This document provides information about unijunction transistors (UJTs) and programmable unijunction transistors (PUTs). It discusses the construction, working principles, characteristics, and applications of both UJTs and PUTs. Some key points include: - UJTs and PUTs are three-terminal semiconductor devices that act as electrically controlled switches. UJTs have one PN junction while PUTs have a four-layer PNPN structure. - The characteristic curve of UJTs and PUTs shows different regions including cutoff, negative resistance, and saturation based on the applied voltage. PUTs allow the peak voltage to be programmed using external resistors. - Common applications of UJ

1. By Ayesha Anum & Marwa Batool
Unijunction&
Programmable
Unijunctiontransistors
Presentation on

2. Unijunction Transistors
“A three-lead semiconductor device with
only one junction that acts like an
electrically controlled switch.”

3. - Constructed from separate P and N-type semiconductor
materials forming a single PN-junction
- Differ from BJTs and FETs despite the name ‘transistor’
- Unidirectional conductivity and negative impedance
characteristics.
- Like N-channel FET’s, the UJT consists of a single solid piece
of N-type semiconductor material forming the main current
carrying channel
- Two outer connections: Base 1 and 2
- Third connection: Emitter ( E ) is located along the channel.

4. Types of
UJTs

5. Original UJT
“ A bar of n-type semiconductor
material into which p-type
material has been diffused
somewhere along its length”
Ex: 2N2646 model
Complementary UJT
“A bar of p-type semiconductor
material into which n-type material
has been diffused somewhere
along its length”
Ex: 2N6114 model
Programmable UJT
“A multi-junction device with two
external resistors with similar
characteristics to the UJT”
Ex: 2N6027, 2N6028 and BRY39
models

6. Symbol of UJT
- Three terminals of unijunction transistor
are emitter, Base 1 and Base 2.
- Arrow is bent and shown that it is in the
direction which is towards the channel.

7. Construction:
- The construction resembles diode but it
consists of three terminals instead of two.
- The higher resistance value is present at the
bar that is of n-type.
- Maximum resistance is formed in between the
terminals of the base 1 and the emitter.
- Reason : Positioning of the emitter is closer to
the base 2 rather than base 1.

8. Working principle of UJT
- Basic functionality depends on the voltage applied.
- If voltage applied between the emitter and the base 1 is zero,
then UJT doesn’t conduct.
- Thus N-Type material tends to acts as a resistor.
- As the applied voltage increases at the emitter, the resistance
tends to increase and the device begin to conduct.
- In the whole process the conduction is completely dependent
on the majority of the charge carriers.

9. - Two resistors RB2 and RB1 in series with a diode, D representing the p-n
junction connected to their center point.
- Emitter p-n junction is fixed in position along the ohmic channel.
- Resistance RB1 is given between the Emitter, E and terminal B1, while
resistance RB2 is given between the Emitter, E and terminal B2.

10. - A voltage divider network is formed by the series resistances RB2
and RB1.
- Emitter (E) will act as input when employed in a circuit
- Terminal B1 will be grounded.
- The terminal B2 will be positive biased to B1, when a voltage
(VBB) applied across the terminals B1 and B2.

11. Characteristic Curve:

12. We have the 3 regions in the characteristic curve of UJTs as given below:
1. Cutoff region:
“Area where the UJT doesn’t get enough voltage to turn on.”
The applied voltage hasn’t reached the triggering voltage, thus making transistor
to be in off state.
2. Negative Resistance Region:
- When the transistor reaches the triggering voltage, UJT will turn on.
- If the applied voltage increases to the emitter lead, it will reach VPEAK.
- The voltage drops from VPEAK to Valley Point even though the current
increases.
3. Saturation Region:
“Area where the current and voltage raises with increase in applied voltage”.

13. 4. Peak-Point Emitter Current: (Ip)
“It is the emitter current at the peak point.”
- It represents the minimum current that is required to trigger UJT
- It is inversely proportional to interbase voltage VBB.
5. Valley Point Voltage: (VV)
“Emitter voltage at valley point.”
- It increases with increase in the interbase voltage VBB.
6. Valley point current: (Iv)
“Emitter current at the valley point.”
- It increases with the increase in inter-base voltage VBB.

14. Pros & cons
of UJTs

15. Advantages
1. Very low-cost device but wide variety of uses.
2. Good electrical and temperature characteristics.
3. A very good switching response.
4. Provides unidirectional conductivity so good
for reverse current blocking circuits.
5. Low-power absorbing device under normal
operating conditions.
6. Negative resistance characteristic
7. High pulse current capability.
8. Low value of triggering current.
Disadvantages
1. Cannot be used in linear amplifiers due to
inability to produce decent amplification.
2. Can work with only low to moderate
frequency so it is not suitable for very high
frequency.
3. Negative impedance property that's why it is
not suitable to use in some circuits.

16. Applications of UJT
• Switching Device
• Triggering Device for Triacs and SCR’s
• Timing Circuits
• For phase control
• Sawtooth generators
• Simple relaxation oscillators

17. Programmable
Unijunction Transistors

18. Programmable Unijunction
Transistors
“Type of three-terminal thyristor that is
triggered into conduction when the
voltage at the anode exceeds the voltage at
the gate.”

