The undoped AlGaN/GaN heterostructure (Ga-face) produces a polarization induced two-dimensional electron gas (2DEG) at the heterointerface. The as-grown structure has a fixed positive charge on its surface to oppose the negative polarization charge on the top AlGaN layer's surface. Although sensitive to atmospheric conditions this positive surface charge is not sufficient in density to prevent significant depletion of the 2DEG. The ideal surface passivation is an insulator, acting as an encapsulant, which provides a fixed positive charge to neutralize the AlGaN polarization charge. We demonstrate that certain types of silicon nitride thin films can result in a charge balance and virtually eliminate depletion of the underlying 2DEG. A simple polarization model is used to explain the Hall and capacitance-voltage data on AlGaN/GaN heterostructures grown by organometallic vapor phase epitaxy. By changing the passivation layer thickness, the surface depletion can be systematically controlled, allowing the dependence of electron mobility on two-dimensional charge densities to be determined for samples grown on sapphire and SiC substrates. A metal-insulator-semiconductor structure of this type can be used to deplete the electrons induced at the AlGaN/GaN interface allowing insulated gate transistors to be realized. (C) 2002 Elsevier Science B.V. All rights reserved.