Herein, a recessed-gate AlGaN/GaN metal oxide semiconductor heterojunction field-effect transistor (MOS-HFET) with an AlGaN back-barrier layer fabricated by a selective area regrowth (SAG) technique is investigated. A recessed-gate structure enables normally off operation required for power-switching applications. A thin AlGaN/GaN channel and the AlGaN back-barrier structures are fabricated on a Si substrate by metal–organic chemical vapor deposition. The 50 nm thick, thin GaN channel layer with a smooth surface is grown on the AlGaN back-barrier layer. A recessed-gate structure is successfully formed by SAG of an AlGaN/GaN layer on the thin AlGaN/GaN layer. The regrown AlGaN/GaN high electron mobility transistor structure shows lower sheet resistance owing to high concentration and high mobility of a two-dimensional electron gas. Transfer characteristics of the thin AlGaN/GaN channel MOS-HFETs show normally off operation as a consequence of using the AlGaN back-barrier structure. Channel mobility becomes five times higher than that of GaN channel in the case of using the thin AlGaN/GaN channel. These results indicate that the regrown thin AlGaN/GaN channel MOS-HFET has the potential to realize low on-resistance and normally off operation.