What is a Transistor?
Transistor is a semiconductor material which is used to regulate current or voltage and is used as switch,amplifier in most of electronic circuits.Transistor stands for Transfer of resistance and has at least three terminals for connection to the external circuit.
Transistor is basically made up of semiconducting materials such as P-type materials and N-type materials. P-type semiconductor is formed when intrinsic semiconductor is doped with trivalent elements such as Al, Ga and Indium. N-type semiconductor is formed when intrinsic semiconductor is doped with pentavalent impurities such as Phosphorous, Arsenic or Antimony.A transistor is nothing but combination of two coupled diodes.
There are many types of transistors(BJT,FET,UJT etc.,) are available in today's market like and these are based up on the arrangement of P-type and N-type materials ,way of operation .
Bipolar Junction Transistor(BJT) is the most commonly used Transistor.In Bipolar Junction Transistor the conduction is due to two types of charge carriers i.e. both electrons and holes are responsible for current conduction.
Transistor is composed by three parts . They are 1) Emitter 2) Base 3)Collector
Emitter is heavily doped and it is used to emit charge carriers and referred as negative lead.
Base region is very thin and lightly doped .Charge carriers are injected into base and pass on to the collector and is responsible for activating the Transistor.
Collector region is lightly doped and is referred as positive lead.
There are two basic types of transistors. they are PNP Transistor and NPN Transistor.
PNP and NPN transistors are differed by the current direction.
The direction of arrow that is shown in the symbol of transistor indicates the direction of current through flowing it. In NPN transistor, current comes out from emitter of the transistor where as in PNP, the current flows into the emitter.
PNP transistor is formed by sandwiching thin layer of N-type semiconductor between two P-type semiconductors.
Transistor is a semiconductor material which is used to regulate current or voltage and is used as switch,amplifier in most of electronic circuits.Transistor stands for Transfer of resistance and has at least three terminals for connection to the external circuit.
TYPES OF TRANSISTORS
There are many types of transistors(BJT,FET,UJT etc.,) are available in today's market like and these are based up on the arrangement of P-type and N-type materials ,way of operation .
1.Bipolar Junction Transistor
Transistor is composed by three parts . They are 1) Emitter 2) Base 3)Collector
Emitter is heavily doped and it is used to emit charge carriers and referred as negative lead.
Base region is very thin and lightly doped .Charge carriers are injected into base and pass on to the collector and is responsible for activating the Transistor.
Collector region is lightly doped and is referred as positive lead.
PNP and NPN transistors are differed by the current direction.
| Symbol | Name | Description |
 | NPN Bipolar Transistor | Allows current flow when high potential at base (middle) |
 | PNP Bipolar Transistor | Allows current flow when low potential at base (middle) |
PNP transistor is formed by sandwiching thin layer of N-type semiconductor between two P-type semiconductors.
A PNP transistor is like two PN junction diodes which are placed back to back. At each junction there is a depletion region which leads to potential barrier. Here The Emitter-Base junction is Forward biased and Collector-Base junction is reverse biased .
As the Emitter-Base Junction is Forward biased more no of holes will cross the junction and enter into the base region.At the same time very few amount of electrons will flow from the base to emitter.
These electrons will recombine with the equal no of holes and electron -hole pairs are generated. The loss of total no of holes in the emitter is made by the flow of equal no.of electrons from the emitter to the positive terminal of the battery. The flow of holes from Emitter to Base is leads to emitter current and we have IE=IB+IC.
An NPN transistor is formed by sandwiching thin layer of P-type semiconductor between two N-type semiconductors.
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In NPN transistor, current comes out from emitter of the transistor And transistor can be operated in 3 regions .They are 1) Active region 2) Saturation region 3)Cutoff region
In Active region the transistor acts as an Amplifier .In Saturation its acts as closed switch and in cutoff regions it acts as open switch .
1.Transistor can be used as an amplifier.
2.In digital circuits these Transistor is used as ON/OFF switches.
3.Transistors are used as buffer or impedance matching between mis-matched electrical circuits.
4.They are used to regulate current,voltage or power.
These electrons will recombine with the equal no of holes and electron -hole pairs are generated. The loss of total no of holes in the emitter is made by the flow of equal no.of electrons from the emitter to the positive terminal of the battery. The flow of holes from Emitter to Base is leads to emitter current and we have IE=IB+IC.
An NPN transistor is formed by sandwiching thin layer of P-type semiconductor between two N-type semiconductors.
In Active region the transistor acts as an Amplifier .In Saturation its acts as closed switch and in cutoff regions it acts as open switch .
Applications of a Transistor
1.Transistor can be used as an amplifier.
2.In digital circuits these Transistor is used as ON/OFF switches.
3.Transistors are used as buffer or impedance matching between mis-matched electrical circuits.
4.They are used to regulate current,voltage or power.
