sentences:  The IV curve for Zeners above 5.6 V (being dominated by avalanche), is much sharper at breakdown.The Zener diode's operation depends on the heavy doping of its p-n junction.The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.