its depletion layer width is high). Avalanche breakdown usually occurs when we apply a high reverse voltage across the diode (obviously higher than the zener breakdown voltage, say Vz). So as we increase the applied reverse voltage, the electric field across junction will keep increasing.
If applied reverse voltage is Va and the depletion layer width is d; then the generated electric field can be calculated as Ea =Va/d
This generated electric field exerts a force on the electrons at junction and it frees them from covalent bonds. These free electrons will gain acceleration and it will start moving across the junction with high velocity. This results in collision with other neighboring atoms. These collisions in high velocity will generate further free electrons. These electrons will start drifting and electron-hole pair recombination occurs across the junction. This results in net current that rapidly increases. We learned that avalanche breakdown occurs at a voltage (Va) which is higher than zener breakdown voltage (Vz). The reason behind this is simple. We know, avalanche phenomena occurs in a diode which is moderately doped and junction width (say d) is high. A zener break down occurs in a diode with heavy doping and thin junction (here d is small). The electric field that occur due to applied reverse voltage (say V) can be calculated as E = V/d.
So in a Zener breakdown, the electric field necessary to break electrons from covalent bond is achieved with lesser voltage than in avalanche breakdown. The reason is thin depletion layer width. In avalanche breakdown, the depletion layer width is higher and hence much more reverse voltage has to be applied to develop the same electric field strength (necessary enough to break electrons free)