  We revise the possibility of having an amplified surface potential in
ferroelectric field-effect transistors pointed out by [S. Salahuddin and S.
Datta, Nano Lett. 8, 405 (2008)]. We show that the negative-capacitance regime
that allows for such an amplification is actually bounded by the appearance of
multi-domain ferroelectricity. This imposes a severe limit to the maximum
step-up of the surface potential obtainable in the device. We indicate new
device design rules taking into account this scenario.
