Photoconductive detection of THz radiation using two-step photoabsorption in LT-GaAs

Summary form only given. Low-temperature-grown GaAs (LT-GaAs) is the most widely used photoconductive (PC) material in the PC emitter or detector for THz radiation because of its unique properties, such as ultrashort carrier lifetime, large resistivity and relatively good carrier mobility. On the other hand, for optical communication using the loss-less 1.55-/spl mu/m wavelength window, a great deal of progress have been-made in developing fast optoelectronic devices toward the terahertz regime. Unfortunately, GaAs is usually not sensitive at 1.55-/spl mu/m because the optical bandgap is 1.43 eV (at room temperature), corresponding to a photon of wavelength 867 nm. While other narrow-gap semiconductors, such as Ge and InGaAs, have been studied and used as the ultrafast PC material for communication wavelength operation, these attempts have not been very successful because of the low dark-resistivity, which is inevitable for those narrow-gap semiconductors. High resistivity is important for performance of both the PC emitter and the PC detector. The emission efficiency increases with the bias electric field and is limited by the breakdown field of the antenna, whereas the noise in the detection process is determined by the thermal noise due to carriers, or equivalently Johnson noise, in the antenna current, which is inversely proportional to the square root of the dark resistivity.