19. Programmable UJT:
- It is an SCR-type device used to simulate a
UJT.
- It is similar in its characteristics to a
unijunction transistor, except that its
behavior can be controlled using external
components.
- A PUT is a more advanced version of a UJT.

20. Construction of PUT:
- The construction is similar to that of SCR.
- Four-layer of semiconductor materials
PNPN or NPNP structure with three
injunction J1 J2 and J3, a terminal which is
connected to the P-type material called the
anode and the terminal connected to the N-
type is called the cathode.
- The gate terminal is connected to the N-
type material near the anode terminal.

21. Working:
- When we bias the PUT properly, the current cannot
flow
- Reason: the gate terminal is positive w.r.t cathode,
when the anode voltage is increased from the
cutoff, the PN junction is forward bias, and the
PUT turns ON.
- The PUT remains in an ON state until the anode
voltage decreases below the cutoff level and at that
time the PUT is turned off.
- The gate can be biased to the desired voltage with
an external voltage divider so that when the anode
voltage exceeds this “programmed” level, the PUT
turns on.

22. Characteristic Curve:

23. - Anode: Connected to a positive voltage, Cathode : Connected to the ground.
- Gate: Connected to the junction of the two external resistors R1 and R2 forming a voltage divider
network.
- It is the value of these two resistors that determine the intrinsic standoff ratio(η) and peak voltage
(Vp) of the PUT.
- When the anode to cathode voltage (Va)is increased the anode current will also get increased and
the junction behaves like a typical P-N junction.
- But Va cannot be increased beyond a particular point.
- At this point, a sufficient number of charges are injected, and the junction starts to saturate.
- Beyond this point, the anode current (Ia) increases, and the anode voltage (Va) decreases.
- This is equal to a negative resistance scenario and this negative resistance region in the PUT
characteristic is used in relaxation oscillators.
- When the anode voltage (Va) is reduced to a particular level called “Valley Point,” the device
becomes fully saturated and no more decrease in Va is possible.
- Thereafter the device behaves like a fully saturated P-N junction.

24. Peak voltage (Vp):
“Anode to cathode voltage after which the PUT jumps into the negative resistance region.”
The peak voltage Vp will be usually one diode drop (0.7V) plus the gate to cathode voltage (Vg).
Peak voltage can be expressed using the equation:
Vp = 0.7V + Vg = 0.7V + VR1 = 0.7V + ηVbb
Vp = VD + ηVbb
Intrinsic standoff ratio (η):
“The ratio of the external resistor R1 to the sum of R1 and R2.”
It helps us to predict how much voltage will be dropped across the gate and cathode for a given
Vbb.
The intrinsic standoff ratio can be expressed using the equation:
η = R1/(R1+R2)

25. Applications of PUT:
Applications for PUTs include:
• Thyristor triggers
• Relaxation oscillators
• Pulse
• Timing circuits with frequencies up to 10 kHz

26. PUT relaxation oscillator :
- The relaxation oscillator is of course the
most common application of a
programmable UJT.
- PUT relaxation oscillator can be used
for generating a wide range of sawtooth
waveforms.
- It is called a relaxation oscillator
because the timing interval is started by
the gradual charging of a capacitor and
the timing interval is terminated by the
sudden discharge of the same capacitor.

27. - Resistors R1 and R2 set the peak voltage (Vp) and intrinsic standoff
ratio (η) of the PUT.
- Resistor Rk limits the cathode current of the PUT. Resistor R and
capacitor C sets the frequency of the oscillator.
- When the supply voltage Vbb is applied, the capacitor C starts
charging through resistor R.
- When V across the capacitor exceeds the peak voltage (Vp) the PUT
goes into negative resistance mode creating a low resistance path
from the anode (A) to the cathode(K).
- The capacitor discharges through this path.
- When V across the capacitor is below valley point voltage (Vv) the
PUT reverts to its initial condition and there will be no more
discharge path for the capacitor.
- The capacitor starts to charge again and the cycle is repeated.

28. The frequency of oscillation of a PUT relaxation oscillator can be expressed by
the following equation:
F = 1/ (RC ln(1/(1-η))
Where F is the frequency, η is the intrinsic standoff ratio, R is the resistance
and C is the capacitance.

29. Advantages
- PUT has high forward conductance, so it can
provide high peak current pulses, even with
a low-value capacitor in the relaxation
oscillator circuit.
- PUT has a fast rise time, which allows a
faster rise in current.
- It provides healthy output pulses as
compared to UJT.
Disadvantages
The main disadvantage of PUT applications i.e.,
relaxation oscillators is that they are unstable and
for good control, features require complex
circuitry.

30. DifferencesbetweenUJT&PUT:
UJT PUT
i. The frequency of the output
pulse of the UJT can be
varied.
i. The frequency of the output
pulse of the PUT is fixed.
i. The intrinsic stand-off ratio
of a UJT is fixed hence
operating characteristics
cannot be altered.
ii. The PUT has operating
characteristics that can be
altered.
ii. The UJT is made up of a
lightly doped n-region known
as the base region which is
joined by a small heavily
doped p-region called the
emitter.
iii.The PUT on the other hand is
a four-layer device similar to
an SCR except that the gate
terminal of the PUT is
connected to the n-region
adjacent to the anode.

31. THANK YOU
FOR WATCHING