Bandstructure and mobility variations in p-type silicon nanowires under electrostatic gate field

Abstract

The sp 3 d 5 s * -spin-orbit-coupled atomistic tight-binding (TB) model is used for the electronic structure calculation of Si nanowires (NWs), self consistently coupled to a 2D Poisson equation, solved in the cross section of the NW. Upon convergence, the linearized Boltzmann transport theory is employed for the mobility calculation, including carrier scattering by phonons and surface roughness. As the channel is driven into inversion, for [111] and [110] NW devices of diameters D>10nm the curvature of the bandstructure increases and the hole effective mass becomes lighter, resulting in a ~50% mobility increase. Such improvement is large enough to compensate for the detrimental effect of surface roughness scattering. The effect is very similar to the bandstructure variations and mobility improvement observed under geometric confinement, however, in this case confinement is caused by electrostatic gating. We provide explanations for this behavior based on features of the heavy-hole band. This effect could be exploited in the design of p-type NW devices. We note, finally, that the "apparent" mobility of low dimensional short channel transistors is always lower than the intrinsic channel diffusive mobility due to the detrimental influence of the so called "ballistic" mobility.

Impacts of orientation and cross-sectional shape on hole mobility of Si nanowire MOSFETs

“…2. The mobility trend for 10-15 nm wide NWs, which is , is in good agreement with a theoretical calculation by Neophytou et al 10) . Moreover, the mobility properties with larger widths than 15 nm may be explained by side-facet model 11) .…”

Gated Si nanowires for large thermoelectric power factors

“…We consider p-type Si NWs of diameters up to 20nm and three different transport as described in detail in Refs [15,23,24]. IIS is only applied for doped/non-gated structures.…”

“…In the case of doped structures the self-consistency is not necessary and only steps i) and iv) are employed. The methodology we employ, as described in Refs [15,23,24], is commonly employed to describe transport in lowdimensional semiconductors. We use appropriate assumptions and approximations such bulk phonon deformation potential scattering [24,25,26], uniformity in the doping profile and the resultant electrostatic potential, and rigid Si electronic bandstructures independent of doping.…”

Use of Field-Effect Density Modulation to Increase ZT for Si Nanowires: A Simulation Study

“…acoustic phonons, optical phonons, surface roughness scattering (SRS), and ionized impurity scattering (IIS) as described in detail in Refs. [1,11]. IIS is only applied for doped/nongated structures.…”