IS PHYSICALLY SOUND AND PREDICTIVE MODELING OF NMOS SUBSTRATE CURRENTS POSSIBLE?

Abstract

Practically all NMOS substrate current models used today, which have been successfully verified for a certain range of NMOS devices and bias conditions, yield for a given homogeneous electric field an impact ionization coefficient which is significantly smaller than the corresponding experimental result extracted from bulk silicon. This effect was named “surface impact ionization”, but no conclusive physical explanation was given for this discrepancy. This obvious inconsistency has led to serious doubts concerning the predictive capabilities of these substrate current models, and in fact none of these models could be verified over the full range of available NMOS technologies. However, recently two different substrate current models, (a full-band Monte Carlo model and a nonlocal soft-threshold lucky electron model) appeared, which are consistent with bulk impact ionization and provide accurate substrate current results for advanced NMOS devices without any modification of the model parameters. In this paper we have demonstrated that these models are precise for several NMOS devices fabricated in very different technologies covering the range from the 256 KBit to 1 GBit DRAM generation. To the best of our knowledge this is the widest technology base used for verification so far, which implies that these two models are the first models which really deserve to be called predictive. On the other hand, it is shown that the two substrate current models used most frequently today have poor modeling accuracy and cannot be called predictive at all. Finally, previous work concerning surface impact ionization is re-investigated with full-band Monte Carlo in the light of the new findings. It is shown that the effect called surface impact ionization cannot be associated with an effect in microscopic physics, but is due to the failure of the impact ionization models which have been applied to extract the impact ionization coefficient by reverse engineering based on experimental NMOS substrate currents. In addition the few theoretical explanations of surface impact ionization are discussed and the proposed effects are shown to be of minor influence.