Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2020 / Paper 1 / Section B / 9 / ii / Solution

A MOSFET is a voltage-controlled three-terminal switch: the insulated gate controls a conductive channel, giving high input impedance and useful current gain at the circuit level. A CMOS inverter actively restores either logic rail, has voltage gain near its switching threshold, and ideally draws no static supply current in either settled logic state. Real devices have leakage and consume energy while switching.

A single p-n junction is a two-terminal diode; its current is controlled by its own terminal voltage, so it does not by itself provide an independently controlled switch, gain and restoring inversion. Cross-coupled CMOS inverters form a bistable memory element. A lone inverter is not itself a complete memory cell, and a p-n junction can still serve useful rectification or capacitance functions.