A large, usually abrupt rise in electric current in the presence of a small increase in electric volt age. Breakdown may be intentional and controlled or it may be accidental. Lightning is the most familiar example of breakdown.
In a gas, such as the atmosphere, the potential gradient may become high enough to accelerate the naturally present ions to velocities that cause further ionization upon collision with atoms. If the region of ionization does not extend between oppositely charged electrodes, the process is corona discharge. If the region of ionization bridges the gap between electrodes, thereby breaking down the insulation provided by the gas, the process is ionization discharge. When controlled by the ballast of a fluorescent lamp, for example, the process converts electric power to light. In a gas tube the process provides controlled rectification.
In a solid, such as an insulator, when the electric field gradient exceeds 106 volts/cm, valence bonds between atoms are ruptured and current flows. Such a disruptive current heats the solid abruptly. In a semiconductor if the applied backward or reverse potential across a junction reaches a critical level, cur rent increases rapidly with further rise in voltage. This avalanche characteristic is used for voltage regulation in the Zener diode. In a transistor the breakdown sets limits to the maximum instantaneous voltage that can safely be applied between collector and emitter.