Steps of working of Unijunction Transistor

(i)  When emitter circuit is closed through switch S but emitter voltage is reduced to zero, and a voltage V_BB is applied between two bases then a voltage gradient is established along  n-type  bar.   This  sets  up  a  voltage  V1  =  V_BB between  emitter junction and B₁ which is more than half the value of V_BB as the value of intrinsic stand-off ratio  (η)  lies  between  0.51  to  0.8.   This  voltage  establishes  a  reverse  bias  on  pn junction and emitter current is cut-off.

(ii) When a +ve voltage is applied at emitter, pn junction  will  remain reverse biased so long as the input emitter voltage is less than V₁(=ηV_BB). If input voltage applied to the emitter is increased, a stage reaches when it exceedsV₁ by V_D i.e. V_E= V₁+ V_D. This will bring pn junction in forward biased condition. At this stage, p-type material injects holes into n-type bar. These holes are repelled by positive B₂ terminal and are attracted towards negative B₁ terminal of the bar. This accumulation of holes  in the emitter to  B₁ region decreases resistance  of this section  (i.e.  R_B1)  of  the  bar.   This  results  internal  voltage  drop  from  emitter  to  B1 and consequently  increases  the  emitter  current  I_E.   This  process  goes  on  and  ultimately a condition of saturation is reached. This is a stage where emitter current is limited by only emitter power. The device is now said to be in ON state.

(iii) Device  can  be  brought  to  the  OFF  state,  if  a  negative  pulse  is  applied  to  the emitter. Under this condition, pn junction is reverse biased and emitter current is cut-off.