We have developed a highly sensitive method, thermally stimulated voltage recovery (TSVR), to investigate traps responsible for the threshold voltage shift (Delta V-th) in gate oxides of metal-oxide-semiconductor field effect transistors (MOSFETs). TSVR provides the contribution of traps with energy level E-t to Delta V-th just after trapping events [Delta V-thi(E-t)]. We applied TSVR to stressed MOSFETs with SiO2 gate oxides. The traps with several Et were observed and Delta V-thi(E-t) showed significant dependence on the gate length, reflecting the influence of fabrication processes on traps. (C) 2012 Elsevier Ltd. All rights reserved.