In recent years, the fabrication and physical characterization of photonic crystals have been met with increasing interest. In our contribution, theoretical dispersion studies of Si-based photonic crystals are presented. Solving Maxwell's equations by `plane wave expansions', band structures and densities of states are computed for triangular 2D lattices of air columns in silicon. A step-by-step variation of the pore size provides batches of band structures combined to comprehensive gap maps. Main concern of the paper are investigations of band gap modi®cations (TM and TE polarization) for pore shapes of deteriorating roundness, approximated by ellipses of varying eccentricity.

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