  The Curie temperature TC is investigated as a function of the hole
concentration p in thin films of ferromagnetic semiconductor (Ga,Mn)As. The
magnetic properties are probed by transport measurements and p is varied by the
application of an external electric field in a field-effect transistor
configuration. It is found that TC is proportional to p^{\gamma}, where the
exponent \gamma = 0.19 \pm 0.02 over a wide range of Mn compositions and
channel thicknesses. The magnitude of gamma is reproduced by a p-d Zener model
taking into account nonuniform hole distribution along the growth direction,
determined by interface states and the applied gate electric fields.
