UJT(Unijunction Transistors)

A Unijunction Transistor (UJT) is a semiconductor device that consists of an n-type silicon base with a p-type emitter embedded in it. 

It has three terminals:

1. Emitter (E), 
2. Base-one (B₁), and 
3. Base-two (B₂). 

Between bases B₁ and B₂, the unijunction behaves like an ordinary resistance.

RB₁ and RB₂ are the internal resistances respectively from bases B₁ and B₂ eta point A.

The behavior of a UJT can be understood using its symbolic representation and equivalent circuit.

When a voltage V_BB is applied across the two base terminals (B₁ and B₂), the potential of point A with respect to B₁ is given by:

V_AB1 = [V_BB / (R_B1 + R_B2)] R_B1 = [R_B1/(R_B1 + R_B2)] V_BB =  ηV_BB

Here, η = [R_B1 / (R_B1 + R_B2)] is called the intrinsic stand-off ratio, where R_B1 and R_B2 are internal resistances from bases B₁ and B₂, respectively.

Typical values of η range from 0.51 to 0.82. The interbase resistance R_BB = R_B1 + R_B2 is usually in the range of 5-10 kΩ.

If a voltage V_e is applied between the emitter (E) and base (B₁), and E is positive with respect to B₁, the UJT operates as follows:

• When V_e < ηV_BB, the E-B₁ junction is reverse biased, and the emitter current I_e is negative.
• As V_e is increased, when it reaches ηV_BB + V_D at point B, the E-B₁ junction starts to conduct,I_e is positive and V_D is the forward voltage drop of the E-B₁ junction. Point B is called the peak point, represented by V_p (peak-point voltage) and I_p (peak-point current).
• As the emitter voltage V_e continues to increase beyond ηV_BB + V_D, the number of carriers injected into the lower base region increases, reducing the resistance R_B1 of the E-B₁ junction. This leads to a decrease in the potential of eta point A, causing an increase in current I_e and voltage V_e. The device exhibits a negative resistance region, shown as BC in the V-I characteristics.In this region, an increase in current I_e is accompanied by a decrease of emitter voltage V_e.
• At point C, the entire base region is saturated and resistance R_B1 does not decrease any more.A further increase in I_e is accompanied by a rise in voltage V_e.This curve is given by CQ. Point C is called the valley point; V_v and I_v represent the corresponding emitter potential and current.
• The negative resistance region between the peak and valley points provides the UJT with switching characteristics, making it useful in SCR triggering circuits.

Basic structure of UJT- Symbol and Equivalent circuit 

V-I characteristics of UJT

mcmurray-bedford-full-bridge