The present invention recites a new method for manufacturing Group III-N field-effect devices, such as HEMT, MOSHFET, MISHFET devices or MESFET devices, grown by Metal-Organic Vapor Phase Expitaxy, with higher performance (power), by covering the surface with a thin SiN layer on the top AlGaN layer, in the reactor where the growth takes place at high temperature, prior cooling down the structure and loading the sample out of the reactor, as well as a method to produce some HEMT transistors on those heterostructures, by depositing the contact on the surface without any removal of the SiN layer by MOCVD. The present invention recites also a device.