Ultra high speed photodetectors

In this paper we will discuss the design and characterization of an ultra-high speed GaAs Schottky barrier photodiode whose -3 dB power bandwidth is 100 GHz or equivalently in the time domain a full width half maximum (FWHM) of 5.4 picoseconds. The 100 GHz bandwidth is attained at an operating reverse bias of only 5 volts. The device consists essentially of a semitransparent Pt-film forming the Schottky barrier on n GaAs an n+ GaAs epitaxial layers grown on semi-insulating CaAs substrate. Ohmic contact is formed on the top surface completing the device. Details of the fabrication process will be presented. Parasitic capacitance of such a structure is less than 15 femtofarads and the junction capacitance is 20 femtofarads for a photosensitive area of 25 square microns. Beam-leads bare been added to the structure to minimize inductances. It will be shown that this structure is particularly adapted for monolithic integration with a field effect transistor (9,39). High speed photodetectors, mostly infrared to far infrared, have being in existence for many years and these will he briefly mentioned and referenced. To limit the scope of this presentation, only photon-effect devices (devices whose operation depends on the creation of an electron/hole in the semiconducting material) in the photoconductive and photovoltaic modes will be considered. The characterization of the speed of response of these ultra-fast photodetectors requires the development of a novel measuring technique known as the electro-optic sampling method