A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch. The UJT is not used as a linear amplifier. It is used in free-running oscillators, synchronized or triggered oscillators, and pulse generation circuits at low to moderate frequencies (hundreds of kilohertz). It is widely used in the triggering circuits for silicon controlled rectifiers. The low cost per unit, combined with its unique characteristic, have warranted its use in a wide variety of applications like oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies.[1] The original unijunction transistor types are now considered obsolete, but a later multi-layer device, the programmable unijunction transistor (PUT), is still widely available.

1. Unijunction Transistor (UJT)
• UJT is a breakover-type switching device
• Useful in industrial circuits: timers,
oscillators, waveform generators, gate
control circuits for SCRs and triacs

2. • Is a special transistor that has two bases and
one emitter
• Has two states: completely on or completely
off
• Part of thyristor family which include SCR,
triac, and diac
P
Unijunction Transistor (UJT)
Lightly-doped
Heavily-doped

3. • UJT consists of a bar of N-type silicon
material (lightly-doped) and a small amount
of diffused P-type material (heavily-doped)
• An emitter terminal E is connected to the P
material to form the PN junction
• Two paths for current flow: B2 to B1; E to
B1
• Normally current does not flow in either
path until Emitter voltage is about 10 volts
higher than B1 voltage
Unijunction Transistor (UJT)

4. • When a voltage, called standoff voltage VP,
applied to the emitter is about 10 volts
higher than the voltage applied to B1 , UJT
turns on and current flows through the B2-
B1 path and from the emitter-B1 path
UJT Operation

5. • Current will continue to flow through the
UJT until the voltage applied to the emitter
drops to a point that is about 3 volts higher
than the voltage applied to B1
Common UJT circuit Pulse produced by capacitor discharge
UJT Operation

6. • When emitter voltage drops to this point,
the UJT will turn off and will remain off
until the voltage applied to the emitter again
reaches a level about 10 volts higher than
the voltage applied to B1
UJT Operation

7. • Since the N-type silicon bar is lightly-
doped, it has a high resistance and can be
represented by two resistors:
UJT CHARACTERISTICS
B2
B1
E
RB2
RB1

8. • Resistor RB1is shown as a variable resistor
• Its ohmic value depends on the amount of
emitter current flowing
• The internal resistance ratio of RB1 to RB2 is
called the intrinsic standoff ratio, η (eta)
• η =
UJT CHARACTERISTICS
RB1
RB1 + RB2

9. • The amount of emitter voltage required to
switch the UJT on is called the standoff
voltage, VP
• It is found be multiplying the applied
voltage VBB, by η
• VP = ηVBB + VF
• VF is the P-N junction forward-voltage drop
UJT CHARACTERISTICS

10. • When VP is reached, the UJT latches in the
conducting state and current flows between
E and B1
• The UJT is a latching device that will
remain in the conducting state as long as the
emitter current remains above a minimum
value called the holding current
• RB1 varies as a function of the emitter current
• As emitter current increases, RB1 decreases
UJT CHARACTERISTICS

11. • VP is the peak voltage, VV is the valley
voltage
UJT CHARACTERISTICS
UJT CHARCTERISTIC CURVE

12. • The UJT appears to the ohmmeter as a
connection of two resistors connected to a
diode as above
• Positive lead of ohmmeter is connected to
emitter, a diode junction from emitter to B1
or B2 is seen
• Reversing leads we have reverse bias, no
continuity
Testing the UJT

13. Some Packages and Terminal Identification