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We report the electrical characteristics at room temperature of (Al,Ga)N/GaN high-electron-mobility transistors with different gate lengths Lg ranging from 0.15 to 2 μm. Static measurements demonstrate an increase of leakage current, whereas the maximum drain–source current and maximum transconductance decrease with increase of the gate length. In addition, drain current–voltage measurements for gate voltage Vgs = −3 V reveal a kink effect, which is pronounced for Lg = 0.15, 0.25, and 0.5 μm and is absent for Lg = 1 and 2 μm. Furthermore, a self-heating effect on the drain current is observed at Vgs = 0 V for a short gate length. By means of deep level transient spectroscopy, we detected one hole trap H1, which is considered to be the main cause of the observed kink effect. Also, the effect of gate length on the static output characteristics and the deep level transient spectroscopy results for Al0.25Ga0.75N/GaN high-electron-mobility transistors is also discussed.