We demonstrate electrical control of ferromagnetism in field-effect transistors with a trilayer quantum well (QW) channel containing an ultrathin n-type ferromagnetic semiconductor (In, Fe) As layer. A gate voltage is applied to control the electron wave functions phi(i) in the QW, such that the overlap of phi(i) and the (In, Fe) As layer is modified. The Curie temperature is largely changed by 42%, whereas the change in sheet carrier concentration is two to three orders of magnitude smaller than that of previous gating experiments. This result provides an approach for versatile, low power, and ultrafast manipulation of magnetization.