The behavior of GaN-based high-electron-mobility transistors (HEMTs) as microwave zero-bias detectors is very dependent on the configuration of the bias (current or voltage), the operation temperature, and whether the radio-frequency power is fed in the drain or the gate terminal. When the signal is injected into the drain, the negative current responsivity shows a bell-shape dependence on VGS centered slightly above the threshold voltage for all the studied frequencies (1–43 GHz) and temperature ranges (8–400 K). In the case of the voltage responsivity, depending on the temperature range (associated with the presence or absence of drain leakage current), an increase or decrease of the responsivity is observed in subthreshold conditions. For the gate-injection (GI) configuration, as expected by the capacitive gate–drain coupling, the voltage responsivity at low frequency is null, but only for V_GS above threshold. Surprisingly, in subthreshold conditions, it is very high and positive, contrary to the negative values intuitively expected for this configuration. The origin of this unexpected behavior, taking place in both gate and drain injection (DI) configurations, is that the drain terminal is self-biased at the zero-current operating point ( V_DS being largely negative). An analytical model based on static coefficients obtained from dc measurements can explain the mechanisms behind the observed dependencies of the experiments.