UJT &

PUT
UNIJUCNCTION TRANSISTOR
A Unijunction Transistor (UJT) is a three-terminal
semiconductor device. The main characteristic of UJT is when it
is triggered, the emitter current increases re-generatively until it
is limited by an emitter power supply. Due to this characteristic
feature, it is used in applications like switching pulse generators,
saw-tooth wave generators, and etc.

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 Construction of ujt
The UJT consists of an n-type silicon semiconductor bar with an
electrical on each end. The terminals of these connections are
called Base terminals (B1 and B2). Near base B2, a PN-junction is
formed between a p-type emitter and the n-type silicon bar. The
terminal of this junction is called the emitter terminal (E).
Since the device has three terminals and one PN-junction, for this
region this is called a Unijunction Transistor (UJT).

The device has only a PN-junction so it forms a diode. Because the

two base leads are taken from one section of the diode, hence the
device is also called a Double-Based Diode.
The emitter is heavily doped while the n-region is lightly doped.
Thus, the resistance between base terminals is very high when the
emitter terminal is open.

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 Equivalent circuit of ujt

• The resistance of the silicon bar is called the inter-base resistance (which has a value from 4 kΩ to 10
kΩ).
• The resistance RB1 is the resistance of the bar between the emitter and B1 region. The value of this is
variable and depends upon the bias voltage across the pn-junction.
• The resistance RB2 is the resistance of the bar between the emitter and B2 region.
• The emitter pn-junction is represented by a diode.
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• With no voltage applied to the UJT, the value of inter-base resistance is given by

• The intrinsic stand-off ratio (ƞ) of UJT is given by

• The voltage across RB1 is

• The value of ƞ generally lies between 0.51 and 0.82.

• The Peak Point Voltage (VP) of the UJT

 Characteristics of ujt

The curve between emitter voltage (VE) and emitter current (IE) of UJT, at a given
value of VBB is known as emitter characteristics of UJT.

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Important points from the characteristics are:

• At first, in the cut-off region, when the emitter voltage increases from zero, due to the minority charge carriers, a small
current flows from terminal B2 to the emitter. This is called leakage current.

• Above the definite value of VE, the emitter current (IE) starts to flow and increases until the peak (VP and IP) is reached
at point P.

• After point P, an increase in VE causes a sudden increase in IE with a corresponding decrease in VE. This is
the Negative Resistance Region of the curve as with the increase in IE, VE decreases.

• The negative resistance region of the curve ends at the valley-point (V), having valley-point voltage VV and current IV.
After the valley-point, the device is driven to saturation.

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 UJT
SAMPLE
PROBLE
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PROGRAMMABLE UNIJUCNCTION
TRANSISTOR
Programmable unijunction transistor or PUT is a close relative of the thyristor
family. It has a four-layered construction just like the thyristors and has three
terminals named anode(A), cathode(K), and gate(G) again like the thyristors.
Yet some authors call it a programmable UJT just because its characteristics
and parameters have many similarities to that of the unijunction transistor. It
is called programmable because the parameters like intrinsic standoff ratio
(η), peak voltage(Vp), etc can be programmed with the help of two external
resistors. In a UJT, the parameters like Vp, η, etc are fixed and we cannot
change them. The main applications of programmable UJT are relaxation
oscillators, thyristor firing, pulse circuits, and timing circuits.

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From the above figure, you can see that the PUT has a four-layered construction. Topmost P-layer
is called the anode (A). The N-layer next to the anode is called the gate (G). The P-layer next to the
gate is left alone. The bottommost N-layer is called cathode (K). Ohmic contacts are made on the
anode, cathode, and gate layers for external connection.

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 Characteristics of put
PUT characteristics is essentially a plot between the anode voltage Va and anode current Ia
of the PUT. The typical biasing diagram and characteristics plot of a PUT is shown below.

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Typically the anode of the PUT is connected to a positive voltage and the cathode is
connected to the ground. The gate is connected to the junction of the two external resistors R1
and R2 which forms 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.

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Peak voltage (Vp): It is the 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 . Where η is the intrinsic standoff ratio and
Vbb is the total voltage across the external resistor network.

Intrinsic standoff ratio ( η) : Intrinsic standoff ratio of a PUT is 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).
 puT
SAMPLE
PROBLE
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VBB = 13.714 V

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 EN
D
Programmable UJT. Circuits Today, 2014,
https://www.circuitstoday.com/programmable-ujt.
Accessed 09 September 2022.

Unijunction Transistor – Construction, Working Principle,

and Characteristic Features. Tutorials Point,
https://www.tutorialspoint.com/unijunction-transistor-const
ruction-working-principle-and-characteristic-features
. Accessed 09 September 2022

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