Definition
A bipolar junction transistor made of three semiconductor layers arranged Positive–Negative–Positive, with a thin N-type base sandwiched between a P-type emitter and a P-type collector. It is turned on by drawing a small current out of the base, which then allows a much larger current to flow from the emitter to the collector. In a PNP, the emitter is normally held at a higher voltage than the collector, so conventional current flows from emitter to collector when the transistor is on.
Plain English
A small electronic switch or amplifier built from three layers of specially treated material. A tiny signal at one terminal lets a much bigger current flow through the other two terminals. The PNP version is wired so that it switches on when its control terminal is pulled to a lower voltage than the input side.
Context Anchor
Seen in aircraft electrical and avionics maintenance, especially when reading circuit diagrams or troubleshooting electronic components.
Derivation
The name describes the layer order of the semiconductor material inside the device: P-type, then N-type, then P-type. P stands for positive (material with a surplus of positive charge carriers, called holes), and N stands for negative (material with extra electrons). Knowing the layer order tells you which way current flows and which way the device must be wired.
Why Pilots Care
Maintenance technicians need to identify PNP versus NPN transistors correctly when troubleshooting or replacing components. Installing the wrong type, or wiring a PNP backwards, will prevent the circuit from working and can damage the part or the equipment around it.
Analogy
Think of it like a small electrical valve. A small control action at one connection can allow or stop a larger flow through the other two connections.
Intuition Check
PNP does not mean “plug and play.” It names the positive-negative-positive layer arrangement inside the transistor.
Example Sentence 1
The schematic showed a PNP transistor controlling the landing light relay, so the technician confirmed the emitter was connected to the positive supply.
Example Sentence 2
A failed PNP transistor prevented the landing light circuit from receiving power during the preflight check.