In this paper we have developed a passivation technique with silicon-nitride (SiNX)film that requires no surface pre-treatment, and is fully compatible to monolithic microwave integrated circuits (MMICs). The nitride depositions were carried out by ECR-CVD ( electron cyclotron resonance-chemical vapor deposition) directly over InGaP/GaAs heterojunction structures, which are used for heterojunction bipolar transistors (HBTs). Optical emission spectrometry (OES) was used for plasma characterization, and low formation of H and NH molecules in the gas phase was detected at pressure of 2.5 mTorr. These molecules can degrade III-V semiconductor surfaces due to the preferential loss of As or P and hydrogen incorporation at the substrate. The substrates were cleaned with organic solvents using a Sox-let distillate. The ECR depositions were carried out at a fixed substrate temperature of 20 degrees C, SiH4/N-2 flow ratio of 1, Ar flow of 5 sccm pressure of 2.5 mTorr and microwave (2.45 GHz) power of 250 W and RF (13.56 MHz) power of 4 W. We have applied this film for InGaP/GaAs HBT fabrication process with excellent results, where two major contribuiton is related to this passivation technique, the enhancement in the transistor dc gain beta and the improvement in the signal-to-noise ratio when compared unpassivated and passivated devices. (c) 2008 Published by Elsevier B.V.