The current gain cutoff frequency of very high frequency AlGaAs/GaAs heterojunction bipolar transistors has been measured at 300 and at 20 K using the picosecond optoelectronic technique. These devices showed cutoff frequencies of 72 and 84 GHz at room temperature and at a fixture temperature of 20 K, respectively. Optical response measurements were also obtained at visible and infrared wavelengths. The visible optical response shows a FWHM of 14 and 9.5 ps at 300 and 20 K, respectively. The infrared (λ=850 nm) optical response, corresponding to deeper penetration, showed a FWHM significantly larger (≳50 ps) than the visible optical response.