Modeling and Simulation of Subthreshold Characteristics of Short-Channel Fully-Depleted Recessed-Source/Drain SOI MOSFETs

Abstract

Non-conventional metal-oxide-semiconductor (MOS) devices have attracted researchers‟ attention for future ultra-large-scale-integration (ULSI) applications since the channel length of conventional MOS devices approached the physical limit. Among the non-conventional CMOS devices which are currently being pursued for the future ULSI, the fully-depleted (FD) SOI MOSFET is a serious contender as the SOI MOSFETs possess some unique features such as enhanced short-channel effects immunity, low substrate leakage current, and compatibility with the planar CMOS technology. However, due to the ultra-thin source and drain regions, FD SOI MOSFETs possess large series resistance which leads to the poor current drive capability of the device despite having excellent short-channel characteristics. To overcome this large series resistance problem, the source/drain area may be increased by extending S/D either upward or downward. Hence, elevated-source/drain (E-S/D) and recessed-source/drain (Re-S/D) are the two structures which can be used to minimize the series resistance problem. Due to the undesirable issues such as parasitic capacitance, current crowding effects, etc. with E-S/D structure, the Re-S/D structure is a better choice. The FD Re-S/D SOI MOSFET may be an attractive option for sub-45nm regime because of its low parasitic capacitances, reduced series resistance, high drive current, very high switching speed and compatibility with the planar CMOS technology. The present dissertation is to deal with the theoretical modeling and computer-based simulation of the FD SOI MOSFETs in general, and recessed source/drain (Re-S/D) ultra-thin-body (UTB) SOI MOSFETs in particular. The current drive capability of Re-S/D UTB SOI MOSFETs can be further improved by adopting the dual-metal-gate (DMG) structure in place of the conventional single-metal-gate-structure. However, it will be interesting to see how the presence of two metals as gate contact changes the subthreshold characteristics of the device. Hence, the effects of adopting DMG structure on the threshold voltage, subthreshold swing and leakage current of Re-S/D UTB SOI MOSFETs have been studied in this dissertation. Further, high-k dielectric materials are used in ultra-scaled MOS devices in order to cut down the quantum mechanical tunneling of carriers. However, a physically thick gate dielectric causes fringing field induced performance degradation. Therefore, the impact of high-k dielectric materials on subthreshold characteristics of Re-S/D SOI MOSFETs needs to be investigated. In this dissertation, various subthreshold characteristics of the device with high-k gate dielectric and metal gate electrode have been investigated in detail. Moreover, considering the variability problem of threshold voltage in ultra-scaled devices, the presence of a back-gate bias voltage may be useful for ultimate tuning of the threshold voltage and other characteristics. Hence, the impact of back-gate bias on the important subthreshold characteristics such as threshold voltage, subthreshold swing and leakage currents of Re-S/D UTB SOI MOSFETs has been thoroughly analyzed in this dissertation. The validity of the analytical models are verified by comparing model results with the numerical simulation results obtained from ATLAS™, a device simulator from SILVACO Inc.