Study Of Compact Modeling For Strained Si NMOSFET

Silicon devices play a decisive role in the semiconductor industry. Scaling downthe feature size has been the main method of moving silicon device’s performance.However, as the device is scaled down to the nanometer scale, serious challenges areposed by the material properties, device physics and fabrication technologies. Suchchallenges have impelled researchers to find for various alternatives and as a result,Silicon-based strain technology has emerged. By inducing stress into Si, the carriermobility can be enhanced and the I-V characteristics of MOSFET based on thistechnology can be improved. Therefore, Silicon-based strain technology is regarded asan important method to extend the Moore Law in the next two decades. In thisdissertation, the compact modeling of strained Si MOSFET is investigated under thebiaxial stress. The primary work and achievements are as follows:1. The structure and working principle of strained Si NMOSFET has been studiedunder the biaxial stress. The equivalent circuit of strained Si NMOSFET has beenestablished for the first time, which can describe the microscopic essence of deviceaccurately. The intrinsic region, source/drain parasitic regions, and substrate parasiticresistance of the equivalent circuit are proposed respectively.2.Based on the structure of strained Si/SiGe NMOSFET, regional solutions of thewell-known Poisson’s equation have been formulated, then various potentials andcharges properties have been developed in the two-terminal of strained Si NMOSstructure when a voltage is applied between gate and body. A physics-based single-piecetwo-terminal small signal capacitance model from accumulation to strong-inversionregions is presented. Moreover, small-signal equivalent circuits are presented, and theperformance of the “Plateau” in these structures is proposed, which is the fundamentalto model parameters.3. Based on the analysis of the basic equation and equivalent circuit of strained SiNMOSFET at direct current (DC) and alternating current (AC) working conditions, DCand AC model parameters have been established, which include threshold voltage, draincurrent, substrate current, intrinsic capacitances, parasitic capacitances, parasiticresistances and parasitic diodes. The development of threshold voltage, drain currentand substrate current models are based on Poisson’s equation using quasi-twodimensional (2D) analysis, taking into account of small size effects. Moreover, thesmoothing function has been used, which guarantees the continuities of the drain current and its derivatives. Based on the analysis of the shortcomings of the Meyer model,charge conservation has been guaranteed by using charge as the state variable.Meanwhile, the smoothing function has been used, which guarantees the continuities ofcharges and capacitances.4. Furthermore, the plug-in package for EDA simulation has been developed,which contains equivalent circuit models of biaxial strained Si NMOSFET, usingVerilog-A, a language to describe analog behavior. Model parameters have beenextracted from the experimental results by software, the evidence for the validity ofequivalent circuits and model parameters have been derived from the comparison ofexperimental results. The model can be used in SPICE simulator, achieving thesimulation of biaxial strained Si NMOSFET successfully.