MOSFET transistors have metal gates which are insulated from the semiconductor by a layer of SiO2 or other dielectric. In enhancement type MOSFETs, the application of a gate voltage activates the channel (by inducing a layer of carriers between source and drain under the gate, Figure 121). In depletion type MOSFETs, there is a small strip of semiconductor of the same type as that of the source and drain, and the gate voltage can either reduce (by depleting carriers) or increase (by increasing carriers) the channel current (Figure 122). In an n channel MOSFET, the conducting channel exists in a p type substrate.
Note the additional B terminal on the substrate, which is often connected directly to the source.
ANU Engineering - ENGN2211