The variation of the electron carrier occupation and the mobility in the subband as a function of the spacer layer thickness in modulation-doped AlxGa1-xAs/InyGa1-yAs/GaAs strained single quantum wells was investigated by Shubnikov-de Haas (S-dH) and Van der Pauw Hall-effect measurements. The results of the fast Fourier transform (FFT) for the S-dH data and those of the Hall-effect data showed that the magnitude of the electron carrier density in the sub-band increased as the spacer layer became thinner, and the increase in the carrier density with decreasing spacer-layer thickness resulted from an increase in the distance between the Fermi energy level and the top of the depletion layer. The full width at half maxima of the FFT results for the S-dH data and of the results of the Hall-effect measurements indicated that the value of the electron mobility increased as the spacer-layer thickness increased, and the increase in the electron mobility originated from a decrease in the Coulomb interaction between the ionized donors and the electrons, The electronic sub-band energies, corresponding wave functions, and the Fermi energies in the InyGa1-yAs quantum wells were calculated by a self-consistent method taking into account exchange-correlation effects together with the strain and nonparabolicity effects. These present results can help to improve the understanding for the application of AlxGa1-xAs/InyGa1-yAs/GaAs strained single quantum wells in electronic devices such as high-frequency and high-speed field-effect transistors.