The unijunction transistor (UJT) is a three-terminal semiconductor device that has two operating states: on and off. It consists of three layers of semiconductor material. Current flows through two paths - from the emitter to base 1, and from base 2 to base 1. The UJT turns on when the emitter voltage is about 10V higher than the base 1 voltage, and turns off when the emitter voltage drops to about 3V higher. UJTs are used in relaxation oscillators where a capacitor charges until the UJT turns on and discharges the capacitor, then the UJT turns off and the process repeats.

1. Chapter 2
Unijunction Transistor

2. Basic Operation
(a) symbol (c) Equivalent circuit
(b) construction
 UJT is an electronic semiconductor device that only has only one junction
 UJT is a digital device because its only have two state that is on and off
 Made by combining three layers of semiconductor material as shown in
figure (b) above.
 Current flows in two path through the UJT. One is through Base 2 to Base
1 and the other path is through Emitter to the Base 1.
 In normal state, current does not flow through either path until the voltage
applied to the emitter is about 10V higher than the voltage applied to the Base
1.
when it turn ON, the current will continue flow until the voltage applied to the
emitter drops to a point that is about 3V higher than voltage applied to Base 1.
UJT will off in this condition and will turn ON again when voltage at emitter
higher 10V that the voltage applied at the Base 1. EAD 3043 (Industrial Electronic)
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3. Characteristic of UJT
The static emitter characteristic (a curve showing the relation between emitter voltage
VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure.
From figure it is noted that for emitter potentials to the left of peak point, emitter current
IE never exceeds IEo . The current IEo corresponds very closely to the reverse leakage
current ICo of the conventional BJT. This region, as shown in the figure, is called the cut-
off region. Once con-duction is established at VE = VP the emitter po-tential VE starts
decreasing with the increase in emitter current IE. This Corresponds exactly with the
decrease in resistance RB for increasing cur-rent IE. This device, therefore, has a
negative resistance region which is stable enough to be used with a great deal of
reliability in the areas of applications listed earlier. Eventually, the valley point reaches,
and any further increase in emitter current IE places the device in the saturation region,
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as shown in the figure. Nor Aida Idayu Binti Abdullah. 2012

4. Characteristic (cont)
Three other important parameters for the UJT are IP, VV and IV and are
defined below:
Peak-Point Emitter Current. Ip. It is the emitter current at the peak
point. It repre-sents the rnimrnum current that is required to trigger the
device (UJT). It is inversely proportional to the interbase voltage VBB.
Valley Point Voltage VV The valley point voltage is the emitter voltage
at the valley point. The valley voltage increases with the increase in
interbase voltage VBB.
Valley Point Current IV The valley point current is the emitter current at
the valley point. It increases with the increase in inter-base voltage VBB.
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5. Operation of UJT
importan
t
The amount of capacitance that
can be connected to the UJT is
limited
Most UJT should not connected
to the capacitor larger than 10µF
because the UJT may not be able
to handle the current spike
produced by the larger capacitor,
and UJT could become damaged
 The variable resistor is control the capacitor’s rate of charge time.
 When the capacitor is been charged to about 10V, the UJT turns ON
and discharges the capacitor through Emitter and Base 1.
 When the capacitor is been discharges to about 3V, the UJT will turn off
and permits the capacitor to begin charging again.
 By varying the resistance connected in series with the capacitor, the
amount of time needed for charging the capacitor can be changed, thereby
controlling the pulse rate of the UJT (T =RC) EAD 3043 (Industrial Electronic)
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6. Advantages of UJT
It is a Low cost device
It has excellent characteristics
It is a low-power absorbing device under normal operating conditions
A stable triggering voltage (VP)— a fixed fraction of applied inter base
voltage VBB.
A very low value of triggering current.
A high pulse current capability.
A negative resistance characteristic.
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7. Model for UJT
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8. Model for Vp
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9. Operation
The relaxation oscillator in Figure below is an application of the
unijunction oscillator. RE charges CE until the peak point. The unijunction
emitter terminal has no effect on the capacitor until this point is reached.
Once the capacitor voltage, VE reaches the peak voltage point VP, the
lowered emitter-base 1 resistance quickly discharges the capacitor.
Once the capacitor discharges below the valley point VV, the E-RB1
resistance revert back to high resistance, and the capacitor is free to
charge again.
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10. Example of calculation of 2n2647
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11. Example
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12. Exercise
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13. Exercise
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14. Quiz
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15. Assignment
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