A PNP transistor is a type of bipolar junction transistor (BJT) consisting of a layer of N-doped semiconductor (the base) between two P-doped layers (the emitter and collector). In a PNP transistor, current flows from the emitter to the collector when a negative voltage is applied to the base relative to the emitter. The transistor is 'on' when the base is more negative than the emitter, allowing holes (positive charge carriers) to move from the emitter to the collector. PNP transistors are commonly used in electronic circuits for switching and amplification. Their operation is complementary to NPN transistors, making them useful in push-pull amplifier designs and other applications requiring both types.