Comparative study of phonon-limited mobility of two-dimensional electrons in strained and unstrained Si metal–oxide–semiconductor field-effect transistors

Abstract

The phonon-limited mobility of strained Si metal–oxide–semiconductor field-effect transistors (MOSFETs) fabricated on a SiGe substrate is investigated through theoretical calculations including two-dimensional quantization, and compared with the mobility of conventional (unstrained) Si MOSFETs. In order to match both the mobility of unstrained Si MOSFETs and the mobility enhancement in strained Si MOSFETs, it is necessary to increase the coupling of electrons in the two-dimensional gas with intervalley phonons, compared to the values used in conventional models. The mobility enhancement associated with strain in Si is attributed to the following two factors: the suppression of intervalley phonon scattering due to the strain-induced band splitting, and the decrease in the occupancy of the fourfold valleys which exhibit a lower mobility due to the stronger interaction with intervalley phonons. While the decrease in the averaged conductivity mass, caused by the decrease in the occupancy of the fourfold valleys, contributes to the mobility enhancement in bulk strained Si, it is not necessarily adequate to explain the mobility enhancement for two-dimensional electrons in strained Si. This is suggested by the fact that the mobility limited by intravalley acoustic phonon scattering, which is the dominant scattering mechanism, has almost the same value in the two- and the fourfold valleys, because the difference in the conductivity mass is compensated by differences in the inversion-layer thickness and the valley degeneracy.

Temperature Dependence of Electron Mobility in Uniaxial Strained nMOSFETs

“…The strain changes the electronic band structure resulting in the change of carrier mobility [1][2][3][4][5]. While the electrostatics of strained MOSFETs has been studied, several research groups have reported the mobility characteristic in strained MOSFETs [1,6] or in unstrained MOSFETs [7,8]. It is also reported that the intervalley phonon scattering is required to accurately explain the behavior of inversion layer mobility in both strained and unstrained Si MOSFETs [9].…”

“…F i,j is the form factor determined by the wave functions of the i th and the j th subbands [6]. The momentum relaxation rate for intervalley phonon scattering (τ i inter ) from the i th subband to the j th subband is given by…”

“…In addition, the phonon scattering mobility (µ ph ), coulomb mobility (µ coul ) and surface roughness mobility (µ sr ) on the (100)/<110> Si surfaces were also calculated through our solver. To consider the physical mechanism of mobility enhancement in strained nMOSFETs, we take the intravalley and intervalley phonon scattering into account [6,8]. The traditional theory of intravalley and intervalley phonon scattering has already been developed and expressions for the momentum relaxation rate have been obtained.…”

“…In (3), f and g are the type of intervalley scattering. The upper or lower sign must be taken for the process of emission and absorption of phonons, respectively [6]. The physical parameters used in equations are listed in Table 1.…”

Mobility enhancement in polymer organic semiconductors arising from increased interconnectivity at the level of polymer segments

“…[6][7][8][9][10] A 2D model is appropriate in the case that electronic coupling in the direction perpendicular to stacked lamellae is negligible. This is the case for polymers such as P3HT and PBTTT, where the lamellae are separated by electronically insulating alkyl chains.…”