Abstract <p>The article considers the features of heat propagation released by a single photon in a thermoelectric sensor with a surface area of 0.25 µm² consisting of a dielectric substrate (Al₂O₃), a heat sink (Mo), a thermoelectric layer (La_0.99Ce_0.01B₆), and an absorber (W) sequentially located on each other. The results of heat propagation simulation in the sensor with an operating temperature of 0.5, 0.8, 1, 1.2, and 1.5 K upon absorption of photons with energies of 0.8 and 1.65 eV are presented. The equivalent power of Johnson and phonon noise is calculated. The power of the signal arising on the sensor and the signal-to-noise ratio are determined. A comparison is made with the characteristics of a sensor of the same design with a surface area of 1 μm². The calculations were performed using the three-dimensional matrix method based on the equation of heat propagation from a limited volume. It is shown that decreasing the surface area of the sensor leads to an increase in the signal/noise ratio, and therefore to an increase in the efficiency of recording the already absorbed photon. This result is especially important for single-photon detection in the near infrared region.</p>