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A simulation methodology for ultra-scaled InAs quantum well field-effect transistors (QWFETs) is presented and used to provide design guidelines and a path to improve device performance. A multiscale modeling approach is adopted, where strain is computed in an atomistic valence-force-field method, an atomistic sp3d5s* tight-binding model is used to compute channel effective masses, and a 2-D real-space effective mass-based ballistic quantum transport model is employed to simulate three-terminal current-voltage characteristics including gate leakage. The simulation methodology is first benchmarked against experimental I-V data obtained from devices with gate lengths ranging from 30 to 50 nm. A good quantitative match is obtained. The calibrated simulation methodology is subsequently applied to optimize the design of a 20 nm gate length device. Two critical parameters have been identified to control the gate leakage current of the QWFETs, i) the geometry of the gate contact (curved or square) and ii) the Schottky barrier height at the gate metal contact. In addition to pushing the threshold voltage toward an enhancement mode operation, a higher Schottky barrier at gate metal contact can help suppress the gate leakage and enable aggressive insulator scaling.