Working of UJT
1. If voltage is applied at VBB
with emitter open, a voltage gradient is established along the n-type bar. Since
the emitter is located nearer to B2, more than half of VBB
appears between the emitter and B1, the voltage V1
between 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 B1
terminal of the bar. This accumulation of holes in the emitter to B1
region 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