A layer of yttrium (Y) and aluminum nitride (AlN) is employed as a back-barrier to improve confinement of electrons within a channel layer of a high electron mobility transistor (HEMT). As HEMT dimensions are reduced and a channel length decreases, current control provided by a gate also decreases, and it becomes more difficult to “pinch-off” current flow through the channel. A back-barrier layer on a back side of the channel layer improves confinement of electrons to improve pinch-off but does not cause a second 2DEG to be formed below the back-barrier layer. The YAlN layer can be lattice-matched to the channel layer to avoid lattice strain, and a thin layer of YAlN provides less thermal resistance than HEMTs made with thicker back-barrier materials. Due to its chemical nature, a YAlN layer can be used as an etch stop layer.