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Working of UJT


Working of UJT

1. If voltage is applied at VBBwith emitter open, a voltage gradient is established along the n-type bar. Since the emitter is located nearer to B2, more than half of VBBappears between the emitter and B1, the voltage V1between emitter and B1 establishes a reverse bias on the pn junction and the emitter current is cutoff.
2. If a positive voltage is applied at the emitter, the pn junction will remain reverse biased so long as the input voltage is less than V1. If the input voltage to the emitter exceeds V1, the pn junction becomes forward biased. Under these conditions, holes are injected from p-type material in to the n-type bar. These holes are repelled by positive B2 terminal and they are attracted towards B1terminal of the bar. This accumulation of holes in the emitter to B1region results in the decrease of resistance in this section of the bar. The result is that internal voltage drop from emitter to B1 is decreased and hence the emitter current IE increases. As more holes are injected a condition of saturation will eventually be reached. At this point, the emitter current is limited by emitter power supply only. The device is now in the ON state.

                                                                              EQUIVALENT CURCUIT OF UJT

3. If a negative voltage is applied to the emitter, the pn junction is reverse biased and the emitter current is cutoff. The device is then said to be in the OFF state.
Interbase Resistance: The resistance of silicon bar is called the Interbase resistance RBB
                 RBB = RB1+ RB2
The value of RBB lies between 4 KΩ to 10 KΩ
Intrinsic Stand Off Ratio: The ratio of V1 to VBB known as intrinsic stand off ratio.

Ƞ =  RB1/RB1 + RB2

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