. Basic building block of all microchips and even CPU.
  . Developed by John Bardeen, Walter Brattaian and William Shockley in 1947.
  . Made by semiconductor (conduct and insulate) devices like Germanium, Silicon etc.
  . First computer had 2000 transistors, now powerful processors about 7 billion transistors

What it contains?

  It has three pins namely:
     - Emitter
     - Collector
  . These three pins can be made of p-type or n-type semiconductors.
  Arrow mark shows the direction of current flow.
  . One of the pins is common to both input and output forming 3 modes of configuration:
     - Common Emitter mode
     - Common Base mode
     - Common Collector mode

How it works?

  . It works on fundamental of - ' Movement of electrons and holes between p-type and n-type semiconductors'.
  p-type means Group III element is added as impurity, n-type means Group V element is added as impurity to semiconductor.

  . Speed at which computer perform calculations depend on transistor ON/OFF.
  . Faster the transistors, the faster computer works.
  It is what creates binary 0's and 1's that computer communicate and deal with Boolean logic.
  . It replaced use of vacuum tubes.
  Moore's Law: Number of transistors contained in IC doubles for every 18 months.

Types of Transistors:

   . There are two types of Transistors called BJT (Bipolar Junction Transistor) and FET (Field Effect Transistor).
   . BJT's are suitable for low current applications.
   . FET's are suitable for low voltage applications.
   . In FET's instead of Emitter, Base and Current, it has Source, Drain and Gate.

1. p-n-p transistor:

  . Base is made of n-type semiconductor.
  . Emitter and Collector are made of p-type semiconductors.
  . In p-n-p transistor, majority charge carriers are holes.
  . Mobility of electrons is less, so less preferred.

2. n-p-n transistor:

  . Base is made of p-type semiconductor.
  . Emitter and Collector are made of n-type semiconductors.
  In p-n-p transistor, majority charge carriers are electrons.
  . It is mostly preferred because of higher mobility of electrons than holes.

3. Field Effect Transistor:

   . In many applications, FET's are mostly used rather than BJT's.
   . FET's are voltage controlled devices.  
   . Heating is less when compared to BJT's.
   . In FET's only majority charge carries flows.

Applications of transistors:

  . Transistors makes logic gates.
   . These gates are used to make flip-flops .
   . These are then used to make different building blocks like ALU, microchips etc.
   . Transistor can be used as switch.
   . When input voltage is enough to drive transistor to ON, VCC flows to ground making output zero.
   . When input voltage is low, transistor is OFF, it is open making VCC to flow to output and hence it is high.
   . This concept is used to make LED bulb ON/OFF is most of applications like calculator, watches etc.
   . When the circuit is designed as Amplifier using transistors, output is large compared to input.
   . This concept is used in amplifying sound signals in Microphones and feeding to loudspeakers.