In this work, by means of Monte Carlo (MC) simulations, the current responsivity of AlGaN/GaN HEMTs operating as zero-bias detectors is analyzed, reaching the THz frequency range. Two approaches are used for the calculation of the responsivity, trying to get physical information on the detection mechanisms from the comparison of their respective results. First, we determine the responsivity directly from the MC values of dc drain current shift originated by an input ac drain voltage excitation. As second approach, the responsivity is estimated from a closed-form expression involving the MC calculation of both the dc I–V curves of the transistor (to determine Taylor expansion coefficients) and the ac response in terms of the Y parameters (to be converted into S parameters). Both approaches coincide at low frequency, but the closed-form expression starts to deviate from the correct result at frequencies around 1 THz. Moreover, a modest plasma resonance is visible in the 2–5-THz range.