Temperature-Bias Aging of (HCl) MOS Structures

The study of the transient flat-band shifts during the drift of sodium ions from the metal to the silicon interface in (HCl) MOS structures revealed that the rate limiting process in the neutralization of positively charged ions is the charge exchange between the ions and the silicon electrode; therefore, the magnitude and duration of the temporary flat-band shifts depend upon the rate of emission of ions from the metal interface as well as upon the rate of the charge exchange. Assuming that the rate of charge exchange is proportional to the instantaneous number of non-neutralized ions, their "lifetime" can be determined experimentally. This has been found to be approximately 4 seconds at 120°C and exhibits an activation energy of 0.8 eV. Although the measurements have been carried out only down to 75°C, it has been definitely established that the neutralization mechanism is operative at temperatures as low as room temperature. Based upon the lifetime data and on the emission characteristics of ions from the metal interface, estimates can be arrived at for the appropriate conditions of temperaturebias aging of MOS devices to avoid the appearance of instabilities during actual operation.