(Invited) Manufacturing Microwave AlGaN/GaN High Electron Mobility Transistors (HEMTs) on Truly Bulk Semi-Insulating GaN Substrates
Authors: Piotrowska Anna Barbara , Kaminska Eliana Anetka , Wojtasiak Wojciech , Gwarek Wojciech , Kucharski Robert , Zajac Marcin , Prystawko Pawel , Kruszewski Piotr , Ekielski Marek , Kaczmarski Jakub , Kozubal Maciej , Trajnerowicz Artur , Taube Andrzej 
Published: February 2020

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 2x104 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 Al0.27Ga0.73N 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=0,0090o. Sheet resistivity, sheet carrier concentration and Hall mobility were 417 Ω/□, 1.23×1013 cm-2, 1308 cm2/Vs, respectively. Fabrication of transistors starts by formation of ohmic contacts by using the regrown n+-GaN:Si layers. Isolation of adjacent devices was done by using Al+ double ion implantation. By this approach the contact resistance Rc of 0.3-0.6 Ωmm was obtained. The fabricated transistors had rectangular Ni/Au gate with 0.8µm length and 2x200 µm width. The source to gate and gate to drain distance was 1.2 and 4 µm respectively. The devices were passivated by 100 nm SiNxlayer deposited by PECVD. Finally, contact pads were opened and thickened by Au deposition. The transistor was designed as a test fixture and its topology was adapted to requirements of the "on-wafer" measurement station Cascade M150.
The measurements performed have shown that the maximum output current density of fabricated transistors reaches 1000 mA/mm at VGS=2V. The extracted on-state resistance Ron is 4.4 Ω/mm. Transistor transconductance is about 220 mS/mm and reaches maximum value for the expected range of operating points of transistors. No measurable leakage current through the buffer layers or the substrate has been observed. The frequency performances of fabricated transistors were evaluated by measuring the S-matrix parameters over 45 MHz to 24 GHz frequency range at quiescent point VDSQ=28V and IDSQ=46 mA. The maximum frequency (fmax) and cut-off frequency (fT) was 30 GHz and 21 GHz as obtained by linear extrapolation with -20dB/dec slope of unilateral gain (U) and small-signal current gain |H21| characteristics. The insertion gain |S21| attain 0dB for frequency (fs) of 22 GHz. The maximum available gain (MAG) and |S21| was 22.7 dB and 15.3 dB at 2GHz and 19.8 and 12.7 dB at 4 GHz. The microwave measurements indicate the lack of significant parasitic elements and confirm the high quality of fabricated HEMTs.
In overall the GaN HEMT transistor manufactured on a truly bulk monocrystalline GaN wafer was shown to have similar parameters as the ones manufactured on a SiC wafer available from a leading commercial manufacturer. In particular we have obtained similar current densities and thermal resistivity. The frequency characteristics were limited mostly by a relatively long gate resulting from application of the photolithography. That limitation is expected to vanish in the planned next round of experiments were electron-lithography is to be applied.
The research was supported by the National Centre for Research and Development PolHEMT Project, Contract Number PBS1/A3/9/2012.