Nonlinear charge modeling for FET large-signal simulation and its importance for IP3 and ACPR in communication circuits

Nonlinear charge-storage (capacitance) characteristics are significant contributors to FET large-signal performance, determining, along with the well-known current-source nonlinearities, the figures of merit important for communication circuits such as PAE, IP3, and ACPR. This paper reviews the general theory and practice of nonlinear two port charge-storage modeling for FET devices. Topics covered include identification of nonlinear capacitances from measured data, transcapacitances, the modeling constraints of terminal charge conservation and energy conservation and their consequences, and the necessary and sufficient conditions for the construction of unique device-specific nonlinear charge-based models from experimental device data. This approach is largely independent of process and technology, and is therefore generally applicable to FET devices from Si JFETs and MOSFETs to GaAs and InP HEMTs. The fundamental theory and general methodology are independent of the explicit functional form for the capacitance constitutive (C-V) relations, so they apply equally well to closed form analytical empirical models and table-based models based on interpolation or approximation of measured device characteristics.