AlGaN/GaN based high power, high frequency high electron mobility transistors (HEMTs) have been in development for over a decade. Although much progress has been made, AlGaN/GaN HEMT technology has yet to be commercialized. The choice of silicon as the substrate for the growth of GaN-based epi layers will enable commercialization of AlGaN/GaN based HEMTs, because of its maturity, scalability, reproducibility and economy. One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the understanding of parasitic losses that can adversely impact the RF device performance. The effect of the III-N MOCVD process on the resistivity of the Si substrate, and correlations between the Si substrate resistivity and AlGaN/GaN HEMT RF characteristics are presented. Optimization of the MOCVD growth process led to a reduction in parasitic doping of the Si substrate. This resulted in the following improvements: (a) small signal gain increased from 17 to 21dB, (b) the cut-off frequency increased from 7 to 11GHz and (c) the maximum frequency of oscillation improved from 12 to 20GHz. This optimized process will enhance performance of AlGaN/GaN HEMTs at higher frequencies.
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We would like to acknowledge the Office of Naval Research (ONR) for supporting this work under contracts N00014-00-M-0159 (Phase I, Colin Wood contract monitor) and N00014-01-C-0253 (Phase II, John Zolper and Harry Dietrich contract monitors).
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Rajagopal, P., Roberts, J.C., Cook, J.W. et al. Symposium Y: Gan and Related Alloys MOCVD AlGaN/GaN HFETs on Si: Challenges and Issues. MRS Online Proceedings Library 798, 695–700 (2003). https://doi.org/10.1557/PROC-798-Y7.2