Advanced MOSFET Designs and Implications for SRAM Scaling

Continued planar bulk MOSFET scaling is becoming increasingly difficult due to increased random variation in transistor performance with decreasing gate length, and thereby scaling of SRAM using minimum-size transistors is further challenging. This dissertation will discuss various advanced MOSFET designs and their benefits for extending density and voltage scaling of static memory (SRAM) arrays. Using threedimensional (3-D) process and design simulations, transistor designs are optimized. Then, using an analytical compact model calibrated to the simulated transistor current-vs.-voltage characteristics, the performance and yield of six-transistor (6-T) SRAM cells are estimated. For a given cell area, fully depleted silicon-on-insulator (FD-SOI) MOSFET technology is projected to provide for significantly improved yield across a wide range of operating voltages, as compared with conventional planar bulk CMOS technology. Quasi-Planar (QP) bulk silicon MOSFETs are a lower-cost alternative and also can provide for improved SRAM yield. A more printable "notchless" QP bulk SRAM cell layout is proposed to reduce lithographic variations, and is projected to achieve six-sigma yield (required for terabit-scale SRAM arrays) with a minimum operating voltage below 1 Volt