Effect of the Twist Crystallinity of N‐Polar AlN Underlayer on the Electrical Properties of GaN/AlN Structures

Recently, N‐polar GaN/AlGaN/AlN high‐electron‐mobility transistors (HEMTs) have been demonstrated using molecular beam epitaxy (MBE). However, the MBE method is not suitable for mass production because of the need for ultrahigh vacuum, small chamber size, and long deposition time. This study fabricates N‐polar GaN‐ or AlGaN‐channel HEMTs on AlN with polarization directions opposite to those of Ga‐polar GaN‐based HEMTs without any cap layer and demonstrates field‐effect transistor static characteristics. In this article, the focus is on the crystal quality of the underlying layer and the effect of crystalline quality on electrical properties is investigated. The crystalline quality of the twist component of the N‐polar AlN layer is improved by combining the Al‐polar tiny‐pit layer and polarity inversion from Al‐ to N‐polar. Finally, it is shown that the crystalline quality of the twist component is a crucial factor for improving the electron mobility of the N‐polar GaN channel and simultaneously increasing I DS .