By using the zone-melting method, we have grown a series of narrow-gap, semiconducting single-crystalline ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ alloys ($0.09\le x\le 0.20$) with centimeter-scale single grains of uniform composition, and systematically investigated their thermoelectric properties. Due to the combination of a significant Seebeck coefficient and reduced thermal conductivity, the in-plane thermoelectric figure of merit $ZT$ achieved approximately 0.35 at 150 K. This $ZT$ value shows minimal variation—less than 30%—across the entire composition range. Furthermore, the $ZT$ value of the single-crystalline alloy reaches an outstanding 0.48 at 100 K when both electric current and heat flow are aligned with the direction of the $c$ axis. We establish a universal scaling relation between the energy gap and the temperature-dependent Seebeck coefficient for the entire series of alloys within their intrinsic semiconducting temperature range. This scaling relation indicates that the large Seebeck coefficient arises from the pronounced asymmetry in the conductivity and effective masses of the electron and hole bands.

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• Electrical conductivity
• Electron thermal conductivity
• Lattice thermal conductivity
• Seebeck effect
• Thermal conductivity
• Thermoelectric effects
• Narrow band gap systems
• Semiconductor compounds
• Semiconductors
• Single crystal materials
• Solid solutions
• Crystal growth
• Resistivity measurements
• Thermometry