A transistor is a three terminal semiconductor device. The word Transistor is an acronym, and is a combination of the words Transfer Resistor used to describe their mode of operation way back in their early days of development.
Types of transistors:
- Bipolar junction Transistor(BJT)
- Junction field effect transistors(JFET)
- Metal-Semiconductor Field Effect transistors(MESFET)
- Metal Oxide Semiconductor Field Effect Transistor(MOSFET)
Bipolar Junction Transistor
If we join together two individual diodes end to end giving two pn junctions connected together in series, we have three layers, two junctions, three-terminal device known as BJT.
BJT is composed of three doped regions of which the two end regions is of same doping and the middle region has the opposite doping. So we can have two types of BJTs viz.,
- NPN
- PNP
The three terminals of a transistor are:
- Emitter
- Base
- Collector
The diagrams below show the construction and the symbols of NPN and PNP transistor
