NPN versus PNP

A simplified drawing of the NPN transistor, and the schematic symbol of it, is shown in the figure given below. The P-type, or center, layer is called as base. The thinner of N-type semiconductors is emitter, and thicker is collector. At times these are labeled B, E, and C in the diagrams although the transistor symbol is enough alone to tell you which is which. A PNP bipolar transistor is opposite of an NPN device, having 2 P-type layers, one on either side of a thin, N-type layer. The emitter layer is thinner, in most of the units, than collector layer.

You can always tell whether bipolar transistor is NPN or PNP. With the NPN, the arrow points outward; with PNP it points inward. The arrow is always at emitter.

Generally, PNP and NPN transistors can do same things in electronic circuits. The only difference is the polarities of voltages, and directions of currents. In most of the applications, an NPN device is replaced with the PNP device or vice versa, and power supply polarity reversed, and circuit will still work as long as new device has the suitable specifications.

Figure   At point A, pictorial diagram of an NPN transistor. At point B, the schematic symbol. Electrodes are E-emitter, B- base, C-collector.

Figure: At point A, pictorial diagram of a PNP transistor. At point B, schematic symbol; E emitter, B base, C collector.

There are different kinds of NPN or PNP bipolar transistors. Some are used for the radio frequency amplifiers and oscillators; others are intended for the audio frequencies. Some can handle high power, others cannot, being made for the weak-signal work. Some bipolar transistors are manufactured for purpose of switching, instead of signal processing.  If you look through the catalog of semiconductor components, you will find hundreds of different bipolar transistors, each with its unique set of specifications.

There be two different types of bipolar transistor (NPN and PNP), if they do exactly same things? At times engineers are required to have both kinds in one circuit. Also, there are some differences in behavior between the 2 types. These considerations are beyond scope. But you should know that NPN/PNP duality is not just whimsy on part of people who want to make the things complicated.