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 modifications (TM and TE polarization) for pore shapes of deteriorating roundness, approximated by ellipses of varying eccentricity.