The excited states of nanosized CdS semiconductor crystals with bulk doping have been studied. It was demonstrated that the giant dipole resonance dominated in their photoabsorption spectra. Varying the potential barrier height at the nanoparticle boundary was shown to be able to cause a change in the character of the collective mode from the plasmonic-type electron density fluctuations to the dimensional quantization mode depending on the particle size and the number of free charge carriers.