Symposium Y: Gan and Related Alloys MOCVD AlGaN/GaN HFETs on Si: Challenges and Issues
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SYMPOSIUM Y: GAN AND RELATED ALLOYS MOCVD AlGaN/GaN HFETs on Si: Challenges and Issues Pradeep Rajagopal, John C. Roberts, J. W. Cook, Jr., J. D. Brown, Edwin L. Piner, and Kevin J. Linthicum. Nitronex Corporation, 628 Hutton Street, Suite 106, Raleigh, NC 27606 Abstract 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. Introduction Successful growth of high quality AlGaN/GaN high electron mobility transistors (HEMTs) on Si offers a pathway to commercialize GaN-based transistors for many applications, including those in the RF microelectronics area. As a result of this potential, several groups have focused attention to growth1, 2, 3 of GaN-based epi layers on silicon substrates and the fabrication of various devices4, 5, 6, 7, 8 thereof. The resistivity of the Si substrate used to grow AlGaN/GaN HEMTs is an important parameter that impacts high frequency operation of the resultant power amplifiers. Highly resistive (ρ > 104 ohm⋅cm) Si (111) substrates offer an effective platform for the growth and operation of high frequency of AlGaN/GaN HEMTs. A non-optimized MOCVD process can lead to the undesirable formation of a localized Al and Ga doped, p-type conductive layer in the Si substrate at the Si/III-N interface. We report on the development of an optimized MOCVD process that reduces the formation of the parasitic p-type layer at the Si/III-N interface. The dramatic reduction of the lossy parasitic p-type layer resulted in improved RF performance of AlGaN/GaN HEMTs on Si. Experimental The experiment consisted of two parts. In the first part, the effect of the growth conditions on the silicon substrate resistivity was monitored. In the second part, AlGaN/GaN HEMT devices were fabricated to correlate appropriate device performance parameters to the substrate
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conductivity. In all cases, the substrate starting resistivity was > 104 ohm⋅cm and all epitaxial layers were grown on 100 mm
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