Compared to epitaxial gallium arsenide (GaAs) grown at regular temperature, low-temperature epitaxial growth of GaAs results in a dramatically shorter carrier lifetime. This produces subpicosecond device responses and enables ultra-high-speed photodetection. However, low-temperature-grown GaAs (LT-GaAs) photodetectors usually exhibit poor responsivity due to the lower carrier mobility in LT-GaAs. Here, the properties and characteristics of the low-temperature-grown material are investigated. Carrier lifetime as a function of growth and annealing temperatures are explored as well as nonlinear optical characteristics and optical absorption well below the bandgap energy of GaAs. Several photodetector styles (photoconductive, PIN, MSM, and waveguide photodetectors) utilizing LT-GaAs are presented. These photodetectors enable a variety of high-speed photonic systems. Their technology and the future use of LT-GaAs are explored, especially in the field of THz emitters and receivers.