Using n-channel TFTs without LDD structures for high stabilities of 1.2-V high-density SRAMs

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