This paper presents measurement results providing the mapping of the opto-microwave transfer function performed on an SiGe microwave heterojunction phototransistor (HPT). This measurements will be used to extract a guideline for designing phototransistors. A mapping of the HPT's gain in low frequency helps to estimate the shape of the optical beam used for the measurement. The study also focuses on the cutoff frequency mapping of the device in phototransistor mode. Finally, these results are used to determine the general optimization rules in the SiGe HPTs design.
