To better understand the formation mechanism of ohmic contacts to GaN-based heterostructure field effect transistors, we have compared in detail Ti/Al/Ti/Au and Ti/Al/Ti/TiN contacts. Transmission electron microscopy and electron dispersive X-ray spectroscopy revealed that following anneal, TiN islands penetrated through the AlGaN barrier, as already well known, in the gold-based ohmic contacts but not in the gold-free contacts. We hence conclude that gold facilitates the formation of the TiN islands and propose that the role of gold is extraction of gallium from the semiconductor, providing a gallium depleted region for TiN island formation. For the case of the gold-free contacts, a 8 nm thick semi continuous TiN layer was formed following 900 °C anneal. A 2 nm thick TiN layer was observed in the as deposited samples and remained intact after anneal up to 825 °C. The different ohmic contact formation mechanism of gold-based and gold-free contacts is also manifested by our finding that a discontinuous AlN nitride spacer layer between the barrier and the bulk may lead to non-uniformity in contact behavior across the wafer in the case of gold-free contacts. For gold-based contacts, ohmic contact behavior was uniform across the wafer.