How to Construct and Operate a Uni-Junction Transistor(UJT)

Introduction to Uni-Junction Transistor

Uni-junction transistor
Uni-junction transistor

Uni-junction transistor is also known as double-base diode because it is a 2-layered, 3-terminal solid-state switching device. It has only one junction so it is called as a uni-junction device. The unique characteristic feature of this device is such that when it is triggered, the emitter current increases until it is restricted by an emitter power supply. Owing to its low cost, it can be used in a wide range of applications including oscillators, pulse generators and trigger circuits, etc. It is a low-power absorbing device and can be operated under normal conditions.

There are 3 types of uni junction transistors

  1. Original Uni-junction transistor
  2. Complimentary Uni-junction transistor
  3. Programmable Uni-junction transistor (PUT)

1. Original Uni-junction transistor or UJT is a simple device in which a bar of N-type semiconductor material into which P-type material is diffused; somewhere along its length defining the device parameter as intrinsic standoff. The 2N2646 is the most commonly used version of UJT. UJTs are very popular in switching circuits and are never used as amplifiers. As far as Applications of UJT are concerned, they can be used as relaxation oscillators, phase controls, timing circuits and trigger devices for SCRs and triacs.

2. Complimentary Uni-junction transistor or CUJT is a bar of P-type semiconductor material into which N-type material is diffused somewhere along its length defining the device parameter as intrinsic standoff. The 2N6114 is one version of CUJT.

3. Programmable Uni-junction transistor or PUT is a close relative of thyristor; just like thyristor, it consists of four P-N layers and has anode and cathode placed at first and last layers. The N-type layer near the anode is known as anode gate. It is inexpensive in production.

Programmable Uni junction Transistor
Programmable Uni junction Transistor

Among these three transistors, this article talks about UJT transistor’s working features and its construction in brief.


Construction of UJT

UJT is a three-terminal, single-junction, two-layered device, and it is similar to a thyristor compare to a transistors. It has a high-impedance off state and low-impedance on state quite similar to a thyristor. From off state to an on state, switching is caused by conductivity modulation and not by a bipolar transistor action.

Construction of UJT
Construction of UJT

The silicon bar has two Ohmic contacts designated as base1 and base2, as shown in the fig. The function of the base and the emitter are different from the base and emitter of a bipolar transistor.

The emitter is of P-type, and it is heavily doped. The resistance between B1 and B2 when the emitter is open-circuited is called an inter-base resistance. The emitter junction is usually situated closer to the base B2 than the base B1. So the device is not symmetrical, because symmetrical unit does not provide electrical characteristics to most of the applications.

The symbol for uni-junction transistor is shown in the fig. When the device is forward-biased, it is active or is in the conducting state. The emitter is drawn at an angle to the vertical line which represents the N-type material slab and the arrow head points in the direction of conventional current.

Operation of a UJT

This transistor operation starts by making the emitter supply voltage to zero, and its emitter diode is reverse biased with the intrinsic stand-off voltage. If VB is the voltage of the emitter diode, then the total reverse bias voltage is VA + VB = Ƞ VBB + VB. For silicon VB = 0.7 V, If VE gets slowly increases to the point where VE = Ƞ VBB, then IE will be reduced to zero. Therefore, on each side of the diode, equal voltages results no current flow through it, neither in reverse bias nor in forward bias.

Equivalent Circuit of a UJT
Equivalent Circuit of a UJT

When the emitter supply voltage is increased rapidly, then the diode becomes forward-biased and exceeds the total reverse bias voltage (Ƞ VBB + VB). This emitter voltage value VE is called the peak-point voltage and is denoted by VP. When VE = VP, emitter current IE flows through the RB1 to the ground, that is, B1. This is the minimum current required for triggering the UJT. This is called the peak-point emitter current and is denoted by IP. Ip is inversely proportional to the Inter-base voltage, VBB.

Now when the emitter diode starts conducting, charge carriers are injected into the RB region of the bar. As the resistance of a semiconductor material depends upon doping, the resistance of RB decreases due to additional charge carriers.

Then the voltage drop across RB also decreases, with the decrease in resistance because the emitter diode is heavily forward biased. This in turn results in larger forward current, and as a result charge carriers are injected and it will cause the reduction in the resistance of the RB region. Thus, the emitter current goes on increasing until the emitter power supply is in limited range.

VA decreases with the increase in emitter current, and UJT have the negative resistance characteristic. The base 2 is used for applying external voltage VBB across it. The terminals E and B1 are the active terminals. UJT usually gets triggered by applying a positive pulse to the emitter, and it can be turned off by applying a negative trigger pulse.

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