Bipolar Junction Transistors
The bipolar junction transistor, or BJT, is a three-terminal electronic device with high current density made by doped semiconductor material. It was named because its operation involves conduction by two carriers: electrons and holes in the same crystal. The first bipolar transistor was invented at Bell Labs by William Shockley, Walter Brattain, and John Bardeen so late in 1947 that it was not published until 1948.
Two variants of BJT are possible: NPN (base is made of p-type material) and PNP (base is made of n-type material). The NPN BJT is a three layer semiconductor sandwich with an emitter and collector at the ends, and a base in between. It is as if a third layer were added to a two layer diode.
Discrete PNP transistors are almost as high quality as the NPN counterpart. However, integrated PNP transistors are not nearly as good as the NPN variety within the same integrated circuit die. Thus, integrated circuits use the NPN variety as much as possible.