We have studied interband optical transitions, electronic structure and structural quality of p-type (Be) and n-type (Si) δ-doped GaAs/AlAs MQWs designed for selective THz sensing applying differential surface photovoltage (DSPV) spectroscopy. Sharp derivative-like features associated with excitonic optical transitions in GaAs/AlAs MQWs have been observed in the spectra at 300 K and 90 K temperature. The energies and line broadening parameters for a large number of QW related excitonic transitions were determined from the line-shape analysis of the DSPV spectra. The spectroscopic data of transition energies were found to be in a good agreement with calculations within the envelope function approximation which took into account the nonparabolicity of energy bands. Analysis of the dependence of the exciton linewidth broadening on the quantum subband number allowed evaluate line-broadening mechanisms and interface roughness in the MQW structures. It was determined that doping with Si broadens more effectively the optical spectra lines in comparison with the structures of the same design doped with Be.