Polarization induced 2DEG in MBE grown AlGaN/GaN HFETs: On the origin, DC and RF characterization
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Polarization induced 2DEG in MBE grown AlGaN/GaN HFETs: On the origin, DC and RF characterization R.Vetury, I.P. Smorchkova, *C.R. Elsass, *B. Heying, S. Keller, and U.K. Mishra Department of Electrical and Computer Engineering, *Materials Department, University of California, Santa Barbara, California 93106 ABSTRACT The contribution of spontaneous and piezoelectric polarization to the formation of a 2DEG in AlGaN/GaN heterostructures was investigated using undoped AlGaN/GaN structures. Hall measurements of 2DEG density on such structures with varying Al percentage (8%-27%) and varying thickness of the AlGaN layer (30-500 Å) indicated that donor-like surface states at an energy of 1.42eV below the conduction band were the source of electrons in the 2DEG. Field effect transistors were fabricated on such undoped heterostructures. For an AlGaN/GaN structure with 0.27 Al mole fraction, power density in excess of 3.5 W/mm at 6 GHz with corresponding maximum PAE of 33.5% was obtained. These results exceed the best reported power performance of MBE grown GaN HFETs on sapphire, thus demonstrating the excellent capability of MBE grown GaN heterostructures for microwave power applications. INTRODUCTION GaN based heterostructures field effect transistors (HFETs) are of tremendous interest for high power microwave applications. Excellent power output at microwave frequencies has been demonstrated by MOCVD [1-3]. Great progress has been made in MBE grown devices [4,5]. The Alx Ga1-x N/GaN material system has several advantages for such applications such as the high breakdown field strength of GaN arising from the large bandgap and high electron mobility achievable in Alx Ga1-x N/GaN heterostructures. Enhanced charge confinement of the two dimensional electron gas (2DEG) in the channel due to polarization fields and large conduction band discontinuity (∆Ec) leads to high 2DEG sheet charge density and hence high current density. Experimentally, it has been observed in our laboratories that a) a 2DEG is formed in GaN in the lower AlGaN/GaN interface even when there is no intentional doping of the AlGaN layer. b) When the AlGaN is intentionally doped, the charge density in the 2DEG is not proportional to the amount of dopant (Si, a shallow donor in GaN) introduced into the AlGaN layer. This suggests that dopants play only a secondary role in determining the sheet carrier concentration. The major cause of the formation of a 2DEG is the presence of polarization induced charge sheets in bottom and top surfaces of the AlGaN layer. THEORY When a strained crystal of AlGaN is grown psuedomorphically on a bulk GaN crystal as in a HFET structure, the distortion of the AlGaN lattice results in sheet charges of opposite polarity and the same magnitude on the two opposite faces of the AlGaN layer. These sheet charges are piezoelectricity induced sheet charge.Also, since the spontaneous polarization coefficients of T2.5.1 Downloaded from https:/www.cambridge.org/core. University of Arizona, on 24 May 2017 at 07:16:12, subject to the Cambridge Co
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