The primary difference between Zener breakdown and avalanche breakdown is their process of recurrence. Zener breakdown happens due to the high electric field, while an avalanche occurs because of the collision of free electrons with atoms. Zener Breakdown is when the diode is reversed biased.
The kinetic energy of the electrons grows, and they move at high speed, then the high-velocity electrons run into other atoms and give rise to free electrons. These free electrons give rise to a high value of saturation current. An avalanche breakdown happens when a high reverse voltage is employed across the diode.
As we increase, the utilized reverse voltage is applied across the diode. Increasing the reverse voltage, the electric field across the junction elevates. The electric field exerts a force on the electrons at the crossroads and frees them.