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Poly Notes Hub shares a note on Unijunction Transistor (UJT) and their Construction, Working Principles, Characteristics, and Advantages.

Author Name: Arun Paul.

Table of Contents

What is Unijunction Transistor?

It is a three-terminal semiconductor switching device. This device has unique characteristics in that when it is triggered, the emitter current increases regeneratively until it is limited by emitter power supply. Due, to this characteristics, the unijunction transistor can be employed in a variety of applications e.g. switching, pulse generator, saw-tooth generator etc.

Construction of UJT

Below is the basic structure of this transistor. It consists of an n-type silicon bar with an electrical connection on each end. The leads to these connections are called base leads ( Base One B1 and Base Two B2 ). Partway along the bar between the two bases, nearer to B2 than B1, a PN junction is forward between a p-type emitter and the bar. The lead to this junction is called the Emitter Lead (E). Note that the emitter on this transistor is closer to B2 than B1.

Also below shows the symbol of UJT.

construction of ujt symbol of ujt or unijunction transistor

The following points are worth nothing:

Since the device has one PN junction and three leads, for this reason it is called so ( Uni means One ).
With only one PN Junction, the device is a form of diode. Because the two base terminals are taken from one section of the diode, this device is also called a double-based diode.
The emitter is heavily doped having many holes. The n region, however, is highly doped. For this reason, the resistance between the base terminal is very high ( 5 to 10 kilo ohm ) when the emitter lead is open.

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Operation of UJT

Below shows the basic circuit operation of a Unijunction transistor. The device has normally B2 positive with respect to B1.

The Operation of UJT is as follows:

If voltage V-BB is applied between B2 and B1 with emitter open (See Fig i), a voltage gradient is established along the n-type bar. Since the emitter is located nearer to B2, more than half of V-BB appears between the emitter and B1. The voltage V1 between emitter and B1 establishes a reverse bias on the PN Junction and the emitter current is cut off. Of course a small leakage current flows from B2 to emitter due to minority carriers.

If a positive voltage is applied at the emitter (See Fig ii), the junction will remain reverse biased so long as the input voltage is less than V1. If the input voltage to the emitter exceeds V1, the PN Junction becomes forward biased. Under these conditions, holes are injected from p-type material into the n-type bar . These holes are repelled by positive B2 terminal and they are attracted towards B1 terminal of the bar. This accumulation of holes in the emitter B1 region results in the B1 is decreased and hence the emitter current increase. As more holes are injected, a condition of saturation will eventually be reached. At this point, the emitter current is limited by emitter power supply only. The device is now in the ON state.
If a negative pulse is applied to the emitter, the PN Junction is reverse biased and the emitter current is cut off. The device is then said to be in the OFF state.

Characteristics of UJT

Below is the curve between the emitter voltage and emitter current at a given voltage between the bases. This is known as the emitter characteristics of this transistor.

Initially, in the cut-off region, as emitter voltage increases from zero, slight leakage current flows from terminal B2 to the emitter. This current is due to minority carriers in the reverse biased diode.
Above the certain value of Emitter Voltage, the emitter current begins to flow, increasing until the peak voltage and current reach at the point P.
After the peak point, an attempt to increase emitter voltage is followed by a sudden increase in emitter current with a corresponding decrease in emitter voltage. This is a “negative resistance portion” of the curve.

The negative portion of the curve lasts until the valley point is reached with valley point.

Advantages of UJT

It is a low-cost device.
It has excellent characteristics.

It is a low-power absorbing device under normal operation conditions.

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