Mechanisms of spontaneous recovery in DC gate bias stressed power VDMOSFETs

Abstract

The effects of spontaneous recovery on threshold voltage and channel carrier mobility in DC gate bias stressed power VDMOSFETs, as well as the underlying changes in gate oxide-trapped charge and interface trap densities, are analysed in terms of the mechanisms responsible. Various mechanisms, including drift of oxide-trapped charge to the SiO2–Si interface, charge neutralisation, interface trap redistribution and electrochemical reactions near the interface are considered, in order to explain the experimental results. A major difference in the results for post-stress recovery obtained by subthreshold midgap and single transistor mobility techniques, both indicating mostly decrease in interface trap and oxide-trapped charge densities and the charge pumping technique, which signified a remarkable post-stress increase of true interface trap density, is ascribed to both interface trap redistribution within the silicon bandgap and to the inability of the former two techniques to assess the contribution of border traps to the estimated densities of interface traps. Drift of positive oxide charge to the interface, accompanied by subsequent charge neutralisation and interface trap redistribution, is found to dominate in the early stage of post-stress recovery, whereas the chain of electrochemical reactions involving hydrogen species (passivation of interface traps, dissociation of interfacial trap precursors, hydrogen dimerisation and cracking at charged oxide traps) is shown to become more important in the advanced stage of recovery.