We present an ab initio study that identifies the main electron-phonon scattering channels in $n$-type PbTe. We develop an electronic transport model based on the Boltzmann transport equation within the transport relaxation time approximation, fully parametrized from first-principles calculations that accurately describe the dispersion of the electronic bands near the band gap. Our computed electronic mobility as a function of temperature and carrier concentration is in good agreement with experiments. We show that longitudinal optical phonon scattering dominates electronic transport in $n$-type PbTe, while acoustic phonon scattering is relatively weak. We find that scattering due to soft transverse optical phonons is by far the weakest scattering mechanism, due to the symmetry-forbidden scattering between the conduction band minima and the zone center soft modes. Soft phonons thus play the key role in the high thermoelectric figure of merit of $n$-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity. Our results suggest that materials like PbTe with soft modes that are weakly coupled with the electronic states relevant for transport may be promising candidates for efficient thermoelectric materials.

Reducing our overwhelming dependence on fossil fuels requires groundbreaking innovations in increasing our efficiency in energy consumption for current technologies and moving towards renewable energy sources. Thermoelectric materials can help in achieving both goals. Moreover, because of recent advances in high-performance computing, researchers more increasingly rely on computational methods in discovering new thermoelectric materials with economically feasible performance. In this article, significant thermoelectric materials discovered through these computational methods are systematically reviewed. Furthermore, the primary computational tools that aid the design of the next-generation thermoelectric materials are introduced and discussed. These techniques include various levels of density functional theory, electronic transport simulations, and phonon calculations.

We present a first-principles-based model of electron-phonon scattering mechanisms and thermoelectric transport at the $L$ and $\mathrm{\Sigma }$ valleys in $p$-type PbTe, accounting for their thermally induced shifts. Our calculated values of all thermoelectric transport parameters at room temperature are in very good agreement with experiments for a wide range of doping concentrations. Scattering due to longitudinal optical phonons is the main scattering mechanism in $p$-type PbTe, while scattering due to transverse optical modes is the weakest. The $L$ valleys contribute most to thermoelectric transport at 300 K due to the sizable energy difference between the $L$ and $\mathrm{\Sigma }$ valleys. We show that both scattering between the $L$ and $\mathrm{\Sigma }$ valleys and additional transport channels of the $\mathrm{\Sigma }$ valleys are beneficial for the overall thermoelectric performance of $p$-type PbTe at 300 K. Our findings thus support the idea that materials with high valley degeneracy may be good thermoelectrics.

We report a fully ab initio calculation of the temperature dependence of the electronic band structure of PbTe. We address two main features relevant for the thermoelectric figure of merit: the temperature variations of the direct gap and the difference in energies of the two topmost valence band maxima located at L and $\mathrm{\Sigma }$. We account for the energy shift of the electronic states due to thermal expansion, as well as electron-phonon interaction computed using the nonadiabatic Allen-Heine-Cardona formalism within density functional perturbation theory and the local density approximation. We capture the increase of the direct gap with temperature in very good agreement with experiment. We also predict that the valence band maxima at L and $\mathrm{\Sigma }$ become aligned at $\sim 600–700\mathrm{K}$. We find that both thermal expansion and electron-phonon interaction have a considerable effect on these temperature variations. The Fan-Migdal and Debye-Waller terms are of almost equal magnitude but have an opposite sign, and the delicate balance of these terms gives the correct band shifts. The electron-phonon induced renormalization of the direct gap is produced mostly by high-frequency optical phonons, while acoustic phonons are also responsible for the alignment of the valence band maxima at L and $\mathrm{\Sigma }$.

The availability of transistors capable of operating at low supply voltage is essential to improve the key performance metric of computing circuits, i.e., the number of operations per unit energy. In this paper, we propose a device concept for energy-efficient, steep-slope transistors based on heterojunctions of two-dimensional (2D) materials. We show that by injecting electrons from an isolated and weakly dispersive band into a strongly dispersive one, subthermionic subthreshold swings can be obtained, as a result of a cold-source effect and of a reduced thermalization of carriers. This mechanism is implemented by integrating in a metal-oxide-semiconductor field-effect transistor (MOSFET) architecture two different monolayer materials coupled through a van der Waals heterojunction, combining the subthermionic behavior of tunnel field-effect transistors (FETs) with the robustness of a MOSFET architecture against performance-degrading factors, such as traps, band tails, and roughness. A further advantage with respect to tunnel FETs is that only an $n\text{−}\mathrm{type}$ or $p\text{−}\mathrm{type}$ doping is required to fabricate the device. In order to demonstrate the device concept and to discuss the underlying physics and the design options, we study through ab initio and full-quantum transport simulations a possible implementation that exploits two recently reported 2D materials.

The unsatisfactory transmission probability in tunneling junction is a major challenge that restricts the performance and scaling of next-generation nanodevices, such as the tunnel field-effect transistors (TFETs). Here, we propose a strategy utilizing anisotropic electronic structures to enhance the inter-band tunneling performance. In the tunneling process, the sharp energy dispersion in transport direction ensures a high transmission eigenvalue, and the weak transverse energy state can broaden the transverse tunneling window, thus strengthening the tunneling probability. Furthermore, our quantum transport simulations demonstrate that in two-dimensional (2D) group VA-VA TFETs, the stronger anisotropic band structures make 2D BiAs and arsenene exhibit high on-state current several times higher than other systems, and the relative larger bandgap of arsenene also gives rise to a steep subthreshold slope below 60 mV/dec. This work provides a physical understanding of the tunneling transport performance, and the anisotropic 2D electronic structures can be regarded as a target feature to design tunneling transistors.