A high-power AlGaN/GaN heterojunction field-effect transistor

Solid-State Electronics 47 (2003) 589–592 www.elsevier.com/locate/sse A high-power AlGaN/GaN heterojunction field-effect transistor Seikoh Yoshida a a,* , Hirotatsu Ishii a, Jiang Li a, Deliang Wang b, Masakazu Ichikawa b Yokohama Research and Development Laboratories, The Furukawa Electric Company Ltd., 2-4-3, Okano, Nishi-ku, Yokohama 220-0073, Japan b Joint Research Center for Atom Technology (JRCAT) 1-1-4, Higashi, Tsukuba, Ibaraki 305-0046, Japan Abstract We fabricated an AlGaN/GaN heterojunction field effect transistor (HFET) with a very low on-state resistance. An undoped Al0:2 Ga0:8 N(30 nm)/GaN(2 lm) heterostructure was grown on a sapphire substrate using gas-source molecular-beam epitaxy. The undoped GaN layer had a high resistivity (above 10 MX) and the breakdown field of the undoped layer was about 2 MV/cm. Si-doped GaN with a carrier concentration of 5
1019 cm3 was selectively grown in the source and drain regions for obtaining a very low contact resistance. As a result, a very low ohmic below 1
107 X cm2 was obtained. After that, an Al0:2 Ga0:8 N/GaN HFET was fabricated. The gate width was 20 cm and the gate length was 2 lm. The ohmic electrode materials were Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 13 lm. The HFET was operated at a current of over 20 A. A higher switching speed of HFET was obtained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: AlGaN; GaN; HFET; GSMBE; Breakdown voltage; On-state resistance 1. Introduction GaN based heterostructure devices are very promising for high-power and high-frequency devices [1]. That is, wide band gap semiconductors such as GaN-based materials have a large figure of merit for those purposes, since they have a wide band gap, a high breakdown electric field and a high saturation velocity [2]. Especially, it is expected that the specific on-state resistance ðRon Þ of the field effect transistor (FET) is lower than that of Si or GaAs. FETs with a very low on-state re- * Corresponding author. Tel.: +81-45-311-1218; fax: +81-45316-6374. E-mail address: [email protected] (S. Yoshida). sistance are very effective for low-loss power-switching devices, such as an inverter. The AlGaN/GaN heterostructure enables us to fabricate a large-current operation device, since it is expected to have a high electron mobility and a high carrier density of two-dimensional electrons due to a large piezo-electric field effect. Recently, it has been actively reported that high-temperature, high-power and high-frequency devices using GaN and related materials have been operated [3–12]. However, there has not yet been any experiment reported concerning the on-state resistance of GaN devices. We have recently reported on the large-current operation of an AlGaN/GaN heterojunction field effect transistor (HFET) [13]. In this paper, it is reported that an AlGaN/GaN HFET with a lower on-state resistance and a higher breakdown voltage was improved and that its switching speed was measured. 0038-1101/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 2 ) