Notes

1. Unijunction Transistor (UJT)
Miss Sushama V. Nikam
Assist. Professor
Y. C. Institute of Science Satara

2. It is three terminal semiconductor device having
only one PN junction. Hence it is known as
Unijunction Transistor. It can be used more
efficiently in switching application as compare to
BJT.

3. Structure of UJT
B2
B1
E
Schematic Symbol
Basic Structure

4. UJT consist of lightly doped N type semiconductor bar.
On both sides of semiconductor bar, there are two
terminals Base 1(B1) and Base 2 (B2)
A PN junction is formed very close to Base B2 as shown
in fig. The terminal attached to heavily doped P type
region is known as Emitter. Thus UJT has three terminals
B1, B2 and Emitter.

5. UJT Equivalent Circuit
 RB1 = dynamic resistance of the silicon bar between
the emitter and base 1. It varies inversely with
emitter current IE , and therefore it is shown as a
variable resistor.
 RB2 = dynamic resistance between the emitter and
base 2
 RBB = The total resistance between base B1 and B2 is
called as Interbase resistance (RBB).
RBB = RB1 + RB2
B2
B1
E
R B 1
R B 2
The PN junction is formed in such a way that
RB1/RB2 = 2/3

6. Equivalent Circuit
Intrinsic standoff Ratio
The ratio of resistance between emitter and Base 1 to the total
resistance of UJT is defined Intrinsic standoff Ratio. It is
denoted by η.
η = RB1/RBB
= RB1 /(RB1 + RB2)
Normally the value of η is between 0.5 to 0.85

7. Working of UJT
R B 2
R B 1
V BB
0EIBBV
+ V D -I E
+
V E
-
B 2
B 1
+
-

8. When an external voltage VBB is applied between Base B1 and Base B2, it
develops uniform voltage drop. The voltage between Emitter and Base B1
is known as Peak Point Voltage VP and it is given as,
VP = η. VBB
When emitter voltage VE is less than Peak Point Voltage VP, the PN
junction is in reverse bias mode and UJT is off. When emitter voltage VE
exceeds Peak Point Voltage by 0.7 V, the PN junction becomes forward
bias and UJT is turn ON

9. When UJT turn ON it offers low resistance and it’s
Base resistance RB1 decreases with η. Now, UJT is said to be
in Negative resistance.
UJT is also known as Negative resistance device.

10. IV characteristics of UJT

11. Above fig shows the circuit arrangement for plotting IV characteristics of UJT. The DC
voltage is applied between Base 1 and Base 2. The emitter current IE is measured by
varying emitter voltage VE. Following graph shows the IV characteristics of UJT.

12. There are two important points on the curve 1) Peak Point 2) Valley Point
These two points divides the curve in three regions as explained bellow
Cutoff Region: In this region emitter voltage is below peak point voltage VP. Therefore
UJT is in OFF position.
Negative Resistance Region: The region between peak point and valley point is called
negative resistance region. In this region emitter voltage decreases from peak point to
valley point. In this region UJT work as an Oscillator .
Saturation Region: The region beyond the valley point is called as saturation region. In
this region, the device is in it’s ON position. The emitter voltage VE remains constant with
the increasing emitter current.

13. Applications
• Trigger device for SCR’s and triac
• Nonsinusoidal (relaxation) oscillators,
• Sawtooth generators
• Phase control
• Timing circuits