Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials, where a form of electric current multiplication can allow very large currents within materials that are otherwise good insulators. It occurs when a high reverse voltage is applied across a diode, and the electric field across the junction increases, exerting a force on the electrons at the junction and freeing them from covalent bonds. These free electrons start moving with high velocity across the junction and collide with other atoms, creating more free electrons and resulting in a rapid increase in net current. The voltage at which the breakdown occurs is called the breakdown voltage, and there is a hysteresis effect where once avalanche breakdown has occurred, the material will continue to conduct even if the voltage across it drops below the breakdown voltage. Avalanche diodes, commonly encountered as high voltage Zener diodes, are constructed to break down at a uniform voltage and to avoid current crowding during breakdown. These diodes can indefinitely sustain a moderate level of current during breakdown.