A bipolar junction transistor (BJT) is a three-terminal device in which a small current into one terminal, the base, controls a much larger current flowing between the other two, the collector and emitter. Used between its extremes it is an amplifier; driven all the way on or off it is a switch.
Both have three terminals, and one of them controls the current between the other two.
How it works
An NPN transistor is a thin P layer (the base) sandwiched between two N layers. The base-emitter junction is an ordinary forward-biased PN junction, so it conducts once the base is about 0.6 to 0.7 V above the emitter. That injects electrons from the emitter into the base.
The base is very thin and lightly doped, so almost all of those electrons are swept on across the reverse-biased base-collector junction into the collector. Only a small fraction leaves through the base terminal. The result: collector current is a fixed multiple of base current. That multiple is the beta, or current gain.
A PNP is the mirror image: every polarity is reversed and the emitter arrow points inward. In the symbol, the arrow always sits on the emitter and points the way conventional current flows.
Switch or amplifier
Illustrative values (12 V supply, 1 kΩ load). Small base current steers a much larger collector current, until the load limits it.
Cutoff: no base current, so no collector current. An open switch.
Saturation: so much base current that the load, not the transistor, limits collector current. The transistor drops only a few tenths of a volt, like a closed switch.
Active region: in between, collector current follows base current. A varying signal on the base becomes a larger copy at the collector. That is amplification.
Beta is not precisely controlled. It differs from one transistor to the next and changes with temperature, so real circuits use bias networks and negative feedback rather than counting on a particular beta.
Three ways to wire it
Name the leg that is shared by input and output. That is the 'common' one.
Configuration
Voltage gain
Input / output
Typical use
Common emitter
high, output inverted
medium / medium
general amplifier stages
Common collector (emitter follower)
just under 1
high / low
buffer between stages
Common base
high
low / high
VHF and UHF amplifiers
In practice
Where they appear: audio and RF amplifiers, oscillators, mixers, and power-amplifier stages in many radios. At higher power and frequency, MOSFETs are also common.
Heat: the turn-on voltage falls as temperature rises (roughly 2 mV per °C, typically). If the bias is not stabilised, current rises, the part heats further, and it can destroy itself: thermal runaway. Power stages use heatsinks and emitter resistors.
Frequency: gain falls at high frequency. The alpha cutoff frequency is where the common-base gain has dropped to 0.7 of its value at 1 kHz, a measure of how high a transistor is useful.
Limits: every transistor has maximum voltage, current and power ratings. Exceeding any one ends its life.