Transistors

Definition:
"A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit." 
A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, a transistor can amplify a signal.
The transistor is the fundamental building block of modern electronic devices.

Usage:
The bipolar junction transistor (BJT) was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters and motor drivers.
Most common uses of transistors are:

1.Transistor as a switch

2. Transistor as an Amplifier

Types:

Transistors are categorized by

• Semiconductor material: graphene, germanium, silicon, gallium arsenide, silicon carbide, etc.
• Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
• Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF),microwave (The maximum effective frequency of a transistor is denoted by the term f_T, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
• Application: switch, general purpose, audio, high voltage, super-beta, matched pair
• Physical packaging: through hole metal, through hole plastic, surface mount,ball grid array, power modules
• Amplification factor h_fe (transistor beta)

Goto:

PNP Transistor
NPN Transistor

Biasing and Working of PNP Transistor

Biasing and Working of NPN Transistor