Lyding et al. have recently reported significant improvements in the lifetime of metal oxide semiconductor ~MOS! transistors due to incorporation of deuterium ~D!, rather than hydrogen ~H!, at the Si/SiO2 interface. This remarkable achievement indicates that the Si–D bond is more resistant to hot-electron excitation than the Si–H bond. Lyding et al. pointed out that the phenomenon is probably analogous to the observed reduction in desorption of deuterium versus hydrogen from hydrogenated Si~100!:H surfaces using the scanning tunneling microscope ~STM!. In this comment, we propose a specific pathway for the dissociation of Si–H and Si–D bonds, providing a natural explanation for the difference in dissociation rates. Shen et al. have proposed that the low-voltage STMinduced desorption of Si–H bonds from Si~100! proceeds via a multiple-vibrational excitation by tunneling electrons. Electrons excite Si–H vibrational transitions with a rate proportional to the tunneling current. The extent to which vibrational energy can be stored in the bond depends on the lifetime, i.e., on the rate at which energy is lost by coupling to phonons. Because the lifetime of H on Si is long, efficient vibrational excitation is expected. In the quantitative analysis of Ref. 3, it was assumed that the vibrational energy is deposited in the stretch mode of the Si–H bond, which has a frequency around 2100 cm. The same assumption is usually implicitly made in discussions of dissociation of Si–H bonds. Our main purpose here is to point out that both the vibrational lifetime and carrier-enhanced dissociation mechanisms are most likely controlled by the Si–H bending modes. The vibrational frequency of the bending mode for Si–H is around 650 cm, and the estimated frequency for Si–D is around 460 cm. This value is close to the frequency of bulk TO phonon states at the X point ~463 cm21). We therefore expect the coupling of the Si–D bending mode to the Si bulk phonons to result in an efficient channel for deexcitation. While it is quite possible to reach a highly excited vibrational state in the case of Si–H, this will be more difficult for Si–D. Deuterium should therefore be much more resistant to STM-induced desorption and hot-electron induced dissociation, due to the relaxation of energy through the bending mode.