Physical basis and limitation of universal mobility behavior in fully depleted silicon-on-insulator Si inversion layers

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Abstract
The physical basis and the limitation for the universal mobility behavior of fully depleted silicon-on-insulator (SOI) metal–oxide–semiconductor Si inversion layers are shown by means of an analysis of the electronic states (potential profile, subband structure, and electron density distribution). As long as the top Si layer thickness is larger than the inversion layer thickness and the electron density is much higher than the impurity concentration in the inversion region, it is proved that the electronic states of an SOI Si inversion region are equivalent to those of a certain bulk Si inversion region. In this context, the definition of the effective vertical electric field Eeff for SOI inversion layers is derived and it ensures the identical mobility dependence on Eeff for SOI and bulk Si inversion layers. The effective carrier mobility μeff behavior in SOI Si inversion layers is universal, irrespective of structural parameters or back gate voltages, over the Eeff range where the mobility is essentially limited by phonon scattering and/or surface roughness scattering at the upper interface. On the other hand, when the electron density distribution of the inversion region reaches the lower surface of the top Si layer sufficiently, it is predicted that the universal mobility behavior is not maintained, but there exists another kind of mobility behavior which is dependent on the top Si layer thickness. Moreover, self-consistent calculations for the electronic states clarify the range where the equivalence of the electronic states for SOI and bulk Si inversion regions holds as function of the effective field Eeff, the top Si layer thickness, and the top Si layer impurity concentration. The phonon-limited mobility is also evaluated to confirm the equality of the mobility for SOI and bulk Si inversion regions.