<p>Hydrogen’s flammability and explosion risks necessitate the development of advanced sensing technologies to ensure its safe production, storage, and utilization as a renewable energy source. In this work, an ultra-sensitive H₂ sensor based on a GaAs/GaSb heterojunction charge-plasma tunnel field-effect transistor (HJ-CP-TFET) is proposed and systematically optimized using device-level simulations. The proposed sensor architecture integrates three key design strategies to achieve enhanced sensitivity: (1) a heterojunction-based device structure that provides a reduced effective bandgap and strengthens the tunneling response; (2) charge plasma-induced source/channel regions, which eliminate the need for conventional doping processes and thereby simplify fabrication; and (3) a highly sensitive catalytic palladium (Pd) sensing terminal that enables selective H₂ detection through work-function modulation. Upon exposure to H₂, adsorption at the Pd terminal induces a pronounced work-function shift, which directly modulates the electrostatics and key electrical characteristics of the device. The sensing performance is evaluated by analyzing variations in drain current (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(I_{DS}\)</EquationSource> </InlineEquation>), threshold voltage (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(V_{Th}\)</EquationSource> </InlineEquation>), subthreshold swing (<i>SS</i>), and transconductance (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(g_m\)</EquationSource> </InlineEquation>), over a wide range of H₂ concentrations, relative to baseline (gas-free) conditions. The proposed sensor demonstrates an exceptional <i>I</i>_DS sensitivity of 99.99% and a <i>g</i>_m sensitivity of 99.98% for H₂ concentration at 8000 ppm. In addition, the device exhibits a strong response even under partial Pd terminal exposure and maintains robust performance against ambient temperature variations.</p>