Control of Short-Channel Effects in GaN/AlGaN HFETs M J Uren, D G Hayes, R S Balmer, D J Wallis, K P Hilton, J O Maclean, T Martin, C Roff*, P McGovern*, J Benedikt*, P J Tasker* QinetiQ Ltd, St Andrews Road, Malvern, WR14 3PS, UK * Cardiff School of Engineering, Cardiff University, CF24 0YF, UK Abstract — GaN/AlGaN HEMTs can suffer from short channel effects as a result of insufficient buffer doping. We show that controlled iron doping of the GaN buffer during MOVPE growth can suppress all short-channel effects in 0.25µm gate length devices. We show that optimised iron doping has no effect on the RF output power or on the knee walkout (current-slump), but significantly improves the power added efficiency. Index Terms — HEMT, GaN, RF power, power added efficiency, current slump I. INTRODUCTION GaN/AlGaN heterojunction field-effect transistors offer a RF power density at least a factor of five higher than conventional III-V devices, combined with comparable noise performance and dramatically improved robustness[1]. However, GaN/AlGaN HFETs are vulnerable to short-channel effects like all FETs. These take the form of a drain dependent pinch-off voltage and a significant output conductance, as shown for example in the pulse IV characteristics in Fig. 1(a). The short- channel effects result from poor confinement of electrons in the channel. A charged depletion region below the channel, formed by deep level acceptors in the GaN, is required to confine the carriers at the surface and prevent “punch-through” of electrons through the GaN buffer induced by the high drain field[2, 3]. Assessment of devices fabricated using the QinetiQ standard GaN/AlGaN HFET structure of ~25nm Al 0.28 Ga 0.72 N on an unintentionally doped but insulating 1.3µm thick GaN buffer grown on semi-insulating 4H- SiC, has shown that for short gate lengths (<0.4µm) there can be significant short-channel effects. Device simulation (using the Silvaco Atlas 2D device simulator) showed that the experimental effect could be reproduced in the model if the net acceptor (deep level) density in the GaN buffer layer was reduced to about 2x10 16 cm -3 [2]. This is significantly lower than the value of 10 17 cm -3 previously measured using substrate bias with layers grown on conducting SiC[4]. It seems that incremental improvements in the quality of the GaN epitaxy over several years has reduced the intrinsic deep level acceptor concentration below that required to prevent “punch- through” of current through the GaN buffer layer. An effective method of reproducibly controlling the deep level concentration, and hence preventing the “punch-through”, is to introduce an extrinsic impurity such as Fe. Significant development of the doping of III- V semiconductors with transition metals such as Cr and Fe was undertaken during the 1970’s and 1980’s to make electrically insulating substrates and films. More recently, Fe doping has been introduced for GaN-based microwave transistor applications, where a highly insulating buffer layer is critical to high power operation of the device at RF frequencies. Highly insulating Fe doped GaN films grown by MOVPE and HVPE have been reported [5-8] with sheet resistivities greater than 10 10 Ω/□. Indeed, the highest reported output power densities and breakdown voltages were achieved for devices grown on Fe doped GaN buffer layers[9]. In this paper, we report a detailed investigation of Fe doping in GaN buffer layers and its influence on short-channel effects and the control of pinch-off. TABLE I SUMMARY OF SAMPLE WAFER AND DEVICE PROPERTIES Wafer ID Depth at which Fe switched off Channel Fe concentration (atoms/cm 3 ) Electron concentration (cm -3 ) I dss0 (mA/mm) g m (mS/mm) f T (GHz) MAG at 10GHz (dB) A 0.5μm 1.5×10 17 8.2×10 12 630 148 30 15 B 0.8μm 3.6×10 16 9.6x10 12 750 180 30 15 C 1.1μm 7×10 15 9.1x10 12 780 179 30 15 D (control) - - 10.0x10 12 780 170 34 14 Proceedings of the 1st European Microwave Integrated Circuits Conference 2-9600551-8-7  2006 EuMA September 2006, Manchester UK 65