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 VBB is applied between two bases then a voltage gradient is established along n-type bar. This sets up a voltage V1 = VBB between emitter junction and B1 which is more than half the value of VBB 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(=ηVBB). If input voltage applied to the emitter is increased, a stage reaches when it exceedsV1 by VD i.e. VE= V1+ VD. 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. 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 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.