FORMATION AND PASSIVATION OF INTERFACE TRAPS IN IRRADIATED N-CHANNEL POWER VDMOSFETS DURING THERMAL ANNEALING

The annealing of gamma-ray irradiated commercial n-channel power VDMOS FETs at elevated temperature is investigated. Rebound effect, observed in the samples annealed with positive gate bias, is primarily the con sequence of a very long-term postirradiation interface-trap buildup. A t later annealing times, a significant decrease in the number of inter face traps is observed. In order to explain obtained results, we propo se a modification to the hydrogen ion transport model for the formatio n of interface traps. According to our model, H+ ions created during i rradiation are responsible only for the initial fraction of the total interface-trap buildup. The other fraction is attributed to H+ ions pr oduced when molecular hydrogen, released at the Si-SiO2 interface in i nterface-trap creation reaction, diffuses back into the bulk of the ox ide and is cracked at positive charge centres. Simultaneously with the generation of interface traps, their passivation also takes place. Pa ssivation, probably caused by hydrogen atoms and water molecules, beco mes predominant at later annealing times, leading to the decrease in i nterface-trap density.