Which types of bipolar junction transistors exist?

Bipolar junction transistors, commonly referred to as BJTs, exist primarily in two types: NPN and PNP. These classifications are based on the arrangement of the semiconductor materials within the transistor.

In an NPN transistor, a layer of p-type material (positive) is sandwiched between two layers of n-type material (negative). This structure allows for efficient electron flow from the n-type materials through the p-type material when the transistor is in operation. The majority charge carriers in the NPN transistor are electrons.

Conversely, in a PNP transistor, a layer of n-type material is positioned between two layers of p-type material. In this case, the majority charge carriers are holes, which are the absence of electrons and can be considered positive charge carriers. Current flows in a PNP transistor when there is a sufficient positive voltage at the base to allow holes to flow from the emitter (p-type) to the collector (also p-type).

Understanding the characteristics of NPN and PNP transistors is fundamental when studying electronic circuits, as they serve different purposes and have different operational principles.