Band to band tunneling in semiconductors fundamentally depends of the complex bandstructure of the semiconductor within its bandgap. In this work, the orientation dependent complex bandstructure of InSb, InAs, GaSb, InP and GaAs, and the orientation dependent probability of band to band tunneling in these materials have been computed using an sp3d5s* tight binding model. These direct bandgap III-V materials are attractive candidates for Tunnel FETs. Comparison of our results with Kane's two-band model commonly used in TCAD simulation, demonstrates the inaccuracies of the latter. Our results will be useful in the design of better performing Tunnel FETs in these materials. © 2012 SPIE.