A transistor is a three-terminal semiconducting device that can be used to amplify
input signals. Figure 41-19 shows a generalized field-effect transistor (FET); in it,
the flow of electrons from terminal S (the source) leftward through the shaded
region to terminal D (the drain) can be controlled by an electric field (hence
field effect) set up within the device by a suitable electric potential applied to
terminal G (the gate). Transistors are available in many types; we shall discuss
only a particular FET called a MOSFET, or metal-oxide-semiconductor-field-effect
transistor. The MOSFET has been described as the workhorse of the modern
electronics industry.