UJT and its Working

UJT and its Working 

What is UJT?

The full name of UJT is uni-junction transistor. It is a three-terminal semiconductor device. It has only one pn junction so it is called uni-junction transistor.Symbol of UJT is given as below

UJT Symbol

Construction of uni-junction transistor

A less doped n-type material and  heavily doped p-type material is used in UJT. The terminal which connects to the p type region is called the emitter terminal (E) and the two terminals which are connected at opposite ends of the n type region are called base1 (B1) and base2 (B2).

Working of UJT 

When the voltage between the emitter and base1 is zero, the UJT does not operate and the n-type rod acts as a resistor. But we see that a small leakage current flows due to reverse bias junction.

When we gradually increase the emitter voltage, the resistance between the emitter and base1 decreases. When the emitter voltage increases enough that the junction forward bias occurs, the emitter current starts flowing.

Uni Junction Transistor

This is because the holes of the more doped p-type region enter the n-type region and are joined with electrons of the n-type region. Thus the movement of electron start in the uni junction transistor.

 The ammeter current increase will decrease the confluence resistance and will also decrease the ammeter voltage, so the transistor will exhibit negative resistance characteristic.

Negative resistance characteristic is the region where the voltage starts decreasing and current starts increases.

UJT as relaxtion osillator

Unijunction Transistor is three terminal silicon diode its has only P-N junction.It is not used for amplification.It has ability to control a large power with small signal.So let us discuss about UJT as relaxsation oscillator.

Diagram of UJT as relaxation oscillator is given as below it consist of a UJT a capacitor C which charged through R as Vbb is switched ON.

When capacitor voltage Vc increase up to Vp,the UJT fires and rapidly discharge C through B1 terminal.After cut off capacitor start charging again.This cycle is repeated continuously generates a sawtooth waveform across capacitor C.



If we place resistor at B1 and B2 then the waveform will changes to spikes.

Wave form of UJT as relaxtion osillator