A Closed-Form Expression for the Frequency-Dependent Microwave Responsivity of Transistors Based on the I–V Curve and S-Parameters

A generic high-frequency model of two-port RF detectors, based on
$I$
–
$V$
curves and S-parameters measurements, is proposed and applied to the case of AlGaN/GaN field-effect transistors (FETs). The expression of the current responsivity (A/W) of any kind of transistor detector is derived for RF power injection through both the gate and the drain. The main novelty of the proposed model is the adequate consideration of the often neglected gate–drain coupling. The developed formalism also clarifies the voltage–current dc bias and the progressive and regressive power waves used to describe the RF excitation of the two-port nonlinear device. The obtained frequency-dependent closed-form expressions replicate very satisfactorily the current responsivity measurements made in AlGaN/GaN high-electron-mobility transistors (HEMTs) up to 67 GHz. To improve the physical understanding of the frequency dependence of the current responsivity of transistors, it is expressed in terms of the magnitude and phase of drain–gate voltage ratio. The analysis of different contributions to the RF responsivity reveals that the gate–drain capacitive coupling plays a key role in its frequency dependence.