The transistor

A transistor is a semiconductor device that, for the purpose of this discussion, functions as a digital switch. This switching operation is electrically equivalent to changing between very high and very low resistance based on the state of an input signal. One important feature of switching transistors is that the switching input does not need to be very strong. This means that a very small current at the switching input can turn on and turn off a much larger current passing through the transistor. A single transistor's output current can drive many other transistor inputs. This characteristic is vital to the development of complex digital circuits.

Figure 2.2 shows the schematic diagram of the NPN transistor. NPN refers to the construction of the interconnected silicon regions that make up the transistor. An N region of silicon has material added to it (using a process called doping) that increases the number of electrons present, making it somewhat negatively...