How does a gate voltage control current between the source and drain of a MOSFET?
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.
In a metal-oxide-semiconductor field-effect transistor (MOSFET), the gate voltage controls the flow of current by creating or modulating a conductive channel between the source and drain.12
In an n-channel MOSFET, the source and drain are n-type regions embedded in a p-type semiconductor substrate. When no voltage is applied to the gate, the device is typically in an "off" state because the p-n junctions between the source, drain, and substrate block current flow.321
When a positive voltage is applied to the gate, an electric field penetrates the insulating silicon dioxide layer. This field repels holes and attracts electrons to the semiconductor surface directly beneath the gate. Once the voltage reaches a specific threshold, these accumulated electrons form a conductive "inversion layer" or channel that connects the source and drain. This allows current to flow between them. The conductance of this channel—and thus the amount of current—can be precisely modulated by varying the gate voltage. A similar, inverse process occurs in p-channel MOSFETs using negative gate voltages.1324