Abstract: Device structures and methods for reducing current collapse in high electron mobility transistors (HEMT) using aluminum nitride back barrier and in-situ two-step passivation are disclosed. In one aspect, the HEMT includes a back barrier layer including Al and N on a substrate, a channel layer including Ga and N on the back barrier layer, an AlxGa1-xN layer on the channel layer, a first passivation layer on the AlxGa1-xN layer, source and drain ohmic contacts, a T-shaped gate electrode at a location on a surface between the drain ohmic contact and the source ohmic contact, and a second passivation layer on the first passivation layer covering the surface and the T-shaped gate electrode.