UNIJUNCTION TRANSISTOR

Thyristors basically are multilayer semiconducting devices. They are switching devices .they have two stages of operation. Hence are bistable devices. The Two states are ON state and OFF state. the main advantage of thyristors is that they can handle large powers with small controlling power there are four layer, three layer devices in the family of thyristors. They are superior to transistors in switching applications industry. A thyristors can be taken as the semiconductor equivalent of a thyratron the electron tube.

UNIJUNCTION TRANSISTOR (ujt)

A injunction transistor is a three terminal semiconductor switching, devices . This device has a unique characteristic that when it is triggered , the emitter current increases regeneratively until it is limited by emitter power supply. Due to this characteristic ,the UJT can be used in a variety power supply .due to this characteristic , the UJT can be used in a variety of a applications involving switching , pulse generation, sawtooth wave generation etc.

It has one junction and three terminal.It act as a diode so it is called double based diode, heavily doped emitter having many holes.

OPERATION:

• when voltage VB is applied between the bases with emitter open (VE=0), a voltage gradient is established along the N-type bar

• As the emitter is located nearer to the base 2, more than half on VBB appears between emitter and B1. The voltage V1. between emitter and B 1 establishers a reverse bias on the P-N junction and the emitter current is cutoff a smally leakage current flows from emitter.

• When a positive voltage is applied at the emitter the P-N junction remains reverse biased on long as the input voltage is less than V .if input voltage to emitter exceeds V 1, PN junction on becomes forward biased. Under these conditions, holes are injected from P-type materials into N-type bar.

These holes are repelled by the positive B 2 terminal and they are attracted towards B 1 terminal of the bar. This accumulation of holes in the emitter to B 1 region results in the decrease of resistance in this section of the bar.

The results is that internal voltage drop from emitter to B1 is decreased and hence the emitter current I E increases. As more holes are injected, a condition of saturation occurs eventually. At this point , I E increases. As more holes are injected, a condition of saturation occurs eventually. A this point, I E is limited by the emitter. Power supply only. UJT is now in the ON state.

APPLICATIONS:

• Phase control
• Switching
• Pulse generation
• Saw-tooth wave generator(relaxation oscillator)
• Since wave generator
• Timing and trigger circuits
• Voltage or current regulated power supplies