The dominant electron trap D‐X center in AlxGa1−xAs has been clearly identified by us to consist of two closely related levels with thermal activation energies of 0.48 and 0.52 eV. Photoionization properties of the two levels have been explicitly measured by us for the first time. A new model for capacitance transients from modulation doped (MD) structures, which takes into account the heterointerface capacitance, is presented to explain the observed anomalous behavior of trap emission in MD structures. Both the thermal and the optical ionization properties of the two D‐X centers in MD structures are measured and discussed.