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Herein, the wavelength dependence of the efficiency and the lifetime of far UV‐C light‐emitting diodes (LEDs) on a single‐crystal AlN substrate is systematically analyzed from experiments and simulations. A positive relation between efficiency and emission wavelength is observed. The wall‐plug efficiency reaches 0.47, 1.0, 2.1, and 2.4% at 227, 230, 233, and 235 nm wavelengths, respectively. The simulation results show that carrier injection plays a critical role in the efficiency change in the far UV‐C region rather than in the internal quantum or light extraction efficiency. The lifetime test clearly demonstrates a trade‐off between the L70 lifetime and the emission wavelength, although the initial optical power increases with the longer wavelength. The measurement results under various wavelengths and stress currents indicate that the carriers injected in quantum wells motivate power reduction during current stress. Moreover, the internal quantum efficiency affects the degradation rate because the influence of defects becomes stronger during stress. Finally, the reliability of a 231 nm LED is investigated at a reduced current of 35 mA. The projected L70 and L50 lifetimes are 11 000 and 28 000 h, respectively, from the fitting model based on the defect generation during aging. [ABSTRACT FROM AUTHOR]