what's the difference between a npn and pnp transistor?

on photo eye sensors

NPN

The symbol of an NPN Bipolar Junction Transistor.NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors.

NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output.

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

[edit] PNP

The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor. Few transistors used today are PNP, since the NPN type gives better performance in most circumstances.

The symbol of a PNP BJT.PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. PNP transistors are commonly operated with the collector at ground and the emitter connected to a positive voltage through an electric load. A small current entering the base prevents current from flowing between the collector and emitter.

http://en.wikipedia.org/wiki/npn#npn...