AlGaN/GaN HEMTs have established themselves in high speed high power electronic applications, thanks to the large breakdown field, heterostructure design freedom, and unique polarization-induced 2DEG properties. A challenge in improving the HEMT frequency performance is scaling down the device dimension both laterally and vertically at the same time without sacrificing the 2DEG transport properties. Compared with AlGaN/GaN, AlN/GaN structures allow very small gate-2DEG distance (1-4 nm) for the ease of vertical scaling while maintaining decent 2DEG properties: large 2DEG density, high mobility and good charge confinement . Besides downscaling of the device dimension, low source/drain contact resistance is also required for high frequency performance. By far, the ohmic contact resistance on AlN/GaN source/drain areas is usually above 0.4 Ω∙mm due to the large energy barrier height of AlN. In this work, we have explored the source/drain regrowth with heavily-doped (In)GaN materials on metal-face AlN/GaN heterostructures. The AlN/GaN heterostructure sample used in this work was grown by MBE in a Veeco Gen 930 system. The AlN barrier is 4 nm thick; and the 2DEG density and mobility are measured to be 3.1e13 cm-2 and 1193 cm2/V.s, resulting in a sheet resistance of 170 Ω/sq. Other GaN HEMT structures have also been used for developing the device fabrication and regrowth processes. The HEMT source/drain areas were first etched, and then, using tungsten as regrowth mask, heavily doped (Si: ~ 10e20 cm-3) InN(30 nm)/graded-InGaN(50 nm)/GaN (20 nm) were regrown by MBE. Control samples were also prepared by regrowing the same structure on semi-insulating GaN templates for further characterization. After regrowth, the amorphous material deposited on the tungsten mask was removed by molten KOH, and the tungsten mask was removed by H2O2. The XRD scans on the control samples reveal that a linearly graded InGaN from GaN to InN can be successfully regrown. The TLM measurement of non-alloyed Ti-based ohmic contacts deposited on a control sample indicates that contact resistances of ~ 0.2 Ω∙mm with sheet resistances of ~ 100 Ω/sq can be achieved on the regrown InN/InGaN/GaN. In the first regrowth experiment of source/drain on a HEMT, a gap was observed between the regrown material and the 2DEG channel, leading to a large contact resistance. To eliminate the gap, the migration enhanced epitaxy (MEE) technique was applied. After exploring several growth conditions, no gap was seen under SEM and AFM imaging. However, electrical measurements using non-alloyed Ti-based contacts on the regrown source/drain in HEMTs showed unacceptably large resistances. To understand its causes, the transmission electron microscope investigation is under way to interrogate the regrowth-sidewall interface. Studies are also currently being carried out to minimize damages to the 2DEG channel during the sample preparation as well as regrowth.