Modeling and analysis of the spectral response for AlGaAs/GaAs HPTs for short wavelength optical communication

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model.