Dangling bonds with “negative Hubbard U  ”: Physical model for degradation of SiO2 gate dielectric under voltage stress

A model proposed to explain the phenomenon of current increase and its fluctuation under voltage stress in a SiO2 gate dielectric is based on the amorphous nature and presence of dangling bonds in SiO2. Dangling bonds D0 are thought to be negative-U centers, where their neutral state is unstable and therefore a spontaneous reaction of charge disproportionation take place: D0+D0→D++D−. As a result, a SiO2 amorphous network has diamagnetic positive D+ and negative D− centers. Due to a large difference in mobility between electrons and holes, hopping current in SiO2 is mainly electron current on D+ centers. Current increase and fluctuation under a voltage stress is due a conversion of D− into D+ centers by the hole component of current through SiO2 gate dielectric. This conversion is an irreversible process accelerated by temperature and electric field.