In 1947,The world of electronic devices is completely changed by the two physicists

John Bardeen and Walter Brattain when they made the first working device on point contact transistor.

Transistor are the semiconductor  device having three terminals, used as a switch or to amplify a current. They are very light in weight around 1gram or less and very small in size. The phone that you are carrying is light in weight and have small size that fit in your hand  because of the transistors.

Transistors can used as a switch in digital electronics and as an amplifier in the analog electronics. 

Transistors have 3 terminal:

Emitter is the source of charge carrier and that is why it is heavily doped. 

Base is lightly doped and very thin in size 

Transistor

Collector is moderately doped and wider in size than base and emitter region because collector main's job is to collect charge carrier from base and emitter region

When the base emitter region is forward bias and collector and base region is reversed  bias. 

For this condition :

Base voltage must be higher than emitter voltage and Vbe =0.7 Volts

Collector voltage must be higher than base voltage so than collector voltage is higher than the other region voltage. 

The base current is directly proportional to the collector current. 

Transistor can act as a solid state switch that doesn't require any moving part, it act as a on/off switch just by sending a base signal.

For transistor to act as a switch:

• Emitter is connected to the Ground.
• Base is connected to the on/off signal with a  resistor
• Collector is connected to the 5v supply.

Resistor is connected with the base terminal because base current should not be of high ampere, it can damage the transistor.

When a base current is flow through the transistor. So, it will create a path for the Emitter electron to flow through base region  to the  collector region and the current will easily flow through the collector region to the emitter region and act as a short circuit path. Transistor act as ON switch.

When base current is remove the collector current can't cross the base region and act as a open circuit or OFF switch.