First we will describe what avalanche breakdown is. Avalanche breakdown occurs at p-n junction which is moderately doped and has a thick junction means have depletion layer width high. Avalanche breakdown occurs when we apply high reverse voltage across the p-n junction. As we apply more and more reverse voltage across the junction, the electric field across the junction keeps 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.
Avalanche Breakdown in Thyristor:
When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J2 will have an avalanche breakdown at forward break over voltage VBO leading to thyristor turn on. The avalanche breakdown occurs in forward conduction mode of thyristor the mechanism is same as in diode avalanche breakdown because at J2 n—p junction is there which behave like diode in reverse bias. As we increase the avalanche occur same as we have described above for diode.