Because we understand how to transistor work first we should know about transistor .Transistor 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.
Transistor Invented By
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.
Construction and Working
Transistor is basically the two PN junction which is connected back to back.It is three terminal device which is emitter, base and collector. It consist two PN junctions .One PN junction of BJT exist between the emitter and abase region and another PN junction exist between the collector and base region. During normal operation of BJT ,the base-emitter junction is in forward biased and base collector junction is in reversed biased. There are two type of BJT one is NPN transistor and second is PNP transistor .
Let discuss the working of NPN transistor. The NPN emitter is made by n-type material, therefore the majority carriers are electron in n-type material. When we connected the base-emitter junction in forward biased then the electron will move from n-type region toward p- type region and minority carrier holes move toward the n-type region. As a result of meeting these holes and electrons ,the current will flow across the junction. If emitter base junction is connected in reverse biased then the holes and electrons move away from the junction and as a result the depletion region forms between the two regions and due to this depletion region no current flow through the junction.
As a result of current flow between the base and emitter the electrons will leave the emitter and enter in to the base.Electron will combine when they reach at depeletion region.But we know that the doping level of base region is very light and thin in shape therefore the most of the electron across the region without recombining with the holes and as aresult the electrons will drift toward the collector region.
Terminal of transistor
Transistor has three terminals emitter ,base and collector All three terminal are discuss below
Emitter :Emitter are the parts of transistor which supply the majority carrier. The emitter of transistor 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.
Collector –The collector of transistor 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.
Base-The base of transistor 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.
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