Oscillatory structure is observed in thedI/dVandd2I/dV2characteristics of conventional GaAs/GaAlAs high electron mobility transistor samples at liquid‐helium temperature, which can be explained using a Fowler–Nordheim tunneling theory. The position of the oscillations allows a determination of the conduction‐band discontinuity, and the depth of the deep donor levels in the GaAlAs for high aluminum concentrations. The fit of the data gives a value of &Dgr;Ec/&Dgr;Eg=0.61±0.04 for aluminum concentration 30, 36, and 40%. The deep donor level in the GaAlAs was determined to be 130 meV below the conduction band.