Two-dimensional topological insulators (2D TIs) are commonly realized either through intrinsic band-gap inversion within a single material or through interfacial band-offset inversion in narrowly tuned heterostructures. Here we demonstrate, based on first-principles calculations, that these two mechanisms can be unified within a single heteroepitaxial platform. We show that monolayer ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$ grown on GaSb(111) hosts a robust 2D TI phase for all alloy compositions $0\le x\le 1$, with a large topological gap ranging from ∼90 to 240 meV. Layer- and orbital-resolved band analyses show a smooth evolution from interfacial band-offset inversion at low Bi concentration to intrinsic band-gap inversion at high Bi concentration. This evolution is captured by a two-band Dirac (Bernevig-Hughes-Zhang  model-type) Hamiltonian with a continuously inverted Dirac mass, placing all band-alignment regimes within the same topological universality class. Our results establish a realistic III–V semiconductor platform for integrating large-gap 2D TIs into wafer-scale device architectures.

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• Electronic structure
• Solid-solid interfaces
• Topological materials
• Density functional calculations