How it works
In an N-channel enhancement MOSFET, applying positive VGS greater than the threshold voltage VTN induces an n-type channel in the p-substrate under the gate oxide, connecting drain and source. In the linear (triode) region where VDS < VGS − VTN, drain current ID = kn[(VGS − VTN)VDS − VDS²/2], and the device behaves as a voltage-controlled resistor. In the saturation region where VDS ≥ VGS − VTN, ID = (kn/2)(VGS − VTN)², independent of VDS — this is where MOSFET amplifiers operate. For depletion MOSFETs, a channel exists at VGS = 0 and can be pinched off by applying negative VGS (for N-channel). The transconductance gm = ΔID/ΔVGS = kn(VGS − VTN) determines small-signal gain.
Key points to remember
Enhancement MOSFETs (E-MOSFET) need VGS > VTN to create a channel — normally off. Depletion MOSFETs (D-MOSFET) have a built-in channel and are normally on. For N-channel E-MOSFET, VTN is positive (typically 1–4 V); for P-channel, VTP is negative. Saturation region drain current: ID = (kn/2)(VGS − VTN)², and the device pinches off when VDS = VGS − VTN. Body effect occurs when source is not connected to substrate: threshold voltage increases as VSB increases — VTN(VSB) = VTN0 + γ(√(2φF + VSB) − √(2φF)). Gate oxide capacitance Cox and carrier mobility μn determine kn = μnCox(W/L), making W/L the primary design parameter for MOSFET current.
Exam tip
Every university exam asks you to identify the region of operation (linear or saturation) of a MOSFET given VGS, VDS, and VTN — check whether VDS ≥ VGS − VTN for saturation, and write the correct ID equation for that region.