We show that MBE-grown GaAs on Si exhibits only a modest increase in the concentrations of the well-known electron traps typical of MBE-GaAs with no evidence for any new electron deep levels in the upper half of the bandgap in spite of the dislocations and other defects in the material. As shown by Au-GaAs Schottky contacts, the defects are unnoticeable when the device is forward biased but become very active in reverse biased condition, causing large leakage current and low breakdown voltage (although the device is still acceptable for many applications, especially FET's). The defects become more active after hydrogenation and more inactive after a post-growth rapid thermal annealing (RTA). Performance of devices made on thermally annealed GaAs on Si is comparable to those of GaAs on GaAs. Also, following the application of a large- current the device behavior improves, indicating a self-annealing action as a result of internal heating. The reverse current in the as-grown material shows a very weak temperature dependence, indicating its origin is not thermionic emission or carrier generation. It is speculated that a large part of the leakage current in the as-grown GaAs on Si is due to the defect assisted tunneling. After RTA, the average spacing between defect clusters increases, thus reducing the tunneling probability and tremendously improving the device characteristics.