Random doping fluctuation effects on static noise margins of 6-T SRAM cells

The effects of random dopant fluctuations (RDF) on the static noise margins (SNM) of 6-T SRAM cells are analyzed by using a doping sensitivity approach. The approach presented in this article is based on a full circuit perturbation theory at the level of each device transport model. The bias points and the magnitude of random dopant induced fluctuations are computed by solving the Poisson, current continuity, and Density-Gradient equations for all the devices self-consistently (mixed-mode simulation). It is shown that random dopant induced fluctuations can significantly impinge on the yield and reliability of SRAM circuits and constitute a fundamental limit for further scaling unless these devices are properly optimized. Simulation results for a well scaled SRAM cell with 30 nm channel length transistors show that the most sensitive regions to doping fluctuations extend for approximately 10 nm below the oxide/semiconductor interface and are located in the middle of the conduction channels for both p-channel and n-channel transistors.