The field effect has been investigated on the deep levels and DX centers in molecular‐beam epitaxilly‐grown AlxGa1−xAs onn+GaAs for several compositions. We have measured the isothermal and isofield capacitance transients for the major energy levels with the applied potential in the range from −0.1 to −3 V. In samples withx=0.5, two electron trap levels (E1andE2) were detected. WhileE1was strongly field dependent (changed from 0.499 to 0.287 eV)E2was practically unchanged (0.305 eV) with respect to energy, cross section, and peak broadening. Contrary to this in pure GaAs (samplesx=0), we observed three energy levels, one hole trap (E1=0.239 eV), one electron trap (E2=0.512 eV), and the usual EL2 trap (E3=0.72 eV). At this composition, however, levelsE1andE2were absent at low reverse fields (at −1 V) and EL2 remained unchanged at 0.72 eV. We have considered the recent theories of Pool–Frenkel effect and tunneling for electron–phonon interaction to interpret the results. In particular, for AlxGa1−xAs withx=0.5, the electron–phonon coupling factor (S) was found to be about 4 and the deep centers were identified as neutral to repulsive.