We present the results of tight-binding molecular dynamics calculations for studying the equilibrium structures and the bonding properties of Si_n clusters for n up to 18. To prevent unphysically large charge transfer between different atoms in clusters, we employ the atomic charge neutrality constraint that each atom has approximately four valence electrons. With a limited number of parameters in the tight-binding scheme, the structures of minimum energy are well reproduced, as compared with results from previous ab initio quantum mechanical calculations. We find the abundant cluster sizes n=4, 7 and 10, which are in good agreement with other theoretical and experimental results. For n>or=7, surface-like compact structures with a pentagon or a hexagon base are found to be energetically favourable, resulting in the metallic nature of the cluster bonding, while a core-based structure appears first for Si₁₅.