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A room-temperature method to determine the trap energy levels in the carbon-doped (Al)GaN buffers is developed via a transient current measurement on the AlGaN/GaN high electron mobility transistors under back-gate voltages combined with a measurement of the buffer vertical leakage. Under high back-gate voltages, a linear relationship is obtained between the trap energy levels and the square roots of electric field strength, suggesting that the vertical conduction in the C-doped buffer follows the Poole–Frenkel law. The trap energy level in C-doped Al0.07Ga0.93N is finally determined to be 1.1 eV through the established room-temperature approach, while that in C-doped GaN is extracted to be 0.9 eV, both of which are related to the carbon impurities. [ABSTRACT FROM AUTHOR]