UJT Symbol, Construction, Working and Characteristics

What is Unijunction Transistor(UJT)?

Unijunction Transistor
Unijunction Transistor

Unijunction Transistor (UJT) is a three terminal semiconductor device. UJT is formed by a single junction of P type and N type semiconductor material. It is used for switching applications and it cannot be used to amplify signals.

Symbol of Unijunction Transistor:

Symbol of UJT
Symbol of UJT

The symbol of UJT looks similar to the symbol of JFET except the bent arrow. The three terminals of UJT are emitter, Base1 and Base2.

Construction of Unijunction Transistor:

Construction of UJT
Construction of UJT

The channel is formed of N type which is lightly doped and P type material is infused on it and the doping concentration of P type is very high. Thus it forms single PN junction and this is the reason for the name Unijunction. The terminals Base2 (B2) ,Base1 (B1) are taken from the N channel through the Ohmic contacts and Emitter taken from the heavily doped P type material. The Emitter terminal is closer to the Base2 terminal than the Base1 terminal.

The complementary type UJT is made up of P type lightly doped channel and heavily doped N type material infused on it. The terminals Base2 (B2), Base1 (B1) are taken from P channel and Emitter terminal from N type material.

Working of Unijunction Transistor:

Working of UJT
Working of UJT

When no voltage is applied at the emitter, the resistance in the channel is high and the device is turned OFF, till the applied voltage is higher than the triggering voltage.

When the PN Junction is forward biased, positive voltage is applied at the emitter terminal and Base 2 is made positive with the Base1. The majority carrier which is the holes in the P type enters the N channel and since Base2 is positive it gets repelled and attracted towards Base1 terminal. So the resistance decreases. The Emitter current increases and reaches the peak and starts decreasing. After it reaches the valley point again it starts increasing. 

When the PN Junction is reverse biased, the emitter current doesn’t flow and it is cut-off. 

Characteristics of Unijunction Transistor:

Characteristics of UJT
Characteristics of UJT


Cut off Region: 

In this region the applied input voltage is not sufficient to turn the device ON and the applied voltage didn’t reach the triggering voltage.

Negative Resistance Region:

After the applied voltage reach the triggering voltage the device is turned ON and the emitter voltage reaches the peak voltage and it drops to valley point, even though the emitter current increases.

Saturation Region:

In this region when the applied voltage increases, the emitter voltage and the emitter current increase gradually.


  • Low cost and low power absorbing device
  • UJT is good at blocking the reverse current
  • Stable triggering voltage
  • It provides very good switching response


  • It cannot be used for high frequency.
  • Because of its negative impedance property, it cannot be used in all circuits.
  • It does not provide amplification.

Application of Unijunction Transistor:

  • Used in timing circuits
  • Used as switching device
  • Used as oscillators
  • Used for phase control
  • Used in sawtooth generators

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