We report on the study of AlGaN/GaN high electron mobility transistors (HEMTs) incorporating an InGaN back-barrier grown by ammonia-assisted molecular beam epitaxy (NH3-MBE). The structural characterizations confirm the good crystalline quality of the heterostructure and the pseudomorphic growth of InGaN. This is also verified by the Hall effect mobility of about 2130 cm2 V−1 s−1. The associated two-dimensional electron gas carrier concentration is in the range of 8 × 1012 cm−2 for heterostructures grown on GaN:Fe-on-sapphire templates. Normal DC transistor operation is observed. Similar results have been obtained on silicon substrates. To our knowledge, this is the first demonstration of AlGaN/GaN HEMTs with an InGaN back-barrier grown by NH3-MBE.