Infrared Spectroscopic Ellipsometry is presented as a feasible and novel technique for contactless and nondestructive measurement of free-carrier and crystal-structure properties in the characterization of complex semiconductor heterostructures for device applications. Infrared-active lattice modes and coupling of free-carrier plasmons to longitudinal-optical lattice phonon modes strongly affect the infrared-optical response of semiconductor materials. Analysis of ellipsometry data from 2 micrometers to 100 micrometers can provide precise information on phonon mode frequencies and broadening parameters, static dielectric constants, free-carrier concentration, and free-carrier mobility at optical frequencies of III-V compound semiconductors, even for films with thicknesses only a fraction of the probing wavelengths. Alloy composition, strain, crystal quality, and free-carrier properties of constituent layers in thin-film structures, designed for optoelectronic or electronic device applications, can be derived. We demonstrate the characterization of coherent and incoherent light emitter structures based on group-III-nitride materials, where information such as concentration and mobility of free carriers in n- and p-type regions, thickness, composition, and quality of device constituents are accessible.