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A large, usually abrupt rise in electric current in the presence of a small increase in electric voltage. 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, current 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. See Breakdown potential, Transistor
the general name for various types of physical processes that bring about an abrupt rise in electric current through a medium that is ordinarily nonconductive or only weakly conductive.
(1) Electrical breakdown in a vacuum is due to the formation of a conducting path by the particles of the electrode materials that are emitted as a result of secondary electron emission and ion emission (for the most part electron-ion emission). Particles of the residual gases, together with the electrons that are always emitted to some extent by electrodes into a surrounding vacuum, serve as the initiatory particles that accelerate in the electric field, bombard the electrodes, and begin the secondary emission. In very strong fields, tunnel emission (autoelectronic emission or autoionization) can play an important part in this type of electrical breakdown.
(2) Electrical breakdown in a gas-filled space is the initial stage of electrical discharge in the gas (seeSPARK DISCHARGE).
(3) For electrical breakdown in solid and liquid dielectrics and semiconductors, seeDIELECTRICS; SEMICONDUCTORS; and .