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Since this is in the electronics section, I'll assume you mean a PNP transistor. PNP refers to the semiconductor layers used to make up the transistor. The P and N letters refer to positive and negative doping in the semiconductors, which changes the way electrons flow through the layers. This is a bipolar junction transistor (BJT), which can be thought of as a current amplifier. One P layer is called the emitter and the other is called the collector. The N layer is called the base. The emitter and collector are not usually interchangeable because they are made differently. When the voltage on the base is below the voltage on the emitter, usually by about 0.7 Volts, current flows from emitter to base. This allows a much larger current to flow from emitter to collector. This assumes the collector is at a lower voltage than the base, and is the usual connection. Similarly, there are NPN transistors.

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17y ago

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