A multilevel donor model is developed, which takes into account the negative charge and the two-electron occupation probability of DX centers and also considers coexisting hydrogenic states. By solving Schrodinger's and Poisson's equations self-consistently, this model is applied to calculate the sheet carrier density in metamorphic InxAl1-xAs/InxGa1-xAs HEMT structures on GaAs for indium contents x in the range 0.3 less than or equal to x less than or equal to 0.5. By comparing the calculations with reported Hall data, the experimentally observed reduction of free carrier density for compositions with x below 0.4 is attributed to the trapping at DX centers. The ionization energies of DX centers in the InxAl1-xAs system deduced for the investigated range of x are consistent with an extrapolation from data reported for compositions with 0 < x < 0.3. (C) 2002 Elsevier Science Ltd. All rights reserved.