Impact ionization is a process in semiconductors that usually is detrimental to circuit performance. It occurs in regions of very high electric field, such as the depletion region of a p-n junction or the pinchoff region of a MOSFET. The high electric field causes electrons to acquire kinetic energy greater than the bandgap. These high-energy carriers scatter with electrons in the valence band, causing them to jump to the conduction band, which creates an electron-hole pair.
The newly created current carriers are themselves accelerated by the electric field, and a chain reaction occurs. The exponential increase in free carriers results in runaway current. In a semiconductor diode, this breakdown mechanism is known as avalanche breakdown. Semiconductor materials are characterized by a critical breakdown field, defined as the field at which on average one carrier undergoes impact ionization upon travelling one micron.