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A new physics-based model of the gate leakage current in AlGaN/GaN heterojunction field effect transistors (HFETs) is demonstrated. The model predicts accurately the gate-leakage current for a wide range of gate-drain voltage. The model is based on the formulation of tunneling and space charge limited (SCL) current flow. The gate leakage current is shown to flow through two paths: 1) the surface of the device establishes the primary path and the current transport mechanism is modeled using SCL transport in the presence of shallow traps and 2) the traps in the AlGaN layer assist in the tunneling of electrons from the gate to the 2-D electron gas, which flows to the drain electrode constituting the second path for the leakage current. The trap levels extracted from the model are consistent with the reports presented in the literature. The model appropriately explains the gate leakage current of the AlGaN/GaN HFETs for drain voltages up to 170 V and is verified by comparing the model results with the measured gate leakage data of industrial devices.