Q. Explain, with the help of structure, the working of UJT? List its applications

One of the oddest semiconductor devices in use is the unijunction transistor (UJT). As its name suggests, this is a three-terminal device, which nevertheless has only one PN junction. It cannot amplify an applied signal, but it nevertheless can be used as the active element in an oscillator circuit.

Above figure shows the physical construction of a typical UJT. It consists of a bar of N-type silicon with electrical connections at either end, plus an aluminum wire bonded to a point along the length of the silicon bar. At the bonding point, the aluminum creates a P-type region in the silicon bar, thus forming a PN junction. Because there's only one junction, it's not reasonable to use the terms anode or cathode, so designations are taken instead from transistor notations. The P-type connection is known as the "Emitter," while the two N-type connections are designated "Base 1" and "Base 2." For this reason, the device is sometimes called a "double-base diode."