A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction.

In the 1960s, the low cost per unit, combined with its unique characteristic, warranted its use in a wide variety of applications like oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies.

There are three types of unijunction transistor: Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple oscillators to be built using just one active device.

[4] Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used.

In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors (silicon controlled rectifiers (SCR), TRIACs, etc.).

The base is formed by a lightly doped n-type bar of silicon.

If no potential difference exists between its emitter and either of its base leads, there is an extremely small current from B1 to B2.

The structure of a UJT is similar to that of an N-channel JFET, but p-type (gate) material surrounds the N-type (channel) material in a JFET, and the gate surface is larger than the emitter junction of UJT.

This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected.