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• HF pretreatment prior to gate formation to improve gate leakage characteristics. • 3–4 orders of magnitude improvement in gate leakage was observed. • The mechanisms of the improved gate leakage was investigated. • The properties of the improved gate leakage with/without pretreatment were compared. We demonstrate the electrical performances of an AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) with low gate leakage current (I g). A low gate leakage current as low as the order of 10−11 A/mm was achieved from normally-off MIS-HEMT device (V th = 2.16 V) with a partially recessed gate, fluorine treatment, and ALD Al 2 O 3 gate dielectric layer. The gate leakage current decrease is attributed to the pre-treatment of the gate region with hydrofluoric acid (HF) and deionized water (DI) solution, which acts to remove the native oxide layer and thus decrease interface traps. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses demonstrate that the AlGaN surfaces are modified such that the surface roughness and native oxide introduced by the treatments used to achieve normally-off operation are remedied with the use of the pre-treatment. [ABSTRACT FROM AUTHOR]