Using n-channel TFTs without LDD structures for high stabilities of1.2-V high-density SRAMs

摘要

In high-density SRAMs, cell-stability issues have limited thelower boundary of the operation voltage (V_DD) for low-powerapplications. In 1990, p-channel thin-film-transistors (TFTs) wereintroduced to make 3.3-V operation possible. As V_DD droppedto 2.0 V in 1991, however, p-TFTs could not solve the stability issuesand cell β ratios as large as 4.4 had to be used, and 1.2-Voperation was not possible in the analyses at all. In addition tostability issues, hot carrier aging and sodium-ions also limited thememory density through the limits of device dimensions. In this paper wehave demonstrated that 1.2-V SRAM cells with high stabilities can bedesigned with n-channel TFTs and p-MOSFETs. The n-TFTs are also easierto fabricate and have superior characteristics than p-TFTs