There is a fundamental tradeoff between conductance and subthreshold swing voltage in tunnel field effect transistors that achieve a sharp turn off by modulating the tunnel barrier thickness. At high conductivities, the voltage bias has little control over the tunneling probability. Unfortunately, this results in a poor subthreshold swing voltage at high conductivities. In tunnel field effect transistors, the best sub 60mV/decade results occur only at very low current densities around 1nA/micrometer. At higher current densities the subthreshold swing voltage is observed to be much worse than 60mV/decade. We show that this is an inherent problem in quantum barrier thickness modulation, and that a different mechanism, band-edge energy filtering, is needed.