The concentration of occupied deep centers in Si-doped AlxGa1-xAs for x=0.2 has been calculated from a three-level donor model, in which the shallow levels are treated as excited states of the deep (DX) ground state. The deep level is assumed to be tied to the L valley, and the shallow levels to the G and X valleys. The behavior of the free-electron density and the thermal activation energy as function of composition is in good agreement with experimental results reported in the literature. In this model of dependent donor levels the deep-level occupancy can be directly calculated without needing deep-level transient spectroscopy measurements. A two-level donor model is used to calculate the pressure dependence of the deep level from a hydrostatic pressure experiment on a GaAs/Al0.3Ga0.7As heterostructure reported in the literature. We assume a shallow level tied to the G valley and an arbitrary deep level which is not coupled to any of the conduction bands. The calculation of the position of the deep level relative to the G valley as a function of pressure confirms the coupling of the deep level to the L valley. In this dependent donor model no large compensation is needed to fit the experimental data.