2D electron density tuning from 5.3 × 10 12 cm-2 to 8.8 × 10 12 cm-2 by UV illumination has been demonstrated for an ultra-pure GaN/Al0.25Ga0.75N heterojunction. A single subband is occupied in the dark with an electron density of 5.3 × 10 12 cm-2, while in the illuminated sample with an electron density of 8.8 × 10 12 cm-2, characteristic beating patterns appear in magnetotransport data due to the filling of the second quantized subband. Simultaneously, this distribution is unambiguously confirmed by Landau level splitting and intensity oscillations in photoluminescence spectra. The electron densities and quantum lifetimes in each individual subband have been extracted independently, and the intersubband energy spacing amounts to 82 meV. Surprisingly, the quantum scattering time for electrons residing in the second subband is increased compared to its ground state counterpart.