Editing Unijunction transistor (section)

== Construction ==
{{Disputed section|date=October 2021}}
[[Image:UJT struttura.png|thumb|right|130px|Structure of a p-type UJT]]
[[Image:Unijuction transistor KT117 (open).jpg|thumb|130px|UJT die: the larger contact in the centre of the crystal is the emitter, the smaller one is B<sub>1</sub>; B<sub>2</sub> is at the bottom of the crystal]]
The UJT has three terminals: an emitter (E) and two bases (B<sub>1</sub> and B<sub>2</sub>) and so is sometimes known a "double-base diode". The base is formed by a lightly [[Doping (semiconductor)|doped]] [[N-type semiconductor|n-type]] bar of silicon. Two ohmic contacts B<sub>1</sub> and B<sub>2</sub> are attached at its ends. The emitter is of heavily-doped [[P-type semiconductor|p-type]] material. The single PN junction between the emitter and the base gives the device its name. The resistance between B1 and B2 when the emitter is open-circuit is called ''interbase resistance''. The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is not symmetrical, because a symmetrical unit does not provide optimum electrical characteristics for most of the applications.

If no potential difference exists between its emitter and either of its base leads, there is an extremely small [[Electric current|current]] from B<sub>1</sub> to B<sub>2</sub>. On the other hand, if an adequately large voltage relative to its base leads, known as the ''trigger voltage'', is applied to its emitter, then a very large current from its emitter joins the current from B<sub>1</sub> to B<sub>2</sub>, which creates a larger B<sub>2</sub> output current.

The [[schematic diagram]] symbol for a unijunction transistor represents the emitter lead with an arrow, showing the direction of [[conventional current]] when the emitter-base junction is conducting a current. A complementary UJT uses a p-type base and an n-type emitter, and operates the same as the n-type base device but with all voltage polarities reversed.

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. A UJT is operated with the emitter junction forward-biased while the JFET is normally operated with the gate junction reverse-biased. The UJT is a current-controlled [[negative resistance]] device.