Transistor are semiconductor devices with three leads. A very small current or voltage at one lead can control a much larger current flowing through the other two leads. This means transistor can be used as amplifier and switches. There are two main families of transistor: Bipolar and Field-Effect.
Bipolar (Junction) Transistor
Add a second junction to a PN junction diode and you get a 3-layer silicon sandwich. The sandwich can be either NPN or PNP, either way, the middle layer acts like a faucet or gate that controls the current moving through the three layers.
Bipolar Transistor Symbol
Arrows point in direction of hole flow.
Bipolar Transistor Operation
The three layer of a bipolar transistor are the emitter, base and collector. The base is very thin and has fewer doping atoms than the emitter and collector. Therefore a very small emitter-base current will cause a much larger emitter-collector current to flow.
More About Bipolar Transistor Operation
Diodes and transistors share several key feature:
1. The base-emitter junction (or diode) will not conduct until the forward voltage exceeds 0.6-volt.
2. Too much current will cause a transistor to become hot and operate improperly. If a transistor is hot when touched, disconnect the power to it.
3. Too much current or voltage may damage or permanently destroy the semiconductor chip that forms a transistor. If the chip isn't harmed, its tiny connection wires may melt or separate from the chip. Never connect a transistor backwards!
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