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The frequency dispersion in admittance measurements in AlGaN/GaN high-electron-mobility transistors, which is typically interpreted to result from an interface trap density Dit, is also known to be caused by effects other than Dit. To study the origin of two peaks of conductance component "Gp/ω" as a function of frequency in lateral gated diodes using AlGaN/GaN structures on Si wafers grown by metal–organic chemical vapor deposition, we measured capacitance C and conductance Gp/ω–voltage V as a function of angular frequency ω (C–ω–V and Gp/ω–ω–V, respectively) of long-gate field-effect transistors with a varied gate length. We also simulated the C–ω–V and Gp/ω–ω–V curves using an equivalent circuit that consisted of actually measured component parameters without Dit. We confirmed that the Gp/ω–ω curves show two peaks caused by the two-dimensional electron gas channel resistance and the gate current leakage and quantitatively determined the two ωpeak positions and intensities. We also discussed the effect of acceptor concentration in the GaN channel-layer on lowering of ωpeak at weak inversion. [ABSTRACT FROM AUTHOR]