Structure of a transistor
A bipolar transistor is made of three doped regions, giving two common types: npn and pnp. Its three terminals are the base (B), the collector (C) and the emitter (E). The base region is very thin and lightly doped so that it can control a much larger current between collector and emitter.
How it works
A small base current (I_B) controls a much larger collector current (I_C). By conservation of charge, the current leaving the emitter is the sum of the two:
Key formula / 关键公式
Emitter current: I_E = I_B + I_C
Current gain: β = I_C ÷ I_B
A large β means a small base current controls a large collector current.
Uses: switch and amplifier
As a switch, a small base current turns the transistor fully on, allowing a large collector current to flow (for example to light a bulb). A light-dependent resistor (LDR) or thermistor in the base circuit lets a transistor switch a device on automatically when it becomes dark or hot. As an amplifier, small changes in the base current cause large changes in the collector current, so a weak signal is made stronger.
Worked example / 例题
A transistor has a base current of 20 μA and a current gain β of 150. Find the collector current.
I_C = β × I_B = 150 × (20 × 10⁻⁶ A) = 3.0 × 10⁻³ A = 3.0 mA.
Remember / 记住
- Terminals: base, collector, emitter.
- A small base current controls a large collector current.
- I_E = I_B + I_C, and β = I_C ÷ I_B.