We present the electrical characterization of n-type GaAs/AlAs multiple quantum well (MQW) structures designed for terahertz (THz) radiation sensing at cryogenic temperatures. The samples were grown by solid source molecular beam epitaxy (MBE) and were δ-doped with silicon atoms at each potential well center. Temperature dependent Hall effect data show that (i) the conduction in these planar doped structures is thermally activated below 180 K, (ii) the free carriers sheet densities are near the metal-insulator transition, and (iii) the low temperature mobility is controlled by ionized impurities scattering. The study of the magneto-transport properties at 1.3 K further indicates that only the fundamental electronic sub-band of the two-dimensional electronic gas is populated. The MQW samples were then processed into lateral mesa-shaped photodetectors to investigate their spectral response in the THz frequency range. The preliminary experimental results for the proposed detection scheme, which involves transitions in the confined shallow donor impurity states, are described.