Definition of Transistor

Transistor is a semiconductor device that can amplify or switch electronic signals and electrical power. Here's the working principle:

Basic Structure

ransistors (specifically bipolar junction transistors - BJTs) consist of three semiconductor layers forming either:

  • NPN (Negative-Positive-Negative)
  • PNP (Positive-Negative-Positive)

These layers create three terminals:

  • Emitter (heavily doped)
  • Base (thin and lightly doped)
  • Collector (moderately doped)

Working Principle

1.As a swithch

  • OFF State: When no current flows to the base (IB=0), only tiny leakage current flows from collector to emitter (transistor is "off")
  • ON State: When sufficient base current (IB) is applied, it allows much larger collector current (IC) to flow (transistor is "on")

2.As an Amplifier

  • A small base current controls a much larger collector current
  • The current gain (β or hFE) is typically 20-1000 for modern transistors
  • This allows small input signals to control larger output signals

    Detailed Operation (NPN Example)

    1.Forward Biasing (Base-Emitter Junction)

    • When voltage is applied to base (positive for NPN), the P-N junction between base and emitter becomes forward-biased
    • Electrons flow from emitter to base

    2.Current Control

    • Because the base is very thin, most electrons cross into the collector region rather than exiting through the base terminal
    • This creates the collector current that's much larger than the base current

    3.Reverse Biasing (Base-Collector Junction)

    • The collector-base junction is reverse-biased in normal operation
    • This helps sweep the electrons from the base into the collector

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