MOS Field-Effect Transistors (MOSFETs)

Having studied the junction diode. which is the most basic two-terminal semiconductor device, we now turn our attention to three-terminal semiconductor devices. Three-terminal devices are far more useful than two-terminal ones because they can be used in a multitude of applications, ranging from signal amplification to digital logic and memory. The basic principle involved is the use of the voltage between two terminals to control the current flowing in the third terminal. In this way a three-terminal device can be used to realize a controlled source, which as we have learned in Chapter I is the basis for amplifier design. Also, in the extreme. the control signal can be used to cause the current in the third terminal to change from zero to a large value. thus allowing the device to act as a switch. As we also