What is UJT v-i characteristics, working,theory, symbol.

What is UJT (uni junction transistor)?

Hello friends!  Today I will read you in this post about what is UJT ? v-i characteristics of UJT, ujt working, UJT triggering circuit, ujt relaxation oscillator, ujt characteristics theory, ujt symbol. Its structure, and procedure.  So let's start

what is UJT?

Full form of UJT is uni - junction transistor.  It is a three-terminal semiconductor device.  It has only one pn junction so it is called unijunction transistor.

construction of unijunction transistor (structure) UJT uses a low doped n type silicon rod.  
What is UJT (uni junction transistor)?

The more doped p-type material is diffused on one side of this rod.  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 UJT :-
When the voltage between emitter and base1 is zero then UJT does not conduct any operation and 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.  

This is because holes in 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 the uni junction transistor begins.

Symbol of UJT:-
Symbol of ujt.

v-i characteristics UJT:-
v-i characteristics of ujt

UJT characteristics theory:-

UJT Characteristics : The UJT characteristic is as shown in diagram.This is emitter voltage ( on Y - axis ) versus emitter current ( on X - axis ) characteristics .

 For the emitter potentials less than V. ( peak point potential ) the UJT is in the OFF state and the magnitude of le is not greater than IEo. 

The current lEo is very similar to the reverse leakage current ICo of a bipolar transistor . This region is known as the cut off region .

As the emitter potential increases and reaches Vp = ( n  VBB + VD ) the UJT starts conducting . Then with increase in emitter current IE the emitter voltage decreases as shown in Diagram. 

The reduction in voltage across UJT is due to the reduction in resistance RB1 with increase in the value of lE.

This region of operation is known as a " Negative Resistance " region . It is a stable region and can be used in various applications . 
Eventually the " Valley Point " will be reached which marks the end of negative resistance region . If we increase lE further , the UJT will enter into the saturation region . 

The saturation region is a positive resistance region as Ve increases with increase in lE .

UJT triggering circuit:-
Ujt triggering circuit.

UJT relaxation Ocilator:-
Ujt relaxation Ocilator.

applications of UJT (Application):- 

Its applications are as follows: 

  • It is used as a triggering device in SCR and TRAIC.  
  • UJT is used to generate non sinusoidal waves in relaxation oscillator.  
  • It is used to control the speed of AC and DC motor.  
  • UJT CT 3941 saw occurs in tooth wave generator and phase control circuits.  


Request: - If this article has been helpful for you, then definitely share this with your friends and if you have any other questions related to your electronics, then you can comment by commenting.  thanks.

Satish jagtap

Hello iam Satish jagtap Electrical engineer. From: Aurangabad, Maharashtra. Edu: MIT college of Engineering.

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