We propose that the photonic band structure (PBS) of semiconductor-based photonic crystals (PCs) can be made tunable if the free-carrier density is sufficiently high. In this case, the dielectric constant of the semiconductor, modeled as ε(ω) = ε0(1 - ωp2/ω2), depends on the temperature T and on the impurity concentration N through the plasma frequency ωp. Then the PBS is strongly T and N dependent; it is even possible to obliterate a photonic band gap. This is shown by calculating the 2D PBS for PCs that incorporate either intrinsic InSb or extrinsic Ge. © 2000 The American Physical Society.