A UJT is a 3 terminal device popular for its negative
resistance property and its application
as a relaxation oscillator.
UJT
Construction
A silicon base is taken. It s doped n-type and p-type
creating a pn-junction as shown below.
n-type is lightly doped whereas p-type is heavily
doped – a very important point to keep in mind.
Two ohmic contacts are provided which are called base,
B1 and B2 . Third terminal is called emitter E.
With E open, UJT behaves like an ordinary resistor
between B1 and B2 and its resistance is uniformly
distributed between B1 and B2 . Lets divide this
resistance to RB2 and RB1 .
As n-type region is lightly doped therefore,
resistance between B1 and B2 i.e RBB = RB1 + RB2
is very high. This resistance can be found out easily by connecting a
multimeter between B1 and B2 keeping emitter open.
You may have noted
that , in UJT construction diagram, emitter is closer to B2 making RB2
less than RB1 .
By above discussion UJT equivalent diagram can be
drawn as shown below.
UJT
Symbol
Working
of UJT
UJT is normally operated with both B2 and E
terminals positively biased with respect to B1 (reference) as shown
below.
The
complete operation is based on the variation of Ve which
is done by varying RE . VBB is kept constant.
Case – 1 :- VEE = 0V
The circuit would look like
The diode is reverse biased therefore a very small
leakage current would flow through it i.e Ie negative and it can be
neglected.
By voltage division rule,
Case -2 :- VEE > 0 V
Lets increase VEE from 0V.
As VEE is increased the voltage of terminal E
increases while voltage of point A is remains almost constant hence diode
changes state from reverse biased condition to forward bias condition.
During this diode state transition, reverse leakage current
decreases, becomes zero and then increases in other direction.
At the point of transistion where Ie becomes
absolutely zero, VEE = VA
= η VBB
Case-3 :-
Let VEE
is increased further , then , the E-B1 pn-junction gets forward biased
and forward current flows through diode.
By varying RE , Ve is increased
till Ve reaches Vp . At this peak point , VE = VD + ηVBB , the diode is forward biased and begins to
conduct. VD is the cut-in voltage of the diode.
The p-emitter begins to inject holes from heavily
doped emitter to base region B1 . As the n-type is lightly doped,
the injected holes rarely get chance to recombine and therefore base region B1
is filled with large number of holes. As a result, the resistance RB1
decreases. The decrease in RB1
reduces voltage drop VA therefore Ve decreases as Ve = VA + VD
.
This increased Ie injects more holes to B1
thereby further reducing RB1 and so on. This regeneration process
continues till valley point is reached. When Ie reaches Iv
, UJT turns ON. Valley point current is also called holding current of UJT.
When Ie > IV
, UJT is ON.
When Ie < IV
, UJT is OFF.
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