Design trade-off study for delta-doped Si/SiGe heterostructure MOSFET's: The potential nano-MOSFET's

A design trade-off study for n-channel δ-doped Si/SiGe heterojunction MOSFET's has been performed using a combination of numerical simulation and analysis. The design parameters unique to the δ-doped Si/SiGe heterostructure MOSFET's have been studied in terms of their effects on short-channel immunity, off-state leakage and on-state current. Our study shows that cap and channel layer must always be made as thin as possible to reduce the separation of the mobile charge centroid from the surface, in which case better short-channel immunity, better leakage and driving ability will result. On the other hand, the setback layer thickness, potential well depth and δ-doping dose are found to be trade-off parameters. Design windows that based on the trade-off parameters were constructed to obtained optimal designs for 0.2. μm channel length and 1.5 V supply voltage, and 0.1 μm channel length and 1 V supply voltage δ-doped Si/SiGe heterojunction MOSFET's. When compared to similarly configured conventional bulk MOSFET's, the 0.2 μm-1.5V design has approximately the same characteristics while the 0.1 μm-1V design has a 25% advantage in short-channel immunity. The δ-doped Si/SiGe heterojunction MOSFET is then redesigned by removing the cap layer, which results in a smaller effective oxide thickness but lower low-field mobility. The new structure is found to produce a 0.1μm-1V design that has improvement of 22% in on-state current, 54% in off-state leakage and 17% in short-channel immunity over the structure with the cap layer. We further are successful in producing a 70nm-1.2V design with excellent characteristics that cannot be reached by conventional MOSFET's. We conclude that the δ-doped Si/SiGe heterojunction MOSFET's without a cap layer have a high potential as the future high-performance transistors that can deliver high speed, high density and low power applications