Acronym for IMPact ionization Avalanche Transit Time. A semiconductor diode that acts as a powerful source of microwave power. When a p-n junction is reverse-biased into avalanche breakdown, it exhibits negative resistance at microwave frequencies and may be used as a negative-resistance oscillator. The differential current is out of phase with the differential voltage due to two effects: following a voltage increment the current builds up with a delay time, t_A, characteristic of the avalanche; the terminal current increment is further delayed by a time t_t (the transit time) during which the carriers are collected by the electrodes. The diode is usually formed so that the current is delayed by half a cycle with respect to the voltage.

Although any p-n junction diode will exhibit IMPATT mode operation, typical devices used consist of an avalanching region together with a drift region in which no avalanche occurs. Examples of such diodes are shown in the diagrams. Fig. a shows a Read diode with structure p⁺-n-i-n⁺. Although both holes and electrons are produced by the avalanche breakdown, only the electrons are given a drift region and collected. This type of device is known as a single drift device. Fig. b shows a modified Read diode, known as a hi-lo Read diode, in which the intrinsic region is replaced by an n-type region. Fig. c shows a PIN diode in which the avalanche occurs throughout the intrinsic region. An alternative structure where both the holes and electrons are given a drift region is a p⁺-p-n-n⁺ structure. Avalanche occurs at the centre p-n junction and both holes and electrons are collected. Such a structure is termed a double drift device.

(a) Typical IMPATT diode: Read diode

(b) One-sided abrupt p-n junction (c) PIN diode

The diode is mounted in a microwave cavity that can have its impedance matched to that of the diode in order to form a resonant system. In an appropriate circuit spontaneous oscillation will occur.