UJT Working And UJT As a Relaxation Oscillator
Ujt Working And Operation
Ujt working and operation Equivalent circuit

UJT is a three terminal semiconductor switching device. It has only one PN junction. The basic structure and symbol of UJT is above the picture.
It consists of a lightly doped N‑type silicon bar with a heavily doped P‑type material is diffused into the bar, forms a PN junction
The P type region is called emitter. Two ohmic contacts are provided at the ends of N‑type silicon bar called base‑1 and base‑2 the resistance between B1 and B2 is called interbase resistance.
Equivalent circuit of UJT

The diode D represents the PN junction. According to the emitter terminal a interbase resistance is separated into two resistances RB1 and RB2
Hence RBB = RB1 + RB2. The resistance RB1 is variable because it’s value can be varied according to the bias voltage between P and N junction
From the equivalent circuit the Voltage across the resistor
RB1=V1= \frac {\vphantom{\big(}RB1} {\vphantom{\big(}RB1+RB2}×VBB
=\frac {\vphantom{\big(}RB1} {\vphantom{\big(}RBB}×VBB
V1=η VBB(∵η=\frac {\vphantom{\big(}RB1} {\vphantom{\big(}RBB})
where η is known as intrinsic stand off ratio
∵ is Before Resoaning
UJT Operation (UJT WORKING PRINCIPLE)

The terminal B2 is always positive voltage (VBB) With respect to B1. Usually a positive voltage (VEE ) is applied to the terminal E with respect to B1. The operating condition of UJT depends upon their emitter voltage
According to the base voltage VBB Voltage V1 is always developed across the resistor RB1 which is applied to the cathode terminal of the PN junction diode. When the emitter voltage is zero the diode works in reverse bias so no emitter current flows. Now the UJT is in OFF state. Now a small leakage current only flows from B2 to E
When a positive voltage is applied to the emitter the PN junction is also in reverse biased. If the emitter Voltage is linearly increased which reduces the amount of reverse bias of the PN junction
When the applied voltage is increased greater than V1 the PN junction will be forward biased
Now the holes are injected from P‑region into N‑region. The holes are repelled by the terminal B2 and are attracted by the terminal B1. Now the accumulation of holes in E to B1 region reduces the resistance in this section.
Hence emitter current IE is increased and the voltage VE is decreased. Now the device is in the ON state
When a negative voltage is applied to the emitter the PN junction is reverse biased and the emitter current is very low. The device is now in OFF state
characteristics of UJT

The curve plotted between emitter Voltage and the emitter current at a given VBB is called emitter characteristics of UJT
Here upto the peak point the diode is reverse biased. Hence the region to the left of the peak point is called cut-off region. The UJT has a stable firing voltage which depends on VBB after the firing voltage the diode starts conduction.

Now the current IE is linearly increased and VE is linearly decreased which produce a negative resistance region.
This region lies in between peak point and valley point . After the valley point current becomes saturation. The region beyond the valley point is called saturation region. In this region the UJT is ON state
Applications of UJT
- It is used in switching circuits
- It is used in saw tooth generator
- It is used in pulse generator
- It is used to trigger SCR and TRIAC
- it is used in phase control circuits
- It is used in timing circuit
UJT as a relaxation Oscillator

Unijunction transistor can be used to generate saw tooth signal. In sawtooth signal the voltage increases slowly and falls down to zero in short time
A capacitor C is connected in between the terminal E and B1. This capacitor Voltage is used to trigger the UJT and also acts as the output signal of the relaxation Oscillator.
The charges of the capacitor depend upon the conduction level of UJT. Before conduction the capacitor charges through R1 and then after conduction the capacitor discharges through the emitter to base 1 path of UJT

When the supply voltage is given the capacitor charges through R1. As long as the capacitor Voltage is less than the base to emitter 1 voltage VEB1 the junction is reverse biased and the UJT will not conduct. When the capacitor Voltage exceeds the reverse biased voltage the UJT starts conducting.
During conduction the resistance between emitter and base 1 falls to a lower value. Hence the capacitor discharges the voltage in a low time and the UJT again goes to non conduction level.
The capacitor repeats the charge and discharges functions continuously. By this cumulative action a continuous sawtooth signal is produced at its output.
Applications
- Used in television
- Used in oscilloscope
- Used in radar equipment