When a PN diode is subjected to a reverse bias higher than the breakdown voltage (V_Z), the diode can conduct a large amount of current. Alternatively, the PN diode can hold the voltage to V_Z when a fixed current is forced through the diode in reverse bias condition. This behavior of the PN diode can provide a fixed voltage clamp for a wide range of currents and can be used as a crude voltage reference or voltage clamp for circuit protection. There are two major physical phenomena that lead to PN diode breakdown, namely, avalanche breakdown and zener breakdown as shown in Figure 2-2. Irrespective of the breakdown mechanisms at play, the commercially available diodes are colloquially called 'Zener' diodes.

Avalanche breakdown results from impact ionization by high-energy electrons in the depletion region. Every electron that enters the depletion region can multiply and generate exponentially more electron-hole pairs that result in a sudden increase in the current when bias is V_Z or higher in the reverse bias. Zener breakdown results from tunneling of valence electrons from p-region to n-region through silicon bandgap under extremely high junction electric fields. Zener breakdown typically occurs for the PN diodes that are heavily doped in both p- and n-type regions. Avalanche breakdown typically occurs for larger voltages (>6V) whereas zener breakdown primarily occurs at lower bias (approximately 2-5V). Both breakdown phenomena are reversible as long as the PN diode does not undergo a thermal breakdown due to excessive current.