  We acoustically measured the energy level for thermally activated conduction
(TAC) in high-resistivity Fe-doped GaN using the non-contacting
antenna-transmission acoustic-resonance method. The acoustic attenuation takes
a maximum at a specific temperature, where the TAC is accelerated with the help
of phonon energy. The Debye type relaxation is thus observed for acoustic
attenuation, and its activation energy (0.54$\pm$0.04 eV) was determined with
attenuation measurements at various frequencies and temperatures. This value
agrees with the E3 level in GaN, indicating that thermally associated
conduction originates from the E3 trap level. We also measured the five
independent elastic constants at high temperatures.
