Monte Carlo simulation of charge transport in Si-based heterostructure transistors

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Wang, Xin

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Strain and bandgap engineering of strained materials has emerged as an important technique for improving the device performance other than conventional scaling method. The purpose of this work is to develop a Monte Carlo simulation tool to investigate properties of these strained materials and carrier transport in deep submicron novel devices with heterostructures and strained materials. A general full-band Monte Carlo simulation tool with high flexibility about device structure and material profiles is developed for the first time. The transport model is based on energy-dependent scattering rates including inelastic acoustic phonon scattering with longitudinal and transverse modes, optical phonon scattering, impact ionization, surface roughness scattering, impurity scattering and alloy scattering. The full-band treatment for strained material model substantially advances the state-of-the-art method relying on simpler valley model for the scattering rates. The multi-material profiles in devices are treated with parameterization of band structure. The tunneling across a potential barrier is treated with Feynman’s effective potential scheme. An orthorhombically-strained silicon (OS-Si) is reported in this work. The six degenerate valleys in OS-Si near X points break into three pairs with different energy minima due to the orthorhombic strain. Thus the drift velocity is enhanced under an electric field transverse to the longitudinal-axis of the lowest valleys. The OS-Si grown on a compressively-strained Si0.6Ge0.4 sidewall has a mobility almost twice that of bulk Si and electron saturation velocity approximately 20% higher. For homogenous strained silicon on Si0.7Ge0.3 (001), in-plane mobility of 2670 cm2 /(Vs) for electrons is obtained, with enhancement by a factor of 1.8 compared to the unstrained case. Electron transport in a strained Si nMOSFET with 50 nm channel length is also investigated by full-band Monte Carlo. Strained silicon devices exhibit around 60% increase of drain current compared to unstrained silicon. Strained SiGe is also studied with full-band Monte Carol tool. A 90% enhancement in hole mobility is obtained for strained Si0.7Ge0.3 compared with bulk Si. The preliminary investigation of hole transport in vertical pMOSFETs with graded SiGe channel is also reported in this work.