Unijunction transistor

A Unijunction transistor (UJT) is an electronic semiconductor device.

There are two types of unijunction transistors:

• The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. The 2N2646 is an example of such a device.

• The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consist of four P-N layers and has an Anode and a Cathode connected to the first and the last layer, and a Gate connected to one of the inner layers. They are not directly interchangable with conventional UJTs but perform a similar fuction. The 2N6027 is an example of such a device.

For both types, their main use is as a trigger device for thyristors and as the active device in relaxation oscillators. The graph of emitter voltage against emitter current of a unijunction transistor shows an area of negative resistance; this is what makes the UJT useful.