  A theoretical model of high-T_c Josephson Field Effect Transistors (JoFETs)
based on a Ginzburg-Landau free energy expression whose parameters are field-
and spatially- dependent is developed. This model is used to explain
experimental data on JoFETs made by the hole-overdoped Ca-SBCO bicrystal
junctions (three terminal devices). The measurements showed a large modulation
of the critical current as a function of the applied voltage due to charge
modulation in the bicrystal junction. The experimental data agree with the
solutions of the theoretical model. This provides an explanation of the large
field effect, based on the strong suppresion of the carrier density near the
grain boundary junction in the absence of applied field and the subsequent
modulation of the density by the field.
