Transport properties of electrons and phonons, which govern heat dissipation, electrical conduction, and energy conversion, are pivotal in determining the efficiency, performance, and reliability of modern electronic and energy devices, such as high-power electronics, optoelectronic systems, and sustainable energy technologies. The interplay between electrical and thermal transport becomes increasingly complex when electrons and phonons are highly coupled, especially under operating conditions where materials are exposed to strong electric fields, high heat fluxes, and intense optical excitations. In such scenarios, conventional transport models often fail to capture the intricate coupling between electrons and phonons, which can significantly alter the transport behavior and impact device performance.

This thesis aims to provide a comprehensive understanding of coupled electron-phonon transport using both simulation and experimental tools, with a focus on the influence of external stimuli on transport properties. Specifically, we explored the effect of a strong external electric field on the thermal transport properties of GaN. In addition to the direct impact on thermal conductivity, the combined presence of a high electric field and high heat flux can drive electrons and phonons away from their equilibrium state. By analyzing the interactions between these nonequilibrium carriers, we investigated their profound influence on electronic and phonon transport properties. The high electric field can also generate hot carriers occupying higher energy levels, which can also be induced by photoexcitation. To directly visualize the transport processes of these hot carriers, we developed an ultraviolet-pumped scanning ultrafast electron microscopy system, enabling the capture of these ultrafast processes with high spatial and high temporal resolution. These fundamental insights into coupled electron-phonon transport can guide the development of advanced strategies for next-generation energy and electronic materials design.