AlGaN/GaN field effect transistors with C-doped GaN buffer layer as an electrical isolation template grown by molecular beam epitaxy S. Haffouz * , H. Tang, J.A. Bardwell, E.M. Hsu, J.B. Webb, S. Rolfe Institute for Microstructural Sciences, National Research Council Canada, Montreal Rd. M-50, Ottawa, Canada K1A 0R6 Received 5 March 2004; received in revised form 23 November 2004 The review of this paper was arranged by Prof. C. Hunt Abstract The effectiveness of Ammonia Molecular Beam Epitaxy (MBE) grown carbon-doped GaN buffer layer as an electrical isolation template was investigated. AlGaN/GaN field effect transistor structures with a product of sheet electron density and mobility (n s l), linearly increasing from 1.5 · 10 16 V 1 s 1 to 2 · 10 16 V 1 s 1 with n s , were grown on 2-lm-thick carbon-doped GaN buffer layer over sapphire substrates. The measurement of the gate-to-source voltage (V GS ) dependent drain current (I D ) demonstrated excellent dc pinch-off characteristics as revealed by an on-to-off ratio of 10 7 for a drain–source voltage (V DS ) up to 15 V. The gate leakage current was less than 1 lA/mm at the subthreshold voltage (V th = 5.2 V). Inter-devices isolation current (I ISO ) measurements demonstrated I ISO values in the low pico-amperes ranges indicating a complete suppression of the parallel conduction paths. Small-signal rf measurements demonstrated a f max /f t ratio as high as 2.9 attesting the absence of charge coupling effects. Ó 2005 Elsevier Ltd. All rights reserved. PACS: 85.30.Tv; 81.15.Hi; 73.61.Ey Keywords: GaN; FET; MBE; Carbon doping; Heterostructure 1. Introduction With improved growth material quality and fabrica- tion technologies, AlGaN/GaN heterostructure field effect transistors (HFET) have reached nowadays a very advanced position and have clearly demonstrated their capability for high-power and high frequency applica- tions [1–6]. Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) techniques have been successfully used for growth of AlGaN/GaN two-dimensional electron gas (2DEG) structures on various types of substrates. Growth of 2DEG structures with high product of sheet carrier den- sity and electron mobility (n s l) is of great importance for fabrication of high performance field effect transis- tors. In literature, considerable studies have addressed the electron mobility (l) dependence carrier densities (n s ) [7–15]. However, there have been only few data on the growth of 2DEG structures with high n s l values (>10 16 V 1 s 1 ). Achievement of these latter values requires growth of AlGaN/GaN heterostructures with high 2DEG mobility (P10 3 cm 2 /V s) at n s values in the few 10 13 cm 2 ranges. Strong decrease of the 2DEG mobility with increasing the sheet carrier density in the 1–2 · 10 13 cm 2 ranges has been observed in AlGaN/ GaN structures grown by MOCVD technique [13]. On another hand, achievement of highly insulating GaN buffer prior to deposition of AlGaN/GaN struc- tures has not been an easy task. A conductive buffer 0038-1101/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2005.01.012 * Corresponding author. Tel.: +1 613 991 0761; fax: +1 613 990 0202. E-mail address: soufien.haffouz@nrc-cnrc.gc.ca (S. Haffouz). www.elsevier.com/locate/sse Solid-State Electronics 49 (2005) 802–807