We present a systematic investigation of the formation of Schottky barriers between n- and p-GaN(0001)-(1 x 1),grown by metalorganic chemical vapor deposition and a series of high and low work function metals (Mg, Al, Ti, Au, and Pt). Al, Ti, and Mg react at room temperature with nitrogen, whereas Au and Pt form abrupt, unreacted interfaces. We find that the Fermi level movement on both n- and p-GaN is consistent with variations in metal work functions, but limited by surface or interface states. Upon annealing, the incorporation of Mg increases the density of accepters as seen on both n- and p-GaN. In spite of similar work functions and chemical reaction with nitrogen, Ti and Al show drastic differences in Schottky barrier formation due to differences in the nature of the products of reaction. AlN is a wide band gap semiconductor whereas TiN is a metallic compound.