What are transistors?

Transistors are semiconductor devices which can be used as switches (to turn on and off an electrical signal or current path) and also as a signal amplifier in a circuit. There are two broad types of transistors BJT (Bipolar Junction Transistor) and FET (Field Effect Transistors).
- Neeraj Sharma
A transistor is a silicon device with 3 leads, emiter, collector, and base. It is used both as a fast switching device and for signal amplification. It is said to be "saturated" when maximum current is flowing and "cutoff" when no current is flowing from the emiter to the collector. It takes a difference of approx. .7 volts between the emiter and base to turn it "on".
Good switch but actually inferior in sound quality to its predicessor, the "Thermionic" or good ole vacuum tube. If you don't believe me, listen to a guitarist play through both a transistor and vacuum tube guitar amp. You will be quite amazed!!!
For signal amplification the input signal goes into the base and the amplified output signal is taken at the collector with the transistor operating, or biased, between cutoff and saturation. It would be normally biased in between cutoff and saturation for the signal to be "linear" or non distorted. The signal at the collector will be inverted (180 phase shift) with respect to the input signal.
There are other applications where the output is taken at the emiter where the signal is not inverted (in phase) with respect to the input signal. This configuration, generally, has no gain or has what is called "unity gain". Sometimes you see this when separate stages need to be impedance matched for maximum efficiency.
The transistor is first "biased" or configured for a specific amount of gain and the output signal is taken at the collector with a higher amplitude than the input at the base.
A transistor is a semiconductor device
used to amplify and switch electronic
signals and electrical power . It is
composed of 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 through another pair of
terminals. Because the controlled
(output) power can be higher than
the controlling (input) power, a
transistor can amplify a signal.
Today, some transistors are packaged
individually, but many more are found
embedded in integrated circuits .
The transistor is the fundamental
building block of modern electronic
devices, and is ubiquitous in modern
electronic systems. Following its
development in 1947 by John Bardeen ,
Walter Brattain , and William
Shockley, the transistor
revolutionized the field of electronics,
and paved the way for smaller and
cheaper radios, calculators, and
computers, among other things. The
transistor is on the list of IEEE
milestones in electronics, and the
inventors were jointly awarded the
1956 Nobel Prize in Physics for their
achievement.