We present experimental and theoretical results on interband and intersubband transitions in non-intentionally doped and silicon-doped GaN/AlN quantum wells. All the samples have been grown by Molecular Beam Epitaxy on sapphire and 6H-SiC substrates. Structural characterization was carried out by means of Rutherford Back-Scattering and X-ray diffraction to determine the average quantum well thickness and period. Room-temperature photoluminescence, Fourier transform infrared absorption and photo-induced absorption spectroscopy show that strong localization of carriers occurs, even at room temperature, due to monolayer fluctuations combined with the strong internal electrical field. In addition, we observed a blue-shift of similar to 0.1 eV of the intersubband transition energy in the doped samples with respect to the undoped ones due to many-body effects. Good agreement between the simulations and the experimental results is achieved.