Small-Signal Equivalent Circuit Model as a Tool for Optimizing Millimeter-Wave Detection With FETs

The responsivity of sub-THz zero-bias detectors based on GaN high-electron mobility transistors (HEMTs) is measured up to 110 GHz. A predictive model based on static coefficients extracted from the dc output curves together with $\textit{S}$ -parameter measurements is able to replicate the detection results both when the radio frequency power is fed into the drain or the gate terminals. The gate-drain coupling, counteracting the contribution of the direct modulation of the drain voltage to the RF responsivity, is at the origin of the frequency roll-off of the detection, and therefore, the frequency performance is improved when shrinking the gate size of the transistors. The extraction of the small-equivalent circuit allows understanding the influence of the parasitic elements and the scaling of the gate width of the transistors on the detection performance, as well as extrapolating the results of the measurements to predict the performances at higher frequencies. In addition, the analysis of the influence of the different equivalent-circuit elements allows linking the values of the responsivity and noise equivalent power (NEP) to the physics of the transistor operation and thus determining at which frequency the detection roll-off appears as a function of the gate length and width, and confirming that both must be reduced as much as possible to optimize the transistor high-frequency operation, the latter having a more significant influence.