On Apparent Electron Mobility in Si nMOSFETs From Diffusive to Ballistic Regime

The Matthiessen-law combination of diffusive and ballistic mobility provides a framework to explain the decrease of apparent mobility in near-ballistic FETs. While this theory works well for III–V FETs reported to date, it underpredicts the mobility decrease in several Si experiments, raising concerns about the FET performance in the near-ballistic regime. In this paper, recent SOI planar and FinFET nMOSFET I–V data are analyzed in detail, accounting for the effect of channel degeneracy. Assuming channel-length independent mean free path (MFP), it is demonstrated that good agreement can be obtained with the conventional Matthiessen-law combination including the effect of drain-channel backscattering recently proposed by Natori
et al.
On the other hand, good agreement can also be obtained with the Matthiessen-law combination if the MFP is assumed to be channel-length dependent. The full range of
$I_{d}$
(
$V_{\mathrm{gs}}$
,
$V_{\mathrm{ds}})$
is well modeled by incorporating either of these effects in a recent transmission-based compact model, but drastically different critical lengths for backscattering must be used in the Landauer formulation in the saturation region. It is also shown that this apparent anomaly is technology dependent and its effect on the overall device performance is discussed.