MOSFETs which are used as access transistors in DRAM cells need a particularly
small footprint to allow high integration densities [29]. The DRAM
capacitor, which requires a capacitance of approximately 50fF to allow
practicable retention times, is usually built as a trench capacitor. One
approach by which the footprint is reduced drastically is to turn the access
transistor into the vertical direction directly above the trench
capacitor [30,31,32,33,34,35].
A. Gehring: Simulation of Tunneling in Semiconductor Devices
