Compressively strained Si1-xGex band-to-band tunneling field effect transistors (TFETs) with planar structure are fabricated and analyzed. Different germanium concentrations of x = 0.35, 0.50 and 0.65 are investigated. An HfO2/TiN high-kappa/metal gate stack is used for a better electrostatic control. The fabricated Si0.5Ge0.5 devices show the highest on-state current I-on and the smallest subthreshold slope (4 mu A/mu m and 162 mV/dec, respectively), whereas the performance of Si0.35Ge0.65 TFETs is degraded due to partial strain relaxation. The influences of Ge content and elastic strain, via band gap narrowing and effective mass change on the band to band tunneling effect are discussed. Simulations using a nonlocal band-to-band-tunneling model indicate that the transistor performance is enhanced with increasing Ge content and strain, in agreement with our experimental results. (C) 2011 Elsevier Ltd. All rights reserved.