This document discusses the operation and key characteristics of metal-oxide-semiconductor (MOS) devices including MOS diodes and MOS field-effect transistors (MOSFETs). It describes the different operating regions of MOS diodes such as accumulation, depletion, and inversion based on the applied gate voltage. It also explains how a MOSFET operates as a switching device when a positive gate voltage is applied for n-type operation. Important equations for threshold voltage, pinch-off effect, and current-voltage relationships in different operating regions are provided.