Differences between NPN and PNP Transistors
Though both NPN and PNP are bipolar junction transistors, they have more differences than similarities.
Since they are internally constructed very differently, current and voltage must be allocated differently in order for them to work. An NPN transistor receives positive voltage to the collector terminal and positive voltage to the base terminal for proper operation, while a PNP transistor receives positive voltage to the emitter terminal and a negative voltage at the base terminal.
Since voltage allocation is different, how current flow works to turn them on is also different. An NPN transistor is powered on when a sufficient current is supplied to the base of the transistor. Therefore, the base of an NPN transistor must be connected to positive voltage for current to flow into the base. However, a PNP transistor is totally the opposite. In a PNP transistor, current flows out of the base by giving the base terminal a more negative voltage than what is supplied to the emitter terminal. As long as the voltage at the base terminal is lower than at the emitter terminal in a PNP transistor, the correct biasing and negative current effect will be achieved.
Another difference between NPN and PNP transistors is that since voltage is allocated differently, they have opposite current flows at the output. In an NPN transistor, output current flows from the collector to the emitter. In a PNP transistor, output current flows from the emitter to the collector.