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At a glance of UJT

In today’s blog we will discuss UJT, construction of UJT, OFF state UJT and ON state UJT also the Turning OFF of UJT

Introduction

UJT stands for unijunction transistor. Its name suggests that it has only one junction. Basically, UJT is a three-terminal device, namely emitter, base B1, and base B2. It is similar to any layer device. The only difference is that the switching voltage of UJT can be easily varied while designing the circuit.

UJT behaves as a switch similar to a four-layer diode. Don’t confuse it with the four-layer diode. It is our Schottky diode which is also referred four-layer diode.

Construction of UJT

A typical UJT structure consists of a lightly doped N- type silicon bar on which the ohmic contacts are provided at each end connection called the base. B1 and B2.

A P region formed by heavily doped material is alloyed on one side of a bar close to terminal B2. This p- the region is also called an emitter (E).

Suppose you have read the above lines carefully. In that case, you will notice that the bar PN junction is formed due to the collision of N-type and heavily doped p region.

On the other hand, an R_BBis the essential resistance of the N-type bar. R_BB is the combination of two resistance, R_B1, and R_B2 where the R_B1 is the resistance between B1 and emitter whereas R_B2 is the resistance between emitter and base B2

The value of R_B2is small as the base B2 is close to the emitter, base B1 is far from the emitter, R_B1has a slightly higher value of resistance than R_B2.

For better visualization, observe the below figures.

UJT, construction of UJT, OFF state UJT and ON state UJT also the Turning OFF of UJT

In the equivalent circuit, you might have observed the diode-connected at point x. this diode represents the PN junction between the points of emitter and base. The arrow passing through R_B1 indicates a variable resistor that can be varied during the operation.

Circuit operation

The UJT operated in two states that are ON state and OFF state.

In the OFF state, the diode between emitter and base junctions becomes reverse biased; hence only a small emitter current will flow. And we already know that R_B1has a high value, so no conduction will happen at the time; therefore, the UJT is said to be in an OFF state.

In the ON state, the diode between emitter and base junctions becomes forward biased. The R_B1between emitter and base becomes low, which allows the emitter current to flow.

The above figure shows the normal UJT biasing. The base B2 and emitter (E) are made positive concerning the base B1. The B1 is referred to as a reference terminal, and the voltage is always measured relative to B1 in UJT.

The V_BB is a constant fixed voltage that is applied from B2 to B1.where as V_EE is the variable input of the circuit, VEE is a source and a voltage across the capacitor.

OFF state UJT

As I mentioned earlier, in the OFF state, the diode is reverse biased.

To obtain the voltage across point X, we need to apply the voltage divider rule. As you see in the figure that R_B1 and R_B2form a voltage divider that produces a voltage V_X from point X relative to the ground.

V_X= (R_B1/R_B1 +R_B2) × V_BB

Where sum of R_B1 +R_B2is R_BB

V_X= (R_B1/ R_BB) × V_BB

But R_B1/ R_BB is η

V_X= η × V_BB

Where η is the internal voltage divider ratio (R_B1/ R_BB) and is also called intrinsic standoff ratio.

value of η is 0.5 – 0.8.

V_X is voltage at point X. To be specific, it is the voltage on N – side of the PN junction.

This V_X plays an important role in deciding the operating mode of UJT. The V_EE is the variable voltage applied across the emitter from the P- side. If the applied voltage V_EEis less than V_X, then the emitter diode will reverse biased. During this condition, reverse emitter current I_E will be negligible, UJT has a very high resistance between emitter and base B1.

As no IE drop across RE will become zero, the emitter voltage V_E will become equal to the source voltage.

From the V_E – I_E curve, the OFF state of UJR extends to the point where emitter voltage V_E exceeds V_Xby adding the diode threshold voltage V_D. This point is known as the peak point voltage, and this threshold voltage will be needed for forward current to flow through the diode.

V_P= V_X+ V_D = η .V_BB + V_D

ON state UJT

From the curve, we observed that as the voltage V_EE is increasing. Still, the PUT remains in the OFF state until the V_EE reached the peak point value V_P. once the V_EE reaches the peak point value V_P,

the PN junction becomes forward biased and starts conducting in the opposite direction.

From the V_E – I_E curve, you can clearly see that I_E becomes positive at the point V_P, and when V_E becomes equal to the V_Ppoint at that time, I_E also becomes equal to I_P peak point current.

A heavily doped emitter injects holes into N-type bar which is mostly the B1 region. Hence the bar is lightly doped; there will be a little chance for hole recombination at the lower end of the bar. However, the lower end of the bar fills with additional current carriers that are holes results in a reduction of R_B1 resistance.

The value of RB1 decreases, causing V_X to drop, due to which the diode becomes more forward bias. As a result of this, the emitter current I_E increases. Due to the large emitter current IE, more injection of holes into B1 takes place, reducing the value of RB1 further.

This regenerative process ends when R_B1reduces and dropped to a very small value. As the value of R_B1 reduces, the emitter current increases very largely to limit this external resistor R_Eis used.

Turning OFF of UJT

Due to this, UJT operation switched to a low voltage high current region of the curve. In this region, emitter voltage V_E will be small. Still, the emitter current continues to increase until it reached its maximum value.

We know that the current always depends on voltage and resistance same happens in the case of UJT emitter current depends on R_B1and V_EE. A further decrease in V_EE results in a decrease of emitter current I_E. I_Edecreases to the valley point and becomes equal to valley current I_V valley current is the essential current required to maintain the UJT in on condition.

When the I_V current decreases below the valley point, UJT goes from ON state to OFF state.