A STUDY ON HOT-CARRIER EFFECTS ON N-MOSFETS UNDER HIGH SUBSTRATE IMPURITY CONCENTRATION

A systematic investigation of the influences of high substrate doping on the hot carrier characteristics of small geometry n-MOSFET's down t o 9.1 mu m has been carried out. Results indicate that the dependence of substrate current and impact ionization rate on substrate impurity concentration is reversed in long channel and short channel devices. I n the long channel case, both increase with rising substrate impurity concentration, while they decrease in the case of short channel device s. An explanation for this phenomenon based on the lucky electron mode l has been developed. The dependence of other characteristics on impur ity concentration has also been studied. The dependence of off-leakage current has been found to fall as the gate oxide is reduced in thickn ess. Regarding the dependence of hot carrier degradations, the degrada tion of drain currents becomes smaller as the substrate impurity conce ntration increases in the case of short channel devices. Further, in t he extremely high impurity doping region, a new hot carrier degradatio n mode was found, in which the maximum transconductance values of n-MO SFET's increase after hot carrier stress. This new degradation mode ca n be explained in terms of effective channel length shortening caused by electron trapping.