What is a programmable unijunction?

A programmable unijunction is a type of transistor and is also called a programmable unijunction transistor, or PUT. It refers to an electronic semiconductor device having three leads. A PUT is much similar to a unijunction transistor (UJT) and the major difference is that it is programmable.  

How does a programmable unijunction work?

Like a unijunction transistor, the programmable unijunction transistor has a base region, which is divided into two separate parts. These separate parts create a voltage divider, setting an operating point for the transistor. However, unlike a UJT, the voltage divider of the PUT can be programmed. 

Two physical resistors remain connected to the transistor’s gate terminal, allowing the user to establish some control over the transistor’s operating point. The standoff ratio of the programmable unijunction transistor is variable and externally programmable. The structure of the device plays no part in determining the standoff ratio of a PUT. 

Applications

Programmable unijunction transistors or PUTs have a multitude of applications across different electrical devices. For instance, a PUT is commonly used as a trigger for thyristors. It is also used in oscillators, pulse, and timing circuits possessing frequencies up to 10 kHz.