This study proposes a nitrogen-polar (N-polar) AlxGa1-xN/Al0.9Ga0.1N/AlN structure that can generate a large amount of two-dimensional electron gas (2DEG) to enhance the device development of samples. Additionally, we have analyzed the critical thickness of N-polar AlGaN/AlN based on the theoretical calculations of three different values of film thickness. The metalorganic vapor-phase epitaxy method is used to grow N-polar AlxGa1-xN/Al0.9Ga0.1N/AlN on sapphire substrates. The substrates with a misorientation angle of 2° along the m-axis and a-axis directions are selected to determine the effect of the off-cut angle on sample flatness and current-voltage characteristics. Furthermore, we determine the effect of Al contents on N-polar AlxGa1-xN/Al0.9Ga0.1N/AlN under the optimum growth conditions of the growth thickness of the top layer of AlGaN and sapphire substrate. The results indicate that the current throughput increases with a decrease in Al content. Lastly, we have fabricated the N-polar AlGaN/AlN heterostructure field effect transistor (FET) to demonstrated the static FET characteristics.

AlN‐based field‐effect transistors (FETs) enable high‐breakdown voltage, high drain current, and high‐temperature operation. To realize high‐frequency devices, N‐polar AlGaN/AlN heterostructure FETs are focused on. N‐polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN FET is fabricated using metal–organic vapor‐phase epitaxy, and its electrical characteristics are evaluated. An N‐polar AlN layer is grown on a sapphire substrate with a misorientation angle of 2.0° toward m‐axis, on which a 20 nm thick Al0.9Ga0.1N base layer and a 20 nm Al0.1Ga0.9N channel layer are grown. The static FET operation is confirmed to exhibit an n‐channel and pinch‐off. Normally, during operation with a turn‐on voltage of −3.2 V, a high operating breakdown voltage of 620 V and high operating temperature of 280 °C are also confirmed.

The nitrogen‐polar (N‐polar) AlGaN/AlN structure is expected to have higher carrier density than conventional metal‐polar AlGaN/GaN electronic devices, and the AlN substrate offers various advantages, such as high breakdown voltage and high‐temperature operation. Herein, a N‐polar AlGaN/AlN‐heterostructured field‐effect transistor (FET) with static FET characteristics is successfully fabricated. However, the drain current density, IDS, remains significantly small. This study aims to improve IDS by doping Si in the topmost AlGaN channel layer under various conditions.