A transistor is made by joining one Positive Typed semiconductor in between two Negative Typed semiconductors, or one Negative Typed semiconductor in between two Positive Typed semiconductors. The first is called an NPN Transistor . The second is called a PNP Transistor . All transistors have three terminals, called (B)ase , (E)mitter, and (C)ollector . Therefore a transistor can be connected as a two port network.