Bolometers are thermal detectors widely applied in the mid-infrared (MIR) wavelength range. In an integrated sensing system on chip, a broadband scalable bolometer absorbing the light over the whole MIR wavelength range could play an important role. In this work, we have developed a waveguide-based bolometer operating in the wavelength range of 3.72–3.88 µm on the amorphous silicon (a-Si) platform. Significant improvements in the bolometer design result in a 20 × improved responsivity compared to earlier work on silicon-on-insulator (SOI). The bolometer offers 24.62% change in resistance per milliwatt of input power at 3.8 µm wavelength. The thermal conductance of the bolometer is 3.86 × 1 0 − 5 W / K , and an improvement as large as 3 orders magnitude may be possible in the future through redesign of the device geometry.