What is BJT ,MOSFET ,FET ,IGBT?

Bipolar Junction Transistor (BJT) is a Semiconductor device constructed with three doped Semiconductor Regions (Base, Collector and Emitter) separated by two p-n Junctions, 

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices

The FET used in many circuits constructed from discrete components in areas from RF technology to power control and switching to general amplification.

An insulated-gate bipolar transistor is a three-terminal power semiconductor device primarily used as an electronic switch which, as it was developed, came to combine high efficiency and fast switching