  We derive a general result that can be used to evaluate and compare the
transconductance of different field-effect mechanisms in molecular transistors,
both electrostatic and conformational. The electrostatic component leads to the
well-known thermal limit in the absence of tunneling. We show that in a
standard three-terminal geometry and in the absence of strong electron-phonon
coupling, the conformational component can lead to significant advantages only
if the molecular dipole moment \mu is comparable to et_ox, t_ox being the
thickness of the oxide. Surprisingly this conclusion is independent of the
``softness'' of the conformational modes involved, or other geometrical
factors. Detailed numerical results for specific examples are presented in
support of the analytical results.
