Some semiconductor materials exhibit axis-dependent conduction polarity (ADCP), where carrier transport is dominated by electrons along one crystallographic axis and holes along another. This unconventional transport behavior enables transverse thermoelectricity and other functionalities that are inaccessible in conventional isotropic/unipolar semiconductors. However, only a few ADCP materials have been identified, largely because the underlying electronic design principles have remained unclear. Here, we establish a quantitative framework that defines the electronic conditions required for ADCP to emerge and remain robust. Using a minimal two-band tight-binding model, we clarify that ADCP requires two key electronic conditions: a sufficiently small band gap that enables simultaneous electron-hole transport and strong anisotropy in carrier effective masses that causes their transport contributions to differ between axes. These parameters define a chemical-potential window in which axis-resolved Seebeck coefficients take opposite signs, identifying narrow-gap semiconductors and semimetals with anisotropic band edges as prime ADCP candidates. Guided by these criteria, we conduct a first-principles screening of 4282 anisotropic narrow-gap semiconductors and metals and identify 361 ADCP materials, which are predominantly found among chalcogenides, pnictides, and tetrel-based compounds, including 57 potential transverse thermoelectrics. Analysis of two representative materials, AlReGe and ZrSe3, reveals that ADCP originates from anisotropic band-edge states derived from low-dimensional bonding networks, resulting in spatially separated electron and hole transport on different crystal sublattices. These results provide chemically intuitive design rules for ADCP materials and establish a comprehensive data set for accelerating the development of transverse thermoelectrics and other next-generation electronic devices.
Yang et al. (Wed,) studied this question.