Spontaneous Recovery in DC Gate Bias Stressed Power VDMOSFETs

Spontaneous recovery of 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 presented and analysed in terms of the mechanisms responsible. A chain of mechanisms related to a presence of hydrogen species is proposed to explain the observed changes of oxide-trapped charge and interface trap densities. Dominant mechanisms for the decrease of interface trap and oxide-trapped charge densities (as observed during the recovery by means of charge separation techniques based on device transfer characteristics) are found to be passivation of interface traps (equivSi_sdot) due to their reaction with hydrogen atoms and neutralization of charged oxide traps (equivSi_o ⁺ or =equivSi_o ⁺ Si_oequiv) due to hydrogen molecule cracking, respectively. A remarkable increase of true interface trap density, as observed by means of charge pumping technique, is ascribed to both redistribution of interface traps within the silicon bandgap (which dominates in an early phase of recovery in all stressed devices) and dissociation of equivSi _s - H precursors by hydrogen atoms (which becomes important in later recovery phase, especially in devices stressed by lower voltages)