10b.4 Investigation and Reduction of Leakage Current Associated with Gate Encapsulation by SiNx in AlGaN/GaN HFETs

The use of dielectric surface passivation in AlGaN/GaN technology is caused by improvement in dc/rf dispersion characteristics of GaN-based HFETs [1]. This passivation layer induces mechanical strain, which depends on type of dielectric, thickness of the layer, substrate and conditions of deposition [2, 3]. The most common choice of dielectric in GaN technology for microwave and high power applications is silicon nitride. The SiNx is also used for the two step lithography gate (“embedded” gate) realization. The MMIC fabrication process flow and the utilization of field plates in discrete microwave power transistors require encapsulation of the gate with the second dielectric layer. Beside these special requirements an encapsulation is advantageous in general, because of protection of the contacts, including gates, from air ambient. The application of the second dielectric layer over the gate of AlGaN/GaN HFETs, in turn, often results in an increase of the gate leakage current, which can be explained as a consequence of the strain experienced by the gate. In this work changes in fabrication process flow and a number of AlGaN/GaN HFETs modifications were tested in order to reduce gate leakage currents associated with the second SiNx layer.