A transistor is a type of semiconductor device that can be used to both conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we can say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals. A transistors is most commonly used as either electronic switches in digital circuits or as an amplifier.
Eg: AC540 – germanium core, LF, low power,
AF125 – germanium core, HF, low power,
BC107 – silicon, LF, low power (0.3W),
BD675 – silicon, LF, high power (40W),
BF199 – silicon, HF (to 550 MHz),
BU208 – silicon (for voltages up to 700V),
BSY54 – silicon, switching transistors.