Programmable Unijuction transistor – PUT

A programmable unijuction transistor is a pnpn device like an SCR. The difference between the PUT and SCR is that the gate is connected to N type material near to the anode instead of P type material near to cathode as in case of an SCR. The below figure shows the diagram of a PUT along with its symbol.

Programmable Unijuction Transistor

symbol of PUT

V - I characteristics of PUT

In this device the gate terminal is always kept positive with respect to cathode. When the anode potential exceeds the gate potential by about 0.7 V the junction J₁ is forward biased and the device starts conducting. Whereas if the anode voltage is less than that of gate voltage the device will be turned off.

PUTs are used in SCR triggering circuits, time delay and logic circuits. These are available with voltage and current ratings of upto 200V and 1A respectively.