A systematic evaluation of the possibilities of low-voltage logic using carbon-based tunneling field effect transistors (TFET) is performed using the non-equilibrium green function (NEGF) formalism  for device simulation and ring-oscillator and load-driven inverter circuits. We found that the on-current is severely limited by quantum mechanical reflections due to wavefunction mismatch. The lower bound of subthreshold swing (SS) is limited by tunneling through the channel. Although the projected performance is well-below that of a recent prediction , we show that digital logic at VDD = 0.1V is possible. Finally, we compare the TFET with its MOSFET counterpart and show that for VDD=0.1V, the TFET out-performs the MOSFET.