  In a ferroelectric field effect transistor (FeFET), it is generally assumed
that the ferroelectric gate plays a purely electrostatic role. Recently it has
been shown that in some cases, which could be called 'active FeFETs',
electronic states in the ferroelectric contribute to the device conductance as
the result of a modulation doping effect in which carriers are transferred from
the channel into the ferroelectric layers near the interface. Here we report
first-principles calculations and model analysis to elucidate the various
aspects of this mechanism and to provide guidance in materials choices and
interface termination for optimizing the on-off ratio, using BaTiO3/n-SrTiO3
and PbTiO3/n-SrTiO3 as prototypical systems. It is shown that the modulation
doping is substantial in both cases, and that extension of an electrostatic
model developed in previous work provides a good description of the transferred
charge distribution. This model can be used to suggest additional materials
heterostructures for the design of active FeFETs.
