Definition
A bipolar junction transistor built from three layers of semiconductor material in the order N-type, P-type, N-type. The outer N layers form the emitter and collector; the thin P layer in the middle forms the base. A small current flowing into the base allows a much larger current to flow from the collector to the emitter, so the device acts as either an amplifier or an electronic switch.
Plain English
A small electronic component with three connections that uses a tiny control signal on one leg to turn a much larger current on, off, or somewhere in between on the other two legs.
Context Anchor
Seen in aircraft electrical and avionics maintenance when reading circuit diagrams or troubleshooting electronic control circuits.
Derivation
The name comes from the layer order: Negative–Positive–Negative. The N layers are doped with extra free electrons (negative charge carriers) and the P layer with 'holes' (positive charge carriers). Knowing the letters describe the layer stack-up makes it easier to remember how the part is built and how it differs from a PNP transistor, which has the layers reversed.
Why Pilots Care
Maintenance technicians need to know which way an NPN transistor is wired and biased before troubleshooting or replacing it. Installing one backwards, or swapping it with a PNP, will stop the circuit from working and can damage other components on the board.
Analogy
Think of it like a water valve where a small hand pressure on the handle (the base current) controls a much larger flow through the pipe (the collector-to-emitter current).
Intuition Check
Do not read NPN as a brand name or part number by itself. It describes the internal layer arrangement and the way the transistor is normally turned on in a circuit.
Example Sentence 1
The technician traced the fault to a failed NPN transistor on the landing gear indicator board and replaced it with the same part number.
Example Sentence 2
During troubleshooting, the mechanic confirmed the NPN transistor allowed current to flow only when the base received the proper signal.