UJT(Uni-junction transistor): 

A Uni-junction transistor (UJT) is a three terminal semiconductor device having two doped regions.  In the three terminals, it has emitter (E) and two bases (B1and B2).  This device has only one pn junction, therefore, naming it a ‘transistor’ is really misnomer.  However, from the outer look (packages), it resembles to a bipolar transistor and hence the name uni-juction transistor.  This device has a unique characteristic that when it is triggered, the emitter current increases regeneratively until it is limited by the emitter power supply.

Construction: 

    The Structure and symbol of UJT are shown in fig.(a). UJT consists of an n-type silicon bar which is lightly doped. Two ends connections are taken from the bar called base-one (B1) and base-two (B2).  A heavily doped p-type region is diffused to Ntype bar nearer to B2.



Working: 

    Make the circuit arrangement as shown in fig:(b).  Normally, B2 is kept positive w.r.t B1.  A variable positive voltage is supplied to the emitter through a switch S. 

(i) When the emitter circuit is closed through a switch S but the emitter voltage is reduced to zero. This voltage establishes a reverse bias on the pn junction and the emitter current is cut-off. 

(ii) When a +ve voltage is applied at the emitter, the p-n junction will remain reverse biased so long as the input emitter voltage is less than V1 (=η VBB).  If the input voltage applied to the emitter is increased, a stage reaches when it exceeds V1 by VD i.e.VE = V+VD.  This will bring the p-n junction in forward biased condition.  At this stage, p-type material injects holes into the 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 and the resistance of this section (i.e. RB1) of the bar.  This results internal voltage drop from emitter to B1 and consequently increases the emitter current IE.  This process goes on and eventually a condition of saturation is reached.  This is a stage where the emitter current is limited by only emitter power.  The device is now said to be in the ON state.



Applications of UJT : 

(i)Relaxation Oscillator  

(ii)Pulse generators

(iii)Phase control  

(iv)Timing circuits. 

(v)Saw-tooth generators 

(vi)Triggering circuits