We report on a simulation for terahertz aluminum gallium nitride (AlGaN)/gallium nitride (GaN) resonant tunneling diode (RTD) at room temperature by introducing deep-level defects into the polarized AlGaN/GaN/AlGaN quantum well. Results show that an evident degradation in negative-differential-resistance characteristic of RTD occurs when the defect density is higher than ∼106 cm−2, which is consistent with the measurements of the state-of-the-art GaN RTDs. At around 300 GHz, the simulation for a RTD oscillator also demonstrates evident decreases of rf power and efficiency because of the electron trapping effect.