In the early days, PNP bipolar transistors were common, but the bulk of circuits you see today use NPN transistors. As [Aaron Danner] points out, many people think PNP transistors are “backward ...

There are numerous differences between NPN and PNP transistors, and even though both are bipolar junction transistors, the direction of current flow is the name of the game.

Last week, in a remarkable achievement, a team led by researchers at the Massachusetts Institute of Technology (MIT) in Cambridge reported building a transistor that is switched by a single photon.