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. The 2N6027 is an example of such a device.
The original unijunction transistor, or UJT, is a simpler 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.
Unijunction transistors are very rarely used, as not many electronic engineers know about them.
Their main use is as a trigger device for thyristors and as the base for a relaxation oscillator.
Interestingly the graph of emitter voltage against emitter current of a unijunction transistor shows an area of negative resistance 5.