How a Bipolar Junction Transistor Works

Three layers, one small current in control.

A bipolar junction transistor has three doped layers, either NPN or PNP, with terminals called emitter, base and collector. A small current into the base allows a much larger current to flow from collector to emitter.

For an NPN silicon transistor the base-emitter junction must be forward biased by about 0.6 to 0.7 volts. In the active region the collector current is the base current multiplied by the gain, written beta.

• Arrow in the symbol marks the emitter and points along conventional current.
• Typical gain ranges from tens to hundreds.
• Emitter current is the sum of base and collector currents.

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