OBSERVATION OF ANISOTROPIC IMPACT IONIZATION IN SI MOSFETS

Anisotropy in the substrate current has been observed experimentally i n Si MOSFETs on the (100) surface. The impact ionization rate along th e <001> axis becomes larger than that along the <011> axis. The ratio of the ionization rate between the two directions has been found to be as high as 5 at 81 K. This anisotropy can be explained by the anisotr opic threshold energy for impact ionization. Furthermore, it has been found that the anisotropy ratio increases with a decrease in temperatu re and the length of the pinch-off region in MOSFETs. These results su ggest that the non-stationary transport in the pinch-off region can le ad to the enhancement of the anisotropy in the impact ionization.