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Polarization-induced p-type doping is realized in molecular beam epitaxy (MBE) grown ultrathin body GaN/AlN heterostructures. The novel heterostructure consisting of a thin strained GaN layer grown on an AlN template exhibits a hole gas density close to the interface polarization charge (~5 × 1013 /cm2). Both enhancement- and depletion-mode (E/D) p-channel heterostructure field effect transistor (HFETs) are demonstrated. Driven by the high hole density, a 2.4-μm-long D-mode HFET with alloyed ohmic contacts shows improvement of drain current from 150 mA/mm (VGS = 12 V, VDS = 30 V) at 300 K to 270 mA/mm (VGS = 15 V, VDS = 30 V) at 77 K. The extrinsic peak transconductance of the D-mode device increases from 11 mS/mm (VGS = 6 V, VDS = 30 V) at 300 K to 16 mS/mm (VGS = 4 V, VDS = 30 V) at 77 K. Both the drive current and transconductance are recorded in nitride p-channel FETs. MBE regrown heavily Mg-doped p+-GaN is then employed for ohmic contacts of E-mode p-channel HFETs. A 2-μm-long E-mode device with a drain current of 4 mA/mm (VGS = 10 V, VDS = 80 V) and ON/OFF current ratio of 103 is achieved.