in physics, a sudden increase in the flow of an electrical current (electric current) through a nonconducting or semiconducting solid when a sufficiently strong electrical force is applied. The ability of most nonmetallic solids to carry an ordinary electrical current is limited by the scarcity of electrons (electron) free to move in the presence of an externally applied electric field. A sufficiently strong electrical force can break free a large number of electrons from the atoms that form the structure of the solid so that a large current can flow through the material. This avalanche effect is responsible for the phenomenon of breakdown in insulators (insulator) and in semiconductors, where it is called the Zener effect. Because avalanche requires a specific electrical force for each type of substance, it can be used for precise control of voltages in electrical circuits, as in a device called the Zener diode.

At room temperature, even an insulator has a few free electrons. Strong electrical forces cause these electrons to move through the solid rapidly and, if the free electron is moving rapidly enough, it may knock an electron away from an atom in the solid. This ejected electron (referred to as excited) can move freely through the solid and excite other electrons in the same way, in a process resembling an avalanche in which each rolling rock frees others.

When the electrical force is removed, the newly freed electrons are recaptured by the atoms of the solid, which once again becomes a poor conductor of electricity. Such sudden, large currents may alter or even melt the solid.