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The Schottky-source forward current-voltage characteristics were measured and analyzed in a recently reported AlGaN/GaN metal-2-D-electron-gas tunnel junction field-effect transistor (FET) (TJ-FET) in order to characterize the effective Schottky barrier height (SBH) and its gate bias dependence. The SBHs were extracted and found to be gate bias dependent, varying from 0.57 eV at VGS = -0.5 V to 0.3 eV at VGS = 1 V. This gate-induced Schottky barrier lowering (SBL) effect is explained by the image-charge effect. A 2-D device simulation is carried out to analyze the SBL effect and is shown to be in good agreement with the experiment results. The larger SBH at lower gate voltage enables off -state blocking, while the lower SBH at higher gate voltage leads to high on -state current driving capability in the AlGaN/GaN TJ-FETs.