We propose a nonlocal impact ionization model applicable for the drain region where electric field increases exponentially. It is expressed as a function of an electric field and a characteristic length which is determined by a thickness of gate oxide and a source/drain junction depth. An analytical substrate current model for n-MOSFET is also derived from the new nonlocal impact ionization model. The model well explains the reason why the theoretical characteristic length differs from empirical expressions used in a pseudo two-dimensional model for MOSFET's. The nonlocal impact ionization model implemented in a device simulator demonstrates that the new model can predict substrate current correctly in the framework of drift-diffusion model.

Optical and electrical measurements of thin film n-channel SOI-MOSFETs reveal that the exponential tail in photon emission spectra originates from electron-hole recombination. Bremsstrahlung radiation model as a physical mechanism of photon emission was experimentally negated. Negative threshold voltage shift at the initial stage of high field stress is found to be caused by hole trapping in buried oxide. Subsequent turnover characteristics is explained by a competing process between electron trapping in the front gate oxide and hole trapping in the buried oxide. As to the degradation of transconductance, generated surface state as well as trapped holes in the buried oxide which reduce vertical electric field in SOI film are involved in the complicate degradation of transconductance.