We make a comparative analysis of coupled carrier plasmon-polar phonon modes at doped polar semiconductor surfaces in the absence and the presence of a carrier-depletion layer. Our analysis shows how the effect of the depletion layer appears in the spatial structure of each excitation mode visualized in a contour map of the induced charge-density, the coupling character elucidated by the phase relation and the amplitude ratio of the carrier component and the polar-phonon component in the induced charge-density distribution, and the energy-loss intensity due to the surface excitation.