UniJunction Transistor:

(abbreviated as UJT), also called the double base diode, is a 2 layer, 3 terminal solid state switching device. The device has a unique characteristic that when it is triggered, its emitter current increases regenerative until it is restricted by emitter power supply. The low cost per unit, combined with its unique characteristic, have warranted its use in a wide variety of applications. A few include oscillators, pulse generators, saw tooth generators, triggering circuits, phase control, timing circuits and voltage or current regulated supplies. The device is in general, a low power absorbing device is in general, a low power absorbing device under normal operating conditions and provides tremendous aid in the continual effort to design relatively efficient systems.