In this letter we report the Arrhenius‐type dependence of the valley current density on inverse temperature in a triple‐well asymmetric resonant tunneling diode. The activation energy is found to be equal to the effective phonon energy representing the AlAs and GaAs‐like mode of AlAs in the AlxGa1−xAs barrier layers of a triple well asymmetric resonant tunneling (RT) diode operating at temperatures as high as 350 K. The data presented here correlate the longitudinal optical phonons to the excess valley current found in AlxGa1−xAs/GaAs RT devices between 200 and 350 K and raise the possibility of phonon spectroscopy of high quality barrier layers via resonant tunneling transport experiments.