We have found a new degradation mode for threshold voltage (V-th) shift in metal oxide semiconductor field effect transistors (MOSFETs). The electrical properties of MOSFETs, MOS capacitors, and n(+) -p junction diodes revealed that the threshold voltage shift was caused by the introduction of donor impurities into the Si substrate just under the gate oxide in the gate oxidation process step. Secondary ion mass spectroscopy analysis showed that the donor was a thermal donor formed by an extremely high concentration (similar to 5 x 10(18) cm(-3)) of oxygen. The mechanism was discussed briefly from the viewpoint of thermal stress in device processing.