A technology similar to silicon-on-insulator is highly desirable for III-V electronics to support scaling for future generations. This letter reports the first realization of strained GaN quantum-well transistors embedded in unstrained AlN as the insulator. The molecular beam epitaxy (MBE)-grown heterostructure consisting of an ultrathin GaN channel buried in strain-free AlN barriers is favorable for scaling by the suppression of shortchannel effects. Ohmic contacts are realized with

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... heavily Si-doped n + GaN. For long-channel devices, a saturation drain current of ∼0.7 A/mm at V GS = +3 V and a peak extrinsic transconductance of ∼160 mS/mm around V GS = +1 V are measured at V DS = +10 V. No hysteresis is observed in the C-V measurement, indicating the high quality of all binary nitride heterostructures. The demonstrated device structure offers a high promise for high-frequency and high-power applications in the future. The strain-free barrier has the potential to enhance the reliability of GaN transistors. Index Terms-Aluminum nitride (AlN), gallium nitride, molecular beam epitaxy (MBE), quantum well (QW), transistor, ultrathin body (UTB).