Tunnel field-effect transistors (TFETs) exhibiting a minimum subthreshold swing (SS) of 27 mV/decade were successfully fabricated using conventional planar HfO2/Si-gate-stack structures. However, an unexpected SS degradation with increasing equivalent oxide thickness (EOT) was observed compared with the simulated results obtained under the assumption of ideal band-to-band tunneling. We found that the poor subthreshold operation was governed by a thermally activated process, suggesting trap-assisted tunneling that occurs with traps near the metallurgical pn junction. Furthermore, we discuss the effect of the observed EOT-sensitive SS degradation on device production. (C) 2014 The Japan Society of Applied Physics