Nam-Kyun Tak

55 nm capacitor-less 1T DRAM cell transistor with non-overlap structure

This paper presents a capacitor-less 1T DRAM cell transistor with high scalability and long retention time. It adopts gate to source/drain non-overlap structure to suppress junction leakage, which results in 80 ms retention time at 85degC with gate length of 55 nm. Compared to the previous reports, proposed cell transistor shows twice longer retention time even though the gate length shrinks to the half of them. By TCAD analysis, we have confirmed that the improvements are attributed to the superiority of the proposed device structure.

A 31 ns Random Cycle VCAT-Based 4F

A functional 4F2 DRAM was implemented based on the technology combination of stack capacitor and surrounding-gate vertical channel access transistor (VCAT). A high performance VCAT has been developed showing excellent Ion-Ioff characteristics with more than twice turn-on current compared with the conventional recessed channel access transistor (RCAT). A new design methodology has been applied to accommodate 4F2 cell array, achieving both high performance and manufacturability. Especially, core block restructuring, word line (WL) strapping and hybrid bit line (BL) sense-amplifier (SA) scheme play an important role for enhancing AC performance and cell efficiency. A 50 Mb test chip was fabricated by 80 nm design rule and the measured random cycle time (tRC) and read latency (tRCD) are 31 ns and 8 ns, respectively. The median retention time for 88 Kb sample array is about 30 s at 90°C under dynamic operations. The core array size is reduced by 29% compared with conventional 6F2 DRAM.