Material and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
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Material and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates P. Javorka, A. Alam,1 M. Marso, M. Wolter, A. Fox, M. Heuken,1 and P. Kordoš Institute of Thin Films and Interfaces, Research Centre Jülich, D-52425 Jülich, Germany. 1 Aixtron AG, D-52072 Aachen, Germany. ABSTRACT Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm-2 and µ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3−0.7 µm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ~16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 µm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices. INTRODUCTION AlGaN/GaN high electron mobility transistors (HEMTs) have recently been attracting much attention because of their promising uses for high−frequency, –power and −temperature applications. Sapphire and since recently also SiC are commonly used as substrates because of lack of large−area GaN bulk crystals. An output power higher than 100 W at 2 GHz has been achieved on AlGaN/GaN HEMTs using sapphire thinned down to 50 µm (pulsed operation, 10% duty) [1] as well as high-resistive SiC (cw operation) [2]. However, silicon can be a useful alternative because of its low cost and large area availability, good thermal conductivity and potential integration of GaN power devices with advanced Si electronics. Growth of GaN on Si is more difficult than on sapphire due to the higher lattice constant and thermal expansion coefficient mismatches, which produce higher dislocation density and potential generation of cracks. Nevertheless encouraging results on AlGaN/GaN/Si HEMTs concerning their high−frequency and –power performance have been presented recently by various groups [3-6]. The device performance is comparable to that known for devices using sapphire and SiC sub
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