SEMI CONDUCTOR DEVICE

* The transistor operate at low voltage with sufficient amount of current. Hence they are known as current operated devices.

* The power handling capacity of a semiconductor is relatively lower than that of valves .

* In complementary amplifier circuit — one PNP &one NPN transistor in series is used

* The frequency on which the gain of a transistor is reduced to 70.7 % of its value at low frequency is called alpha cut off frequency of that transistor.

* UJT —uni junction transistor (Voltage control device )

- It has three terminal (emitter , base 1 , base 2)

- It has stable negative resistance characteristics

- It is used at low frequency, relaxisation oscillator & timer

- It is also called double base transistor

* SCR — Silicon controlled rectifier (Unidirectional switch )

- It has 4 Layer , 3 junction , 3 terminals

- Terminals are anode , cathode, gate

- Once the conduction of current is started the function of the gate is finished & the magnitude of current is then controlled by varying the load resistor or the supply voltage.

Uses — A SCR can conduct 30 to 100 A of current it is used in switching circuits, Regulated power supply radar modulator , servo system & electronics ignition system.

- SCR can not be used for Amplification purpose

What is the difference between SCR & GTO ?

SCR can not be turn off by gate pulse but GTO is turning off by gate pulse .

* GTO — gate turned off ( uni polar )

- It can be turned to off by applying a negative gate pulse

- It is used in triggering circuit

* SCS — silicon controlled switch

- It has 4 terminals , 3 junction , 4 layer device

- anode , cathode , anode gate , cathode gate

- SCS are used in counters & timing circuits of computer, pulse generators , voltage sensors & oscillator.

* SBS — Static bilateral switch

- characteristics similar to TRIAC

- The device is very useful for uninterrupted power supply (UPS) system

- It’s switching time is of the order of a few micro seconds only ( nearly equal to almost zero)

TRIAC

A TRIAC is equivalent to two SCRs joined together in anti parallel (Inverse parallel , opposite directions polarity )

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