Gating of a molecular transistor: Electrostatic and conformational

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.