We present measurements of the Shubnikov-de Haas (SdH) resistivity on GaAs/AlGaAs heterojunctions. We compare measurements in which in the same sample the electron density was increased to the same value either by applying a back-gate voltage or illumination. We conclude that both the overlap of the electron wavefunctions in the two-dimensional electron gas with positive and negative scattering centers and the number of positive and negative scatterers determine the asymmetry in the SdH-peaks. The asymmetry in the SdH-peaks depends on the electron density because the scattering on remote and background impurities have a different electron density dependence. After illumination the neutralization rate of the acceptors in the GaAs determines whether the asymmetry increases or decreases. We show that the asymmetry of the SdH-peaks can also be modified by applying uniaxial stress.