Physical limitation of p–n junction––statistical variations of p–n junction depth in MOSFET array

Abstract

We have experimentally studied statistical fluctuations of p–n junction depth x j of the drain region due to the dopant density variation near the p–n junction edge, whose x j was measured by the impact ionization rate α, using an 8k MOSFET array test structure, and we have investigated their influence on transconductance G m characteristics. The impact ionization rate fluctuation δ α/ α is experimentally found to be proportional to the square root of the gate oxide thickness T ox and to be about 0 at T ox 1/2=0, and the average α is explained by the usual lucky electron model. Therefore, x j variation δ x j / x j can be obtained by this δ α/ α, because δ α/ α is caused by the fluctuation of the pinch-off length due to the x j variations. Moreover, it was also found that G m fluctuation at the saturation region correlates well to the x j variations.