A new quantum‐confined Stark shift structure, the step‐coupled well, is introduced. The feature of the new structure is using a square well of only one quantum state to enhance the Stark shift of the step well. The design rule of 10‐&mgr;m modulator is suggested. The transfer matrix technique is used for numerical calculation. Numerical results show that the new structure produces a larger Stark shift (13 meV) than the step well (7 meV) under the same bias (20 kV/cm). A lower electric field is thus needed for the step‐coupled well to produce the same Stark shift, so that the leakage problem can be minimized and a high switching speed can be achieved.