Unijunction Transistor (UJT)

Unijunction transistor (UJT), also called the double base diode, is a two layer. three terminal solid state (silicon) switching device. The device has unique characteristics that when it is triggered, its emitter current increases regeneratively (due to negative resistance characteristic) until it is restricted by emitter power supply. The low cost per unit, combined with its unique characteristics, have warranted its use in a wide variety of applications. A few include oscillators, pulse generators, sawtooth generators, trigger circuits, phase control. timing circuits, and voltage or current regulated supplies. The device is, in general, a low power absorbing device under normal operating conditions and provides tremendous aid in the continental effort to design relatively efficient systems.

Construction:

The basic structure of a unijunction transistor is shown in Fig. 1. It is essentially consists of a lightly doped N-type silicon bar with piece of heavily doped P-type material alloyed to its one side to produce single P-N junction. The single P-N junction accounts for the terminology unijunction. The silicon bar, at its ends, has two ohmic contacts designated as base-(B1) and base-2(B2), as shown and P type region is termed the emitter (E). The emitter junction is usually located closer to base-1(B2) than base-1(B1) so that the device is not symmetrical, because symmetrical until does not provide optimum electrical characteristics for most of the applications.

Fig. 1. UJT Basic Structure

The symbol for unijunction transistor is shown in Fig. 2. The emitter leg is drawn at an angle to the vertical line representing the N-type  material slab and the arrowhead points in the direction of conventional current when the device is forward biased, active or in the conducting state.

Fig. 2. UJT Symbol

The basic arrangement for the UJT is shown in Fig. 3.

Fig. 3. UJT Basic Arrangement

A complementary UJT is formed by diffusing an N-type emitter terminal on a P-type base. Except for the polarities of voltage and current, the characteristics of a complementary UJT are exactly the same as those of a conventional UJT.

The important points about UJT are given below:

1. The device has only one junction, so it is called the unijunction device.
2. The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode that it has three terminals.
3. The structure of a UJT is quite similar to that of an N-channel JFET. The main difference is that P-type (gate) material surrounds the N-type (channel) material in case of JFET and the gate surface of the JFET is much larger than emitter junction of UJT.
4. In a unijunction transistor the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 K Ohm when the emitter is open.
5. The N-type silicon bar has a high resistance and the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.
6. UJT is operated with emitter junction forward biased while the JFET is normally operated with the gate junction reverse biased.
7. UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal.
8. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.

Device Operation:

The device has a unique characteristic that when it is triggered, its emitter current increases regeneratively until it is restricted by emitter power supply. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.

The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.

Fig. 4. Static Emitter-Characteristic

for a UJT