Enhanced hotspot thermal resistance in silicon thin films is governed by low modal heat capacity rather than long MFPs alone

Enhanced thermal resistance near nanoscale hotspots is often interpreted as a direct consequence of long mean free path (MFP) phonons. Here, we revisit this interpretation for silicon thin films by solving the phonon Boltzmann transport equation (BTE) using a band-resolved, full phonon-dispersion model (FPDM) that retains an explicit interband scattering network. For a representative silicon-on-insulator (SOI) geometry with a nanoscale hotspot embedded in the Si layer, we vary the partitioning of volumetric heat generation between acoustic and optical branches and quantify the resulting changes in mode-resolved thermal resistance. We find that near-hotspot resistance amplification is dominated by low-frequency acoustic modes, whereas higher-frequency modes are comparatively insensitive. The amplification is therefore not governed by long MFPs alone; rather, it emerges when scattering-mediated spectral routing channels energy into modes with low modal heat capacity and weak intermodal coupling. Thick-film-normalized trends further distinguish hotspot-local nonequilibrium from boundary confinement: the former remains localized near the hotspot, whereas the latter manifests as a spatially distributed renormalization across the film. Within the present silicon/SOI system, this contrast provides a system-specific experimental discriminator and a testable prediction: heating-condition variation ( qa ), interpreted as a change in the effective source spectral distribution, preferentially modifies the hotspot-local excess resistance, whereas film-thickness variation primarily alters the distributed confinement response. Accordingly, in the hotspot-proximal regime, the hotspot temperature rise or apparent thermal resistance is expected to exhibit stronger sensitivity to heating-condition variation than to film-thickness variation. At the aggregate level, these contrasting effects are directly reflected in measurable quantities such as hotspot temperature rise and apparent thermal resistance. The quantitative results are specific to the present system and modeling assumptions; accordingly, the broader implication claimed here is mechanistic rather than quantitatively universal. These findings identify modal heat capacity and scattering-network connectivity as key determinants of hotspot thermal resistance and motivate thermal management strategies based on spectral energy partitioning and intermodal energy-transfer pathways.