Everett X. Wang

Modeling gate leakage current in nMOS structures due to tunneling through an ultra-thin oxide

Abstract For the first time, the tunneling current in silicon nMOS structures with ultra-thin gate oxides has been studied both by numerically solving Schrodingers equation and by using the WKB approximation, which explicitly includes the size quantization effects in the inversion layers. The numerical solution employs first-order perturbation within the one-band effective-mass approximation to calculate the lifetime of an inversion-layer quasi-bound state. The good agreement in the tunneling currents estimated with these two methods justifies the use of the WKB approximation in the direct tunneling regime. The range of validity of the WKB approximation is also discussed.

Quantum mechanical calculation of hole mobility in silicon inversion layers under arbitrary stress

We have developed a quantum anisotropic transport model for holes which, for the first time, allows mobility to be studied under both uniaxial and arbitrary stress in PMOS inversion layers. The anisotropic bandstructure of a 2D quantum gas is computed from a 6-band stress dependent k.p Hamiltonian. Our unique momentum-dependent scattering model also captures the anisotropy of scattering. A comprehensive study has been performed for uniaxial stress, biaxial stress, and their nonlinear interactions. The results are compared with device bending data and piezoresistance data, showing very good agreement.

Inversion mobility and gate leakage in high-k/metal gate MOSFETs

For the first time, we show with simulation that the use of a metal gate/high-k stack offers improved mobility over polysilicon/high-k gates stacks while maintaining decreased gate leakage compared to conventional SiO/sub 2/ stacks, thus allowing high-performance transistor scaling to continue.

Application of cumulant expansion to the modeling of non-local effects in semiconductor devices

The cumulant expansion method is proposed to solve the Boltzmann transport equation (BTE) in semiconductors. This method involves deriving a set of partial differential equations for the expansion coefficients from a Fourier transformation of the BTE. The collision terms for phonon emission and absorption scattering are obtained directly from quantum computed scattering transition rates, without invoking the relaxation time approximation. Unlike the moment expansion method used in hydrodynamic models, the cumulant expansion converges much faster when the distribution function is close to a drifted maxwellian because, for this case, only the first three cumulants are non-zero. This method also provides a way to construct an arbitrary distribution function from the computed cumulants, without being limited to a shifted maxwellian.