Strain in silicon is able to enhance the effective mobility of both n-channel and p-channel MOSFETs by around 70% and 100%, respectively. In SOI and DG MOSFETs the thickness of the Si substrate has a major impact on the device performance. Accurate models for the mobility of DG-, SOI-, or strained Si/SiGe MOSFETs require a simulation of the inversion layer mobility. The strong confinement leads to the quantization of the electron (hole) state normal to the interface and the conduction (valence) bands split into a system of discrete subbands. In the direction parallel to the interface the charge carriers behave like free particles. This system is called 2D electron gas and exhibits different scattering rates than the bulk case and additional scattering mechanisms caused by surface roughness and oxide charges. The scattering mechanisms have been introduced in an MC simulator suited for the simulation of the inversion layer mobility. In order to extract the universal mobility curves, the MC simulator has been coupled to a Schrödinger-Poisson solver.
By means of MC simulations we can deduce the effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering. In the unstrained case the inclusion of the Pauli principle has almost no impact on the effective mobility, whereas degeneracy effects increase the effective mobility of strained inversion layers at high inversion layer concentrations. This can be understood from the following: When using non-degenerate statistics electrons reside at lower kinetic energies and hence experience more effective surface-roughness scattering. On the other hand, having lower kinetic energies, phonon scattering is less pronounced. In unstrained Si these two effects cancel each other, and the difference between a simulation with non-degenerate and degenerate statistics is very small even for high effective fields. In strained Si, where the ratio between phonon and surface roughness scattering is different - due to suppressed intervalley transitions - simulations with degeneracy effects yield higher mobilities.