A bipolar junction transistor, or BJT, is a three-layer semiconductor device used to control electric current. It is one of the basic building blocks of amplifiers, switches, logic circuits, and power control systems. The name bipolar means that both electrons and holes take part in conduction.
Understanding BJTs helps students connect semiconductor physics to practical electronic engineering.
Key Facts
- A BJT has three terminals: emitter, base, and collector.
- For an NPN transistor in active mode, I_E = I_C + I_B.
- Current gain is beta = I_C / I_B.
- In active mode, the base-emitter junction is forward biased and the base-collector junction is reverse biased.
- For a silicon BJT, V_BE is usually about 0.7 V when the transistor is on.
- As a switch, cutoff means nearly no collector current and saturation means maximum collector current limited by the circuit.
Vocabulary
- Emitter
- The heavily doped region of a BJT that injects charge carriers into the base.
- Base
- The thin, lightly doped middle region that controls how many carriers pass from emitter to collector.
- Collector
- The region of a BJT that collects most of the charge carriers that cross the base.
- Current gain
- The ratio of collector current to base current, usually written as beta = I_C / I_B.
- Saturation
- The switching state in which a BJT is fully on and the collector current is limited mainly by the external circuit.
Common Mistakes to Avoid
- Confusing emitter and collector is wrong because they are built with different doping levels and are not usually interchangeable in a real circuit.
- Assuming the base current is zero is wrong because a BJT is current controlled and needs a small base current to support collector current.
- Using beta as a fixed exact value is wrong because transistor gain changes with temperature, current, device type, and manufacturing variation.
- Treating a saturated transistor like an ideal short circuit is wrong because it still has a small collector-emitter voltage drop and power dissipation.
Practice Questions
- 1 An NPN transistor has beta = 120 and base current I_B = 40 microamperes. Find the collector current I_C in milliamperes, assuming active mode.
- 2 A transistor switch drives a 200 ohm load from a 5.0 V supply. If the saturated collector-emitter voltage is 0.2 V, find the load current.
- 3 Explain why a BJT can act as both an amplifier and a switch, and describe how the operating region changes between these two uses.