The passivation of GaAs MESFET’s with PECVD silicon nitride films of both compressive and tensile stress is reported. Elastic stresses included in GaAs following nitride passivation can produce piezoelectric charge density, which results in a shift of MESFET characteristics. The shift of MESFET parameters due to passivation was found to be dependent upon gate orientation. For example, it is found that the shifts of threshold voltage are of opposite sign for MESFET’s oriented along the  and  directions. Our experiments show that nitride of tensile stress is preferable for MESFET’s with  oriented gates. The shifts in VTH, IDSS, and GM of the devices before and after nitride passivation are less than 5 percent if the nitride of appropriate stress states are used for passivation. The breakdown voltage of the MESFET's after nitride deposition was also studied. It is found that the process with higher hydrogen incorporation tends to reduce the surface oxide and increase the breakdown voltage after nitride deposition. In addition, the passivation of double channel HEMT’s is reported for the first time.