Multi-input intrinsic and extrinsic field effect transistor models beyond cutoff frequency

•Drift electron transport is used to model linear FET beyond cutoff for THz detectors.•The effect of multiple AC input signals and their coupling efficiencies were derived.•Formation of symmetry point and parameters affecting it were derived analytically.•These results can be used to avoid symmetry point operation in FET THz detectors.•Symmetry point can be used for characterization of the FET.

This work expands the classical theory of operation of FETs beyond cutoff frequency. Using an electron drift transport model, the responsivity of a FET working in the linear region of operation within the semi-classical transport region is derived. Different DC circuit configurations are included in the analysis, as well as multiple AC input signals. Separating the model into intrinsic and extrinsic parts enables better analysis of the effect of multiple input AC signals. A new treatment of the concept of symmetry/asymmetry in FETs beyond the cutoff frequency is presented. The origin of symmetry and factors affecting it are analyzed. The effects of asymmetries from various origins within the FET system are taken into account. The analytical results are used to qualitatively explain published experimental results of symmetry points in FET THz detectors.