•We demonstrated gallium nitride (GaN) based negative capacitance field-effect transistors (NCFETs) that achieve steep switching operation.•Ferroelectric phase characteristics were successfully demonstrated using a 10 nm thick undoped-HfO2 ferroelectric thin film fabricated through atomic layer deposition.•The HfO2 metal-ferroelectric-metal (MFM) capacitor was connected in series with the gate electrode of a recessed normally-off AlGaN/GaN MOS-HFET.•The forward/reverse subthreshold slopes were dramatically reduced from 104/105 to 22/23 mV/dec. We demonstrated gallium nitride (GaN) based negative capacitance field-effect transistors (NCFETs) that achieve steep switching operation. Ferroelectric phase characteristics were successfully demonstrated using a 10 nm thick undoped-HfO2 ferroelectric thin film fabricated through atomic layer deposition. The HfO2 metal-ferroelectric-metal (MFM) capacitor was connected in series with the gate electrode of a recessed normally-off AlGaN/GaN MOS-HFET. The forward/reverse subthreshold slopes were dramatically reduced from 104/105 to 22/23 mV/dec.