We perform a first-principles investigation of electron-phonon interactions in silicon and germanium, uncovering distinct nonpolaronic spectral and transport fingerprints in these archetypal covalent semiconductors. Using many-body perturbation theory with the retarded cumulant expansion, we compute quasiparticle energies, lifetimes, and phonon satellites beyond the Dyson-Migdal approximation. Short-range crystal fields dominate coupling in both materials, yet their low-temperature spectral fingerprints differ: Si exhibits well-resolved satellites at both band edges, whereas Ge displays strong sidebands mainly at the valence-band maximum (VBM) and much weaker features at the conduction-band minimum (CBM). Phonon-induced satellites in both materials broaden and merge with the quasiparticle peak at elevated temperatures. Doping broadens peaks and compresses satellite-quasiparticle separation, with $n$-type carriers affecting the CBM and $p$-type the VBM. Mobility calculations, combining cumulant-derived phonon scattering with experimentally motivated ionized-impurity scattering models, reproduce measured trends and reveal Ge's consistently higher mobilities than Si, stemming from lighter effective masses and weaker coupling. These results link band-edge asymmetries and phonon energetics to measurable transport differences, providing a unified framework for predicting mobility in nonpolar semiconductors.

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• Electron-phonon coupling
• Transport phenomena
• Elemental semiconductors
• Green's function methods
• Many-body techniques