Lecture - 34: Uni-junction transistor

Uni-junction transistor

The UJT as the name implies, is characterized by a single pn junction. It exhibits negative resistance characteristic that makes it useful in oscillator circuits.

The symbol for UJT is shown in fig. 1. The UJT is having three terminals base1 (B1), base2 (B2) and emitter (E). The UJT is made up of an N-type silicon bar which acts as the base as shown in fig. 2. It is very lightly doped. A P-type impurity is introduced into the base, producing a single PN junction called emitter. The PN junction exhibits the properties of a conventional diode.




Fig. 1

Fig .2

A complementary UJT is formed by a P-type base and N-type emitter. Except for the polarity of voltage and current the characteristic is similar to those of a conventional UJT.

A simplified equivalent circuit for the UJT is shown in fig. 3. VBB is a source of biasing voltage connected between B2 and B1. When the emitter is open, the total resistance from B2 to B1 is simply the resistance of the silicon bar, this is known as the inter base resistance RBB. Since the N-channel is lightly doped, therefore RBB is relatively high, typically 5 to 10K ohm. RB2 is the resistance between B2 and point Ďa', while RB1 is the resistance from point Ďa' to B1, therefore the interbase resistance RBB is

RBB = RB1 + RB2

Fig. 3

The diode accounts for the rectifying properties of the PN junction. VD is the diode's threshold voltage. With the emitter open, IE = 0, and I1 = I 2 . The interbase current is given by

I1 = I2 = VBB / R BB .

Part of VBB is dropped across RB2 while the rest of voltage is dropped across RB1. The voltage across RB1 is

Va = VBB * (RB1 ) / (RB1 + RB2 )

The ratio RB1 / (RB1 + RB2 ) is called intrinsic standoff ratio

h = RB1 / (RB1 + RB2 ) i.e. Va = h VBB .

The ratio h is a property of UJT and it is always less than one and usually between 0.4 and 0.85. As long as IB = 0, the circuit of behaves as a voltage divider.

Assume now that vE is gradually increased from zero using an emitter supply VEE . The diode remains reverse biased till vE voltage is less than h VBB and no emitter current flows except leakage current. The emitter diode will be reversed biased.

When vE = VD + h VBB, then appreciable emitter current begins to flow where VD is the diode's threshold voltage. The value of vE that causes, the diode to start conducting is called the peak point voltage and the current is called peak point current IP.

VP = VD + h VBB.

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