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In spite of having transistor added to its name, a Unijunction Transistor (UJT) working and characteristic curve varies completely from other counterparts. This post will give you a detailed idea about UJT, its basic construction, symbol, how it works, its characteristics curve, various applications, advantages and disadvantages.

What is Unijunction Transistor (UJT)

UJT stands for UniJunction Transistor. It is a three terminal semiconductor switching device. The Unijunction Transistor is a simple device that consists of a bar of n-type silicon material with a non-rectifying contact at either end (base 1 and base 2), and with a rectifying contact (emitter) alloyed into the bar part way along its length, to form the only junction within the device (hence the name ‘Unijunction’).

The Unijunction Transistor is also known as Double Base Diode.

Introduction to Unijunction Transistor (UJT)

Fig. 1 – Introduction to Unijunction Transistor (UJT)

The unique switching characteristics of UJT makes it different from conventional BJT’s and FET’s by acting as switching transistor instead of amplifying the signals. It exhibits negative resistance in its characteristics which employs it as relaxation oscillators in variety of applications.

Symbol and Construction of Unijunction Transistor (UJT)

In Unijunction Transistor, the PN Junction is formed by lightly doped N type silicon bar with heavily doped P type material on one side. The ohmic contact on either ends of the silicon bar is termed as Base 1 (B1) and Base 2 (B2) and P-type terminal is named as emitter.

Basic Construction and Symbol of Unijunctin Transistor (UJT)

Fig. 2 – Basic Construction & Symbol of Unijunction Transistor (UJT)

The emitter junction is placed such that it is more close to terminal Base 2 than Base 1. The symbols of both UJT and JFET resemble the same except the emitter arrowhead represents the direction in which conventional current flow, but they operate differently.

How does a Unijunction Transistor (UJT) works

The simplified equivalent circuit (at Figure 3 below) shows that N-type channel consists of two resistors RB2 and RB1 in series with an equivalent diode, D representing the PN junction. The emitter PN junction is fixed along the ohmic channel during its manufacturing process.

Simplified Equivalent Circuit of Unijunction Transistor (UJT)

Fig. 3 – Simplified Equivalent Circuit of Unijunction Transistor (UJT)

The variable resistance RB1 is provided between the terminals Emitter (E) and Base 1 (B1), the RB2 between the terminals Emitter (E) and Base 2 (B2). Since the PN junction is more close to B2, the value of RB2 will be less than the variable resistance RB1.

A voltage divider network is formed by the series resistances RB2 and RB1.  When a voltage is applied across the semiconductor device, the potential will be in proportion to the position of base points along the channel.

The Emitter (E) will act as input when employed in a circuit, as the terminal B1 will be grounded. The terminal B2 will be positive biased to B1, when a voltage (VBB) applied across the terminals B1 and B2. When the emitter input is zero, the voltage across resistance RB1 of the voltage divider circuit is calculated by

Calculation of RB1 in Voltage Divider Circuit

The important parameter of Unijunction Transistor is ‘intrinsic stand-off ratio’ (η), which is resistive ratio of RB1 to RBB. Most UJT’s have η value ranging from 0.5 to 0.8. The PN junction is reverse biased; when small amount of voltage which is less than voltage developed across resistance RB1 (ηVBB) is applied across the terminal emitter (E).

Thus a very high impedance is developed prompting device to move into non-conducting state i.e., it will be switched off and no current flows through it. The UJT begins to conduct when the PN junction is forward biased.

The forward biased is achieved when voltage applied across emitter terminal is increased and becomes more than VRB1. This results in larger flow of emitter current from emitter region to base region. Increase in emitter current reduces the resistance between emitter and Base 1, resulting in negative resistance at emitter terminal.

The Unijunction Transistor (UJT) will act as voltage breakdown device, when the input applied between emitter and base 1 reduces below breakdown value i.e., RB1 increases to a higher value. This shows that RB1 depends on the emitter current and it is variable.

Characteristics Curve of Unijunction Transistor (UJT)

The characteristics of Unijunction Transistor (UJT) can be explained by three parameters:

  • Cutoff
  • Negative Resistance Region
  • Saturation

Characteristics Curve of Unijunction Transistor (UJT)

 Fig. 4 – Characteristics of Unijunction Transistor (UJT)


Cutoff region is the area where the Unijunction Transistor (UJT) doesn’t get sufficient voltage to turn on. The applied voltage hasn’t reached the triggering voltage, thus making transistor to be in off state.

Negative Resistance Region

When the transistor reaches the triggering voltage, VTRIG, Unijunction Transistor (UJT) will turn on. After a certain time, if the applied voltage increases to the emitter lead, it will reach out at VPEAK. The voltage drops from VPEAK to Valley Point even though the current increases (negative resistance).


Saturation region is the area where the current and voltage raises, if the applied voltage to emitter terminal increases.

Applications of Unijunction Transistor (UJT)

The Unijunction Transistor can be employed in variety of applications such as:

  • Switching Device
  • Triggering Device for Triacs and SCR’s
  • Timing Circuits
  • For phase control
  • In sawtooth generators
  • In simple  relaxation oscillators

Application of Unijunction Transistor (UJT) in Relaxation Oscillator

UJT Relaxation Oscillator can be practically viewed by the following circuit.

Unijunction Transistor (UJT) Relaxation Oscillator

 Fig. 5 – Use of Unijunction Transistor (UJT) in Relaxation Oscillator

The resistance R3 charges the capacitor C1 until the peak point. The UJT’s emitter terminal has no effect on C1 until peak point is reached. When the emitter voltage reaches peak voltage point, the lowered emitter base 1 resistance rapidly discharges the capacitor.

As the capacitor C1 discharges beneath the valley point, the emitter base 1 resistance will return back to high resistance, thus making capacitor free to charge again.

Application of Unijunction Transistor (UJT) in Speed Control Circuit

Speed Control Circuit is one of the typical applications of UJT to produce set of pulses to trigger and control Thyristor. We can adjust the speed of universal motors by using UJT as triggering circuit in combination with SCR and Triacs.

Advantages of Unijunction Transistor (UJT)

The advantages of Unijunction Transistor include:

  • low cost
  • negative resistance characteristics
  • Requires low value of triggering current.
  • A stable triggering voltage
  • Low power absorbing device

Disadvantage of Unijunction Transistor (UJT)

The main disadvantage of Unijunction Transistor is its inability to provide appropriate amplification.

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