III-nitride electronic devices have made significant progress in high frequency and high power applications in the past decade. In this work, nitride heterostructures with >70 % Al composition AlGaN ternary alloy barriers have been grown by molecular beam epitaxy (MBE). With quasi-growth interruption technology the mobility of the two-dimensional electron gas (2DEG) channel has been improved by one order of magnitude and reached ~1400 cm2/Vs. Threshold voltage control by work function engineering has been demonstrated in high Al composition HEMTs, enabling integration of enhancement (E) and depletion (D) mode HEMTs.
To further boost the radio frequency (RF) and power performance of nitride-based electronics, scaling to deep submicron regime is required. Ultra-thin body (UTB) devices offer ways for tight electrostatic and quantum confinement of charge carriers to nanoscale dimensions. With its large polarization charge, wide bandgap and large band offsets, AlN induces the maximal carrier densities while providing the best confinement for nitride channels of all compositions. Several MBE growth technologies have been employed in this work to realize, for the first time, nitride heterostructures consisting of an ultra-thin strained GaN channel sandwiched in relaxed AlN barriers. The density of the 2DEG could be varied by changing the GaN quantum well thickness while fixing the top barrier thickness in the heterostructure. The resulting AlN/GaN/AlN n-channel field effect transistors (nFETs) have been demonstrated on single crystal bulk AlN substrates with regrown ohmic contacts. A maximum drain current drive of ~2.8 A/mm has been achieved. A current cut-off frequency fT of 120 GHz has been measured on 65-nm-long gate devices.
Nitride devices on AlN platform stand also to benefit from the symmetry of electronic polarization: high density hole gases can be generated in much the same way as the high density 2DEG in GaN HEMTs, thus enabling p-channel FETs (pFETs) on the same material platform as nFETs in a logical manner. Polarization-induced p-type doping is realized in a UTB strained GaN channel on AlN template grown by MBE, with a hole gas density close to the interface polarization charge (~5 x 1013 /cm2). High density of 2D hole gases are resistant to freeze out at cryogenic temperature in temperature-dependent Hall-effect measurements. Both E/D-mode operations are achieved based on GaN/AlN heterostructures. Driven by the high hole density, a long channel D-mode pFET shows a drain current of ~150 mA/mm at 300 K which increases to ~270 mA/mm at 77 K.
The UTB GaN nFETs and pFETs on AlN present a compelling case for III-nitride complementary logic and high power applications. Various channels (InGaN, AlGaN etc.) could be explored in the future to improve carrier mobility, induce higher carrier density or enhance breakdown characteristics. Technologies for monolithic integration of nitride channel nFETs and pFETs on AlN have been proposed.