Growth, electron transport, and transistors of N-polar distributed polarization-doped AlGaN channel on single-crystal AlN substrates

We report growth and electron transport of distributed polarization-doped (DPD) AlGaN channels on N-polar single-crystal AlN substrates by plasma-assisted molecular beam epitaxy, along with the initial field-effect transistors (FETs) demonstrated in these AlGaN channels. The AlGaN channels were realized by linearly grading AlxGa1−xN from binary AlN to terminal Al compositions of x=0.07, 0.17, and 0.50 over a thickness of 100 nm without any intentional doping, enabling the formation of polarization-induced three-dimensional electron gases (3DEGs). Reciprocal space mapping confirmed coherent growth on AlN substrates. Hall-effect measurements revealed systematic control of integrated electron sheet density from 3.7×1013cm−2 (x=0.07) to 1.8×1013cm−2 (x=0.50), and capacitance–voltage measurements confirmed the 3DEG spatial distribution. FETs fabricated on the structure with x=0.50 exhibited effective field modulation of the 3DEG, with a maximum drain current density of 32 mA/mm, an on/off ratio exceeding 104, and a breakdown voltage of 445 V at a gate-to-drain spacing of 2.5μm. Comparison between modeled and measured 3DEG mobilities suggests that the moderate mobility achieved can be attributed to alloy scattering and point defects in the high-Al composition (x>0.5) layers in these early demonstrations. Future investigations should focus on improving the high-Al composition epitaxy and transport in order for N-polar DPD-AlGaN FETs on bulk AlN to establish as a viable platform for high-power, ultrawide-bandgap electronics.