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 RBB is the essential resistance of the N-type bar. RBB is the combination of two resistance, RB1, and RB2 where the RB1 is the resistance between B1 and emitter whereas RB2 is the resistance between emitter and base B2
The value of RB2is small as the base B2 is close to the emitter, base B1 is far from the emitter, RB1 has a slightly higher value of resistance than RB2.
For better visualization, observe the below figures.
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 RB1 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 RB1 has 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 RB1 between 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 VBB is a constant fixed voltage that is applied from B2 to B1.where as VEE 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 RB1 and RB2 form a voltage divider that produces a voltage VX from point X relative to the ground.
VX = (RB1/RB1 +RB2) × VBB
Where sum of RB1 +RB2 is RBB
VX = (RB1/ RBB) × VBB
But RB1/ RBB is η
VX = η × VBB
Where η is the internal voltage divider ratio (RB1/ RBB) and is also called intrinsic standoff ratio.
value of η is 0.5 – 0.8.
VX is voltage at point X. To be specific, it is the voltage on N – side of the PN junction.
This VX plays an important role in deciding the operating mode of UJT. The VEE is the variable voltage applied across the emitter from the P- side. If the applied voltage VEE is less than VX, then the emitter diode will reverse biased. During this condition, reverse emitter current IE 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 VE will become equal to the source voltage.
From the VE – IE curve, the OFF state of UJR extends to the point where emitter voltage VE exceeds VX by adding the diode threshold voltage VD. This point is known as the peak point voltage, and this threshold voltage will be needed for forward current to flow through the diode.
VP = VX + VD = η .VBB + VD
ON state UJT
From the curve, we observed that as the voltage VEE is increasing. Still, the PUT remains in the OFF state until the VEE reached the peak point value VP. once the VEE reaches the peak point value VP,
the PN junction becomes forward biased and starts conducting in the opposite direction.
From the VE – IE curve, you can clearly see that IE becomes positive at the point VP, and when VE becomes equal to the VP point at that time, IE also becomes equal to IP 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 RB1 resistance.
The value of RB1 decreases, causing VX to drop, due to which the diode becomes more forward bias. As a result of this, the emitter current IE 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 RB1 reduces and dropped to a very small value. As the value of RB1 reduces, the emitter current increases very largely to limit this external resistor RE is 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 VE 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 RB1 and VEE. A further decrease in VEE results in a decrease of emitter current IE. IE decreases to the valley point and becomes equal to valley current IV valley current is the essential current required to maintain the UJT in on condition.
When the IV current decreases below the valley point, UJT goes from ON state to OFF state.
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