Considerable modulation ratios are achieved for GaAs multiple‐quantum‐well reflector modulators grown on Si by inserting an AlAs/AlGaAs dielectric mirror into the device structure. Modulation ratios of up to 4:1 is attained as the external bias voltage is increased to 9 V and the 1C‐1HH exciton absorption peak undergoes quantum‐confirmed Stark shift. Measurements also indicate that cavity effects arising from the front surface reflection and that of the imbedded dielectric mirror strongly modify the reflectivity spectra.