Unijunction Transistor

A unijunction transistor is a three terminal, one pn junction switching device with a unique characteristic which when triggered, the emitter current increases regeneratively until its limited by emitter power supply.

A unijunction transistor consists of an n - type silicon bar with an electrical connection on each end. The leads to these connections are called base leads, B₁ and B₂.

A pn junction is formed between a p - type emitter and the bar which is n - type.

Applications of a Unijunction Transistor are:

1.   It can be used as a switching device.
2.   It can be used as a pulse generator.
3.   It can be used to produce saw tooth signals.

Note that a unijunction transistor is also called a double based diode because two base terminals are taken from one section of the diode. The emitter of a UJT is

heavily doped (having many holes) and the n - region is lightly doped which makes the resistance between the base terminal very high when the emitter lead is open.

Note that the resistance between the emitter and base B₁ is greater than the resistance between the emitter and base B₂ because the emitter is nearer to B₂ as compared to B₁. Since the emitter is located nearer to B₂ then half of V_BB appears between the emitter and B₁.

Operation of a Unijunction Transistor:

If a positive voltage is applied to the emitter the pn junction will remain reverse biased as long as the input voltage is less than V_P (≈ V₁). When the input voltage then becomes greater than V_P (≈ V₁), the pn junction now becomes forward biased and holes are injected from the p - type material to the n - type bar. These holes are repelled by positive B₂ terminal and are attracted towards B₁ terminal of the bar. The accumulation of holes in the emitter to B₁ results in the decrease of resistance in the section of the bar. This also results to the internal voltage drop from emitter to B₁ and hence emitter current is increased thereby exhibiting negative resistance. At the this negative resistance region, the UJT does not obey Ohm's law because voltage decreases while current increases.
As more holes are injected a condition of saturation (the UJT now begins to obey Ohm's law ===> as voltage increases, current increases) is eventually reached and the emitter current is now only limited by the emitter power supply. [THE DEVICE IS KNOWN TO BE IN ITS 'ON' STATE].
If a negative power is applied to the emitter, the pn junction is reversed biased and the emitter current is cut off. [THE DEVICE IS KNOWN TO BE IN ITS 'OFF' STATE].

Note that the resistance of the silicon bar is known as the interbase resistance, R_BB.

RB1 is variable because its value depends on the bias voltage.
With no voltage applied, R_BB = R_B1 + R_B2
Voltage across R_B1, V₁ = R_B1(V_BB) / (R_B1 + R_B2)
Intrinsic Stand Off Ratio, η = V₁ / V_BB = R_B1 / (R_B1 + R_B2)
Peak Point Voltage, V_P = ηV_BB + V_D

Note that in the cut off region as V_E increases from zero slight leakage current flows from terminals B₂ to the emitter and the current is due to minority carriers in the reverse biased diode.
The valley point voltage is that voltage at which saturation begin to occur in a unijunction transistor.
It is also worthy to note that the difference between the peak point voltage and the valley point voltage is a measure of the switching efficiency of unijunction transistor and can be seen to fall off as V_BB decreases.

There are three regions of a unijunction transistor (UJT):

1.   Cut Off Region
2.   Negative Resistance Region
3.   Saturation Region.

Advantages of a Unijunction Transistor (UJT):

1.   It is a low cost device
2.   It has excellent characteristics
3.   It is a low power absorbing device under normal operating conditions.