Over the last decade, Si1-xGex has increasingly been used as a channel material in MOSFETs. Though many studies have dealt with the real bandstructure of Si1-xGex, the effect of germanium mole fraction x on complex bandstructure has been unexplored. Complex bands fundamentally determine band to band tunneling (BTBT) current. For example, using the orientation dependent complex bandstructure of silicon , it has been shown  that the BTBT current in the  direction is an order of magnitude larger than that along the  direction. BTBT contributes significantly to off-current Ioff in conventional MOSFETs, via the mechanism of gate induced drain leakage (GIDL). Additionally, BTBT determines the on current Ion in tunneling FETs, which have been suggested as next generation devices. Further, BTBT is more dominant in Si 1-xGex than silicon, owing to a narrower bandgap. In this work, we determine the orientation dependent complex bandstructure of Si 1-xGex along common crystallographic directions and predict trends in BTBT current. © 2011 IEEE.