<p>This study presents the fundamental solution and the Green’s function for a semi-infinite orthotropic photothermoelastic medium incorporating diffusion and temperature-dependent material properties within the Moore–Gibson–Thompson (MGT) heat-conduction framework (TDMGTPD). The governing equations are nondimensionalized and reduced to two dimensions, yielding a unified formulation for the coupled thermoelastic–diffusive fields. By introducing suitable harmonic potentials, explicit expressions for the fundamental solution and the Green’s function are derived for point heat and chemical-potential sources located at the surface or in the interior of the medium. Closed-form solutions for displacement, stress, temperature, and carrier density are expressed using elementary functions. Numerical results for silicon demonstrate the influence of temperature-dependent parameters and show clear distinctions from earlier photothermoelastic and thermoelastic models. Several classical cases are recovered as limiting forms. The findings indicate that diffusion effects and temperature-dependent behavior significantly modify the thermomechanical response of orthotropic media, with implications for semiconductor device modeling and high-temperature material design.</p>