Donor binding energies in quantum well systems using position dependent effective masses are obtained. Results are provided for GaAs-Ga1- Al xAs and the semimagnetic CdTe-Cd1-x Mn xTe systems. In the latter the barrier height reduces in a magnetic field. Using the available experimental data the variations of the barrier height with magnetic field has been obtained using a simple model. Our results are in good agreement with similar works in the literature in the case of constant effective mass for the donor electron. However, in the case of the semi magnetic system and in the case of the position dependent effective mass, the results obtained are shown to be appreciably different for narrow well dimensions. The validity of the effective mass approximation in the case of applied magnetic field is also critically examined.