A high-speed and high-sensitivity planar GaAs/Al/sub 0.3/Ga/sub 0.7/As heterostructure Schottky barrier photodiode is discussed. Using a highly doped Al/sub 0.3/Ga/sub 0.7/As buffer layer between the active layer and semi-insulating GaAs substrate, the series resistance and the undesired diffusion tailing are greatly reduced. Studies of surface stabilization and antireflection coating, performed to reduce the reverse-bias dark current and the reflection loss, resulted in significant improvement in the sensitivity of the photodiode. The measured internal quantum efficiency and responsivity are 60-77% and 0.47-0.6 A/W, respectively, for the wavelength range of 0.5-0.84 mu m. A risetime of 8.5 ps and an FWHM of 16 ps were measured by using a sampling/correlation method. >