The electronic characteristics of GaAs grown on Si by molecular beam epitaxy(MBE) has been examined by studying the diode characteristics and deep level transient spectroscopy (DLTS) of Schottky barriers. l-V characteristics of samples show the existence large leakage current. Post-growth rapid thermal annealing(RTA)has been found to be able to significantly improve the diode behaviour. The reverse current in the as-grown material shows a very weak temperature dependence, indicating that its origin is not thermoionic emission or generation-recombination currents. We think that a large part of this current is due to defect-assisted tunneling, which is reduced by RTA. DLTS spectra show that Ec-0.41eV and Ec-0.57eV electron traps are observable. The former is located near the GaAs-Si interface probably, the latter relates to the well-known electron trap M5 typical of MBE GaAs.