AlGaN/GaN HFETs for Automotive Applications
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AlGaN/GaN HFETs for Automotive Applications Ronald Birkhahn*, David Gotthold, Nathan Cauffman, Boris Peres EMCORE Corporation, Corporate Research and Development 145 Belmont Drive, Somerset, NJ 08873 USA *Email: [email protected] Seikoh Yoshida The Furukawa Electric Co, Ltd. Yokohama R&D Laboratories 2-4-3, Okano, Nishi-ku, Yokohama 220-0073, Japan
ABSTRACT AlGaN/GaN heterojunction field effect transistors (HFET) on sapphire substrates have demonstrated ability as power devices operating with high current densities and high breakdown voltages. Additionally, AlGaN/GaN HFET devices have a very low on-state resistance. This makes these devices ideal for automotive applications such as switching relays, DC-DC converters, and power inverters. By 2006, switching devices using GaNbased FETs are anticipated to be employed in luxury automobiles and transitioned to the mass market by 2009. In this presentation, data from AlGaN/GaN HFET’s grown in an Emcore D180 MOCVD system will be presented. Typical production-scale material results (on 2” sapphire substrates) for these wafers were: µ ~ 1000 cm2/Vs, Ns = 1.0x1013 cm-2, and Rs ~ 450 Ω/square with 10 A) and corresponding power (420 W). Targeted initially in luxury automobiles, AlGaN HFET devices should see their introduction in 2006 with transition to the broader automotive market by 2009. EXPERIMENT DETAILS AlGaN HFET epilayers were deposited at Emcore using a D180 rotating disk MOCVD reactor. Trimethylgallium and trimethylaluminum were used as sources of Ga and Al respectively with NH3 as the source of atomic nitrogen. The pressure throughout the entire process was 100 Torr. A low temperature AlN was used to nucleate on sapphire and semi-insulating (SI) GaN and AlGaN films were deposited at 1030oC as measured by pyrometer. The SI-GaN thickness was nominally 2 µm and the AlGaN was calculated at 25-30 nm at 28% Al composition. Average sheet resistivity of the as-grown samples was 450 ohms/square with ≤3% uniformity. These wafers were then processed at Furukawa into FET devices with Al/Ti/Au metallization for the ohmic source and drain contacts and Pt/Au for the gate. The AlGaN surface was passivated using SiO2 with mesa isolation between devices and the entire structure was sealed with an insulator (Figure 1). Individual FET device dimensions were gate length Lg = 2µm, gate width Wg=200µm with a source-drain distance of Lsd=13µm. These individual FETs were assembled into a larger assembly (L-FET) that had an effective gate width of 20cm on a 5x5mm2 chip for large current handling capability. Gate connection electrode (Al/Au)
AlGaN layer 2DEG
Source
Gate
Contact
Insulator High resistive GaN buffer ¥¥¥
GaN buffer
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Sapphire Sub.
Figure 1: Mesa-isolation individual HFETs on sapphire. Device dimensions of Lg = 2µm, Wg=200µm with a source-drain distance of Lsd=13µm.
RESULTS The materials properties were uniform from the as-grown AlGaN HFET wafers as shown in Figure 2. The sheet charge on the samples averaged around 1x1013 carriers/cm2 with a
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