Semiconductor Physics: Bipolar Junction Transistor

What are the three currents in a bipolar junction transistor and how are they related?

What is the most commonly used transistor configuration?

What is the function of a transistor in the forward-active state?

In an NPN transistor, what happens when the emitter-base junction is forward biased?

What are the majority charge carriers in a PNP 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

Test your knowledge of semiconductor physics and the functioning of a bipolar junction transistor (BJT) formed by joining two individual p-n junction diodes. Understand the three regions of a BJT - Emitter, Collector, and Base, and how the transistor transfers signals from low resistance to high resistance regions.