Uni Junction Transistor (UJT) Basically, it is a three terminal silicon diode. As the name indicates, it has only one pn junction. It differs from an ordinary diode in that it has three terminals and it differs from a FET in that it has no ability to amplify. However, it has the ability to control a large a.c. power with a small signal. It also exhibits a negative resistance characteristics which makes it useful as an oscillator. It consists of an n-type silicon bar with an electrical connection on each end. The leads to these connections are called Base 1 B 1 and Base 2 B 2 . Partway along the bar between the two bases, nearer to B 2 than B 1 a pn junction formed. The lead of this junction is called emitter E the below Fig shows the basic structure and the circuit symbol of unijunction transistor.