This paper reports on the use of plasma assisted molecular beam epitaxy of AlGaN/GaN high electron mobility transistors (HEMTs) grown on silicon substrate. Surface passivation effects on AlGaN/GaN HEMTs were studied using SiO2/SiN dielectric layers grown by plasma enhanced chemical vapor deposition. The direct current measurement, pulsed characteristics and microwave small-signal characteristics were studied before and after passivation. An improvement of drain-source current density and the extrinsic transconductance was observed on the passivated HEMTs when compared with the unpassivated HEMTs. An enhancement of cut-off frequency (ft) and maximum power gain (fmax) was also observed for the devices with full SiO2/SiN passivation. A good correlation is found between pulsed and power measurements.