Ultra-Wide Bandgap AlGaN-Based N-Polar High Electron Mobility Transistors: From Materials to Devices

Ultrawide bandgap (UWBG) materials are being widely investigated for high-power and high-frequency electronics [1,2]. While emerging UWBG materials like Ga₂O₃ possess large bandgap and high critical electric field, AlGaN compositions can potentially have larger Figure of Merit owing to higher sheet carrier concentration and larger mobility. Recently, using N-polar all AlGaN-HEMT we showed simultaneously large drive current, low on-state leakage current as well as &gt; 400 V breakdown voltage [3].<br/><br/>In this work, we discuss the growth and optimization as well as the characterization of N-polar AlGaN channel HEMT with varying Al concentrations. We also probe the composition-dependent surface morphology and transport in these materials, aided by simulation and Hall measurements.<br/><br/>For the growth of the N-polar all-AlGaN HEMT structures, we used the metal-organic chemical vapor deposition (MOCVD) method. We used c-plane sapphire substrates with a 4° miscut towards <i>a</i>-plane for achieving smooth and high-quality epitaxy. We demonstrated four different HEMT structures with Al mole fraction x=20%, 30%, 59% and 73% in the Al_xGa_1-xN channel layer. The growth of such MOCVD-grown N-polar all-AlGaN HEMT was optimized at higher temperatures with higher TMAl flow to achieve a crack-free growth and lower dislocation density within the structures.<br/><br/>We measured the stoichiometries of the structures using both symmetric and asymmetric 2θ-ω scans in X-ray diffraction to incorporate strain effect. After KOH etching the N-polar HEMT showed hexagonal hillock-like structures, confirming the N-polarity of our grown samples. Atomic force microscopy imaging confirmed the good surface morphology with a root mean square surface roughness of &lt;1 nm under 5 µm x 5 µm scan size. We also found that the surface roughness increased for HEMTs with the higher Al mole fraction AlGaN in the channel and barrier layers which can be from strain due to larger lattice mismatch between layers.<br/><br/>To develop a fundamental understanding of the material property and device performance, we explored the effect of Al mole fraction in the transport characteristics of the HEMT structures using Hall measurement. We found a decreasing trend in the mobility with increasing Al mole fraction in the AlGaN channel that can be correlated to alloy-scattering [4] and optical phonon-scattering dominated transport, further explored in our TCAD simulations. Our simulation showed that beyond 73% Al mole fraction, the mobility increases again. Comparing our simulation with the Hall mobility numbers, we can discern that mobility degradation due to surface undulation might be more pronounced for HEMT structures with higher Al percentages. We also fabricated N-polar all-AlGaN HEMT devices with 20% Al composition in the channel. A 10 µm channel length device offered a drive current of ~250 mA/mm. The high drive current can be attributed to the formation of better contacts on the lower-bandgap material and the natural formation of a back-barrier.<br/><br/>To summarize, in this work we reported experimental demonstration of device-quality epitaxial growth of N-polar all-AlGaN HEMT. The success of this study which will be discussed at the presentation led to the first demonstration of &gt;400V breakdown voltage N-polar AlGaN HEMTs. We also explored the correlation between the material surface morphology and transport properties for varying Al compositions. This work will deepen our understanding of the optimization of N-polar AlGaN HEMT for high-power applications.<br/><br/>We acknowledge the funding from ONR (Award No N00014-19-1-2611- P00004) and Dr. Paul Maki for supporting this work.<br/><br/>References: [1] R. J. Kaplar <i>et al. ECS J Solid State Sci Technol </i>6, 3061 (2016). [2] J. Y. Tsao, S. Chowdhury <i>et al.</i>, <i>Adv. Electron. Mater.</i> <b>4</b>(1), 1600501 (2018). [3] M. Noshin, S. Chowdhury <i>et al</i>., <i>IEEE Electron Dev. Lett</i>., DOI: 10.1109/LED.2023.3279400 (2023). [4] M.E. Coltrin <i>et al</i>., <i>ECS J Solid State Sci Technol </i>6, S3114 (2017).