NPN versus PNP

THE WORD TRANSISTOR IS A CONTRACTION OF “CURRENT-TRANSFERRING RESISTOR.” A BIPOLAR transistor has two P-N junctions. There are two configurations: a P-type layer sandwiched between two N-type layers, or an N-type layer between two P-type layers.
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At A, pictorial diagram of an NPN transistor. At B, the schematic symbol. Electrodes are E = emitter, B = base, and C = collector.
A simplified drawing of an NPN bipolar transistor is shown in above figure A, and the schematic symbol is shown in above figure B. The P-type, or center, layer is called the base. One of the N-type semiconductor layers is the emitter, and the other is the collector. Sometimes these are labeled B, E, and C in schematic diagrams. A PNP bipolar transistor has two P-type layers, one on either side of a thin N-type layer (below figure A). The schematic symbol is shown in below figure B.
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At A, pictorial diagram of a PNP transistor. At B, the schematic symbol. Electrodes are E = emitter, B = base, and C = collector.
It’s easy to tell whether a bipolar transistor in a diagram is NPN or PNP. If the device is NPN, the arrow at the emitter points outward. If the device is PNP, the arrow at the emitter points inward. Generally, PNP and NPN transistors can perform the same functions. The differences are the polarities of the voltages and the directions of the resulting currents. In most applications, an NPN device can be replaced with a PNP device or vice versa, the power-supply polarity can be reversed, and the circuit will work in the same way—as long as the new device has the appropriate specifications.