Semiconductor Physics: Bipolar Junction Transistor

Bipolar Junction Transistor (BJT)

• A BJT is formed by joining two individual p-n junction diodes back-to-back, creating a two-junction, three-terminal device.
• A BJT consists of three regions: Emitter, Collector, and Base.

Transistor Function

• The transistor transfers a signal from low resistance region to high resistance region.
• This ability to change between two states enables the transistor to have two basic functions:
  • "Switching" (digital electronics)
  • "Amplification" (analogue electronics)

Bipolar Transistor Construction

• The direction of the arrow in the symbol always points from the positive P-type region to the negative N-type region.
• The arrow direction is the same as for the standard diode symbol.

Emitter, Base, and Collector

• Emitter:
  • A region on one side of the transistor
  • Moderate in size
  • Heavily doped (to increase the current)
  • Supplies a large number of majority charge carriers
• Base:
  • The central region of the transistor
  • Thin in size
  • Lightly doped (to reduce recombination)
• Collector:
  • The largest region in size
  • Moderately doped at one end of the transistor
  • Collects a majority of majority charge carriers