In one line
A BJT is a three-layer semiconductor device (n-p-n or p-n-p) with an emitter, a base and a collector forming two p-n junctions; the emitter is heavily doped, the base is thin and lightly doped, and the collector is moderately doped and largest.
Explained simply
Think of a BJT as a crowd passing through a thin gate. The emitter is a packed stadium entrance that sends out lots of people (charge carriers). The base is a very thin, nearly empty gate area, so almost nobody stops there. The collector is a big open ground that collects nearly everyone. A tiny "gatekeeper" signal at the base controls the huge flow from emitter to collector.
Technically, "bipolar" means both electrons and holes take part in conduction. The two junctions are the emitter-base junction (\( J_E \)) and the collector-base junction (\( J_C \)). In the symbol, the arrow on the emitter shows the direction of conventional current: pointing out for NPN, pointing in for PNP.
How it works
| Region | Doping | Size | Reason |
|---|---|---|---|
| Emitter | heavy | moderate | to inject a large number of carriers into the base |
| Base | light | very thin | so that very few injected carriers recombine; most pass to the collector |
| Collector | moderate | largest | to collect carriers and dissipate the heat generated at \( J_C \) |
- NPN: a thin p-type base sandwiched between n-type emitter and n-type collector. Majority carriers are electrons.
- PNP: n-type base between p-type emitter and collector. Majority carriers are holes. All voltages and currents are reversed.
- Because emitter and collector differ in doping and size, they cannot be interchanged even though both are the same type.
- The transistor is usually made of silicon; NPN is more common because electrons move faster than holes.
Key formula
\( I_E = I_B + I_C \)
\( I_E \) = emitter current, \( I_B \) = base current, \( I_C \) = collector current, all in amperes (A), usually mA or µA. This follows from KCL applied to the transistor as a node.
Quick example
Common small-signal transistors such as the BC547 (NPN) and BC557 (PNP) are used in hobby circuits for switching LEDs and amplifying signals. The 2N3055 is a larger power transistor whose collector is connected to its metal case to help remove heat.
Common mistakes
- Saying emitter and collector can be swapped because they are both n-type in an NPN transistor.
- Saying the base is heavily doped.
- Drawing the emitter arrow pointing inward for an NPN transistor.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the construction of NPN and PNP transistors with symbols" (5 marks). For full marks draw both layer structures with E, B, C and junctions labelled, draw the symbols with correct arrow directions, and explain the doping and size of each region with reasons.