What is transistor?

What is Transistor? It is a semi conductor device that transfer weak signal from low resistance circuit to high resistance circuit or in other words it is a switching device that regulates and amplify the electrical signal voltage and current.

what is transistor

Transistor was invented by three scientists at Bell laboratories in 1947 and its replace the vacuum tube in the electronics as a regulator which is used to regulated the electronics signals. It is used to regulated the voltage and current flow in the circuit and act as a switch in electronics circuit.

Transistor is basically the two PN junction which is connected back to back. It consist three terminals emitter ,base and collector. The base is made up of thin layers. The right parts of diode are emitter diode and left part is called collector-base diode. The emitter base junction is connected to forward biased and the collector base junction is connected in reverse bias.



what is transistor ?symbol


There are two type of transistor

1-NPN transistor

2-PNP transistor

NPN transistor are those which have two N-type semi conductor material and one P-types semi conductor material and PNP types re those which have two P type semi conductor material and one N type semi conductor material. In the transistor symbol the arrow show the flow of current in it.


Terminal of transistor

It has three terminals emitter ,base and collector All three terminal are discuss below

terminals of transistor


Emitter are the parts of transistor  which supply the majority carrier. The emitter is always connected in forward biased  with respect to base because the emitter supply the majority carrier to the base. As the base is lightly doped and the emitter –base junction inject heavy amount of majority charge carrier in to the base.


The collector are part of transistor which collects the majority charge carrier supplied by the emitter through base .The collector –base junction is always reverse bias because we need to remove the majority charge carrier from is junction with the base. The collector is moderately doped and size is too much large so it can collect the most of the majority charge carrier which supply from the emitter.


The base is the middle part which is heavy doped..The base connected with two circuit i-e input circuit with emitter and output circuit with collector. The emitter –base circuit is always forward bias offering very low resistance o the circuit and the collector-base junction is always reverse bias offering high resistance to the circuit. The base is very thin and lightly doped.

Transistor Working

During the operation of transistor emitter-base junction is kept in forward biased which create the base current flow through the base region.The current flowing trough base is very small. The base current move the electron to the collector region and create a hole in the base region.

working of transistor

As the base of transistor is very thin and lightly doped therefore it has less no of electrons as compare to emitter region.The small portion of electron combined with the hole in the base region and the remaining electrons are moved forwards to the collection region which create collector current. By this discussion we can conclude that the large collector current can obtained by varying the base region.  

Operating Condition of transistor

If the emitter base junction is in forward biased and collection –base junction is in reverse biased then we can said that it is in active region. The transistor have two junction and by varying these junctions biasing we can operate the transistor

Emitter Junction (EB)Collector Junction (CB)Region of operationCondition
Reversed- biasedReversed-biasedCut-offRR
operation of transistor

Explanation  of conditions

FR condition

In this condition the emitter-base junction of transistor is in forward biased and collector –base junction is in reversed biased. In this case the transistor is in active region and collector current depend upon the emitter current. This type of operation is used for amplification

FF condition

In this condition the both junction of transistor are connected in forward biased. The transistor will be in saturation and the collector current become independent of the base current.In this condition the transistor act like a closed switch.

RR condition

In this condition both junction are in reversed bias connected. In this case the emitter does not supply the majority charge carrier to the base and carrier charges are not collect by the collector..Therefore in this condition the transistor act like a closed switch

RF condition

In this case he emitter-base junction are in reversed biased and collector-base junction are in forward bias connected.As we know that collector is lightly doped as compare to emitter junction therefore it does not supply the majority charge carrier to the base. Hence in this condition the transistor action is very poor.

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