Creation of Two‐Dimensional Electron Gas and Role of Surface Donors in III‐N Metal‐Oxide‐Semiconductor High‐Electron Mobility Transistors
The role of surface donors at the oxide/semiconductor interface of III‐N metal‐oxide‐semiconductor (MOS) high‐electron mobility transistors (HEMTs), by creating a two‐dimensional electron gas (2DEG) and the device performance, are investigated. Al2O3/GaN/AlGaN/GaN MOS HEMTs show the surface donor density (Nd,surf) of 2.2 × 1013 cm−2, which is increased up to 3.4 × 1013 cm−2 after post‐deposition annealing. In the latter, surface donors fully compensate the surface polarization charge and the HEMT threshold voltage decreases substantially with the oxide thickness. On the other hand, an open‐channel drain current is found to be independent of Nd,surf, while marginal trapping is completely removed when Nd,surf increases with annealing. Consequently, ionized surface donors behave like a fixed charge and are clearly distinguishable from trapping states. Open‐channel 2DEG densities of ≈1.1 × 1013 cm−2 are extracted from capacitance–voltage measurements. Similarly, recent data on enhancement‐mode HfO2/InAlN/AlN/GaN MOS HEMTs are analyzed where Nd,surf is reduced down to 1 × 1013 cm−2 while 2DEG densities reach ≈2.7 × 1013 cm−2. It is suggested that under the open‐channel condition, 2DEG is supplied also by an injecting source contact if Nd,surf is lower than the QW polarization charge. Our charge quantifications are supported by calculating energy‐band diagrams.