The effects of the channel electron density on the interband optical transitions of strained (x=0.6 and 0.65) and lattice‐matched (x=0.53) InxGa1−xAs/In0.52Al0.48As/InP high‐electron‐mobility transistor structures have been investigated by phototransmittance at room temperature. Analysis of the ground and first excited transitions for low and high densities, respectively, enabled a separate estimation of the electron densities occupying each one of the first two subbands. It was found necessary to include the modulation of the phase‐space filling in the analysis of the spectra, especially for the samples with a high electron density, in which case this modulation mechanism becomes dominant. ©1996 American Institute of Physics.