What is transistor?

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The word transistor is a combination of Transfer & Resistor.This is because it transfer the resistance from one end of the device to the other end.We can say transfer of resistance,hence the name transistor.Transistors have very high input resistance & very low output resistance. 

A transistor is defined as a semiconductor device that's fundamentally bult with three terminals, for amplifying or switching electronic signal & electrical power purposes.

Transistor developed in the year 1947 by three American physicists John Bardeen,
Walter Brattain & William Shockley.

Types of transistor- There are mainly two types of transistor.
Bipolar Junction Transistor (BJT) - BJT is a three terminal semiconductor device in which the operation depends on both electron & hole charge carriers and hence the name Bipolar.It is a current controlled device. 
The three portions of BJT are Emitter, Base,& Collector shown as E, B & C respectively.
Emitter(E)- It has a moderate size & heavily doped as it's main function is to supply a number of majority carriers (electrons or holes). It emits electron or hole so it called emitter.
Base (B)- Base is lightly doped & very thin. Its main function is to pass the majority carriers from emitter to collector.
Collector (C)- Collector name implies it's function of collecting the carriers.This is bit large in size than emitter and base.It is 
moderately doped.
Types of BJT- BJT are two types.
(1)PNP Transistor-It is formed by semiconductor (Germanium or Silicon crystal), in which a thin layer of N-type semiconductor is sandwiched between two layers of P-type semiconductor.
 Operation of PNP transistor - The Forward bias applied to the emitter-base junction of a PNP transistor causes a lot of holes from the emitter region to crossover to the base region as the base is lightly doped with N-type impurity.The number of electrons in the base region bis very small & hence the number of holes combined with electron in the N-type base region is also very small.
Hence a few holes combined with electrons to constitute a base current. The remaining holes (more than 95%) crossover into the collector region to constitute a collector current. 

(2)NPN Transistor- It is formed by semiconductor (Silicon & Germanium crystal),in which a thin layer of P-type semiconductor is sandwiched between two layers of N-type semiconductor.
Operation of NPN transistor- The forward bias applied bro the emitter base junction of an NPN transistor causes a lot of electrons from the emitter region to crossover to the base region.As the base is lightly doped with the emitter region to crossover to the base region.As the base is lightly doped with P- type impurity,the number of holes in the base region is very small & hence the number of electrons that combine with holes in the P-type base region ia also very small.
Hence a few electrons combine with holes to constitute a base current.The remaining electrons (more than 95%) crossover into the collector region to constitute a collector current. Thus the base & collector current summed up gives the emitter current.
 Operating mode of transistor - There are four possible way of biasing the two P-N junction (emitter junction & collector junction) of Transistor.
(1)Active mode - In this mode, the transistor operates as a current amplifier.
(2) Saturation mode - In this mode,the transistor behaves as a close circuit & current flow from callector to emitter, when the base emitter voltage is high.
(3)Active mode - In this mode, transistor operates as current amplifier.
(4) Reverse active mode - In this mode, transistor work like active mode,the current is proportional to base current, but it flow is reverse. Current flow from emitter to collector.

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