Terahertz radiation associated with the impurity electron transition in quantum wells upon optical and electrical pumping

Radiation in the terahertz (THz) spectral range from structures with GaAs/AlGaAs doped quantum wells is investigated under conditions of the interband optical excitation of electron-hole pairs in $n$-type structures and impurity breakdown in a longitudinal electric field in $p$-type structures. The emission spectra are obtained. Emission is observed at low temperatures and shown to be determined by optical transitions between impurity states and transitions between the band and impurity states. Upon optical interband pumping, the impurity states are depopulated due to the recombination of electron-hole pairs with the involvement of impurities, while, in an electric field, the impurity states are depopulated due to impact ionization.