Compressively strained Si 1−x Ge x band-to-band tunneling field effect transistors with planar structure and HfO 2 /TiN gate stack have been produced and analyzed, with different Germanium concentrations of x = 0.35, 0.50 and 0.65. Simulations using a nonlocal band-to-band-tunneling model have been carried out to understand the switching behavior and its dependence on the material parameters. One would expect an increase of the tunneling currents for the increase of x. However, the Si 0.5 Ge 0.5 devices show the highest I on and smallest S, whereas Si 0.35 Ge 0.65 devices exhibit decreased currents due to partial strain relaxation.