(Invited) Manufacturing Microwave AlGaN/GaN High Electron Mobility Transistors (HEMTs) on Truly Bulk Semi-Insulating GaN Substrates

AlGaN/GaN high electron mobility transistors (HEMTs) seem to dominate the market and research in the area of high-power RF technology. The dominant substrate material for GaN epitaxy is SiC. The 3.5% lattice mismatch between GaN and SiC is relatively small but still may be a considerable source of lattice defects. The main reason for use of foreign substrates was for years, lack of high quality GaN wafers. However, recently it came a breakthrough in this field. A leading GaN substrates manufacturer, Ammono S.A., has shown 2” truly bulk highly-resistive GaN wafers grown by ammonothermal method. The next key problem related to the use of foreign substrates is a management of the heat transfer inside multilayer epitaxial structure of GaN HEMTs. Due to the reliability and performance issues, this aspect is especially important for high power devices. While the thermal conductivity of 4H-SiC is higher than for GaN, the heat flow inside typical GaN HEMT on SiC substrate is significantly limited because of presence of buffer or nucleation layers (e.g. AlN) between epilayers and SiC substrate. That effect is commonly called thermal boundary resistance (TBR). Dislocations at the interfaces have a large share in the TBR. It can be generalized, that buffer layers grown on the other substrates that GaN have a significant contribution to total thermal resistance of GaN HEMT. In case of AlGaN/GaN structure grown on the bulk gallium nitride, the thermal resistance of GaN HEMT is only determined by the conductivity of bulk GaN. This paper presents first AlGaN/GaN HEMT structures prepared on novel generation Ammono-GaN semi-insulating substrates. Such substrates characterized by FWHM value of X-ray rocking curve 20 arcsec, curvature radius of several tens of meters, and dislocation density 2x10 4 cm -2 were chosen for epitaxial growth of GaN-based semiconducting thin film materials by MOCVD method on c-plane. The HEMT epitaxial structure consisted of 1 nm GaN-cap, 27 nm Al 0.27 Ga 0.73 N barrier layer, 0.8 nm AlN spacer, and 3 μm unintentionally doped GaN layers. High resolution 2θ-ω and rocking curve XRD scans of HEMT structure prove excellent crystal quality of epilayers grown on Ammono-GaN with FWHM=0,0090 o . Sheet resistivity, sheet carrier concentration and Hall mobility were 417 Ω/□, 1.23×10 13 cm -2 , 1308 cm 2 /Vs, respectively. Fabrication of transistors starts by formation of ohmic contacts by using the regrown n + -GaN:Si layers. Isolation of adjacent devices w