The ab-initio theory of low-field electronic transport properties such as carrier mobility in semiconductors is well-established. However, an equivalent treatment of electronic fluctuations about a non-equilibrium steady state, which are readily probed experimentally, remains less explored.Here, we report a first-principles theory of electronic noise for warm electrons in semiconductors. In contrast with typical numerical methods used for electronic noise, no adjustable parameters are required in the present formalism, with the electronic band structure and scattering rates calculated from first-principles. We demonstrate the utility of our approach by applying it to GaAs and show that spectral features in AC transport properties and noise originate from the disparate time scales of momentum and energy relaxation, despite the dominance of optical phonon scattering.Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors.

Thermally conductive molecular crystals are of fundamental interest because they are unlike typical complex crystals, which conduct heat poorly owing to their large phonon scattering phase space. While molecular crystals with high thermal conductivity in the range of tens of Wm −1 K −1 have been known experimentally for decades, their intrinsic upper limits for thermal conductivity are unclear. Ab initio methods that have been successfully applied to simple crystals have proved difficult to adapt to molecular crystals due to quantum nuclear motion and their complex primitive cells. Here, we report the thermal transport properties of crystalline polythiophene with 28 atoms per primitive cell using an ab initio approach that rigorously includes finite-temperature anharmonicity and quantum nuclear effects. The calculated room temperature thermal conductivity is 198 Wm −1 K −1 along the chain axis, a high value that arises from exceptional phonon focusing along the chain for both acoustic and optical branches for nearly all wave vectors and despite short lifetimes in the picosecond range. Our finding, along with other recent ab initio studies of polyethylene, suggests that the intrinsic upper bounds for the chain axis thermal conductivity of polymer crystals may exceed 100 Wm −1 K −1 .

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The ab-initio theory of charge transport in semiconductors typically employs the lowest-order perturbation theory in which electrons interact with one phonon (1ph). This theory is accepted to be adequate to explain the low-field mobility of non-polar semiconductors but has not been tested extensively beyond the low-field regime. Here, we report first-principles calculations of the electric field-dependence of the electron mobility of Si as described by the warm electron coefficient, β.Although the 1ph theory overestimates the low-field mobility by only around 20%, it overestimates β by over a factor of two over a range of temperatures and crystallographic axes. We show that the discrepancy in β is reconciled by inclusion of on-shell iterated 2-phonon (2ph) scattering processes, indicating that scattering from higher-order electron-phonon interactions is non-negligible even in non-polar semiconductors. Further, a ∼ 20% underestimate of the low-field mobility with 2ph scattering suggests that non-trivial cancellations may occur in the perturbative expansion of the electron-phonon interaction.

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