The parity anomalous semimetal (PAS) phase is a topological state of matter exhibiting a semimetallic nature and a half-quantized Hall conductance of $e^2/h$ ($e$ is the elementary charge and $h$ is the Planck constant). In this work, we investigate the disorder-driven topological phase transition in a semimagnetic narrow-gap band insulator thin film. We demonstrate that strong disorder induces a transition from the narrow-gap band insulator to a PAS phase, accompanied by the emergence of a single gapless Dirac cone—the hallmark of the half-quantized Hall effect. Calculations of the local density of states reveal the spectral evolution underlying this transition, while finite-size scaling of the real-space Hall conductivity confirms the robustness of the half-quantized plateau over a finite range of disorder strengths. Our findings reveal disorder as a potent tool for engineering topological phases, in which the interplay between topology and localization in quantum materials warrants further exploration.

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• Anomalous Hall effect
• Topological insulators
• Semimetals
• Thin films
• Tight-binding model