Impact of gate-induced drain leakage on retention time distribution of 256 Mbit DRAM with negative wordline bias

A negative wordline bias scheme is utilized to reduce the subthreshold leakage of deep submicron DRAM cell transistors. With excessive negative wordline bias, gate-induced drain leakage (GIDL) could dominate cell leakage and degrade product retention time performance. The dependence of retention time on negative wordline bias for a 256-Mbit DRAM with 0.14μm ground rule is investigated. The retention fail bit count in the tail distribution increases as wordline bias goes more negative and as temperature increases. A cell array with a density of 1.14M is also characterized for the device leakage behavior. The negative wordline bias and temperature dependent GIDL is believed to be due to band to defect tunneling. Hence, elimination of traps near the oxide/silicon interface in the gate to drain overlap region during the DRAM fabrication process is important for the negative wordline scheme.