MOSFETid

MOSFETid is an informal term used in some electronics texts to denote the drain current of a MOSFET, written as ID. The term highlights the gate-controlled current in a metal-oxide-semiconductor field-effect transistor. Although ID is the standard symbol for this current, MOSFETid is not a formal device name; it is a label sometimes used in teaching materials.

MOSFETid depends on the region of operation. In the triode region (small VDS), ID increases roughly linearly with VDS and with the gate overdrive (VGS – Vth). In saturation (VDS large enough), long-channel MOSFETs follow ID ≈ (1/2) μn Cox (W/L)(VGS – Vth)^2, with corrections for channel-length modulation. Short-channel devices show velocity saturation and deviations from the square-law. For PMOS devices, the same relationships hold with sign changes.

Factors affecting MOSFETid include gate overdrive, device geometry, carrier mobility, oxide capacitance, temperature, and body bias. The body effect alters Vth; current direction is defined by convention (drain-to-source is positive for NMOS; PMOS is opposite).

Measurement and use: ID is determined by applying voltages to the gate and drain and measuring the drain current. It is central to transistor characterization, analog amplifier design, and digital switching, and it relates to transconductance gm and output conductance gds.

In sum, MOSFETid is a colloquial way to refer to the MOSFET drain current ID in theory and circuits, not a separate quantity or device name.