High quality passivation silicon nitride films have been obtained requiring no surface pretreatment and being fully compatible with monolithic microwave integrated circuits. The nitride film is deposited by electron cyclotron resonance-chemical vapor deposition directly over GaAs-n substrate and over InGaP/GaAs heterojunction structures, which are used for heterojunction bipolar transistors (HBTs). Metal/ nitride/ GaAs-n capacitors were fabricated for all the samples. Effective charge densities of 3 X 10(11) cm(-2) and leakage current densities of 1 mu A/cm(2) were determined. Plasma analysis showed a reduced formation of molecules such as NH in the gas phase at low pressures, allowing the deposition of higher quality films. The process was used for InGaP/GaAs HBT fabrication with excellent results, such as higher current gain of passivated device comparing to unpassivated HBTs. (c) 2006 American Vacuum Society.