Construction - unijunction transistor, Electrical Engineering

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Construction - UniJunction Transistor:

Construction: The basic structure of uni junction transistor is shown in fig. (a). It essentially consists of a lightly doped N type silicon bar with a small piece of heavily doped P type material alloyed to its one side to produce single P N junction, the single P L junction accounts for the terminology uni junction. The silicon bar, at its ends, has two ohmic contacts designated as base 1 (B1) and base 2 (B2), as shown and the P V; region is termed the emitter (E). The emitter junction is usually located closer to base 2 (B2) than base 1 (B1) so that the device is not symmetrical, because symmetrical units does not provide optimum electrical characteristics for most of the applications. The symbol for uni junction transistor is shown in fig. (b). 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 conduction state. The basic arrangement for the UJT is shown in fig. (c). The worth noting points about UJT are given below :

1.  The device has only one junction, so it is called the uni junction 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 UJT is quite similar to that of an N channel JFET. The main difference is that P type material surrounds the N type material in case of JFET and the gate surface of the JFET is much larger than emitter junction of UJT.

4. In a uni junction 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 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.


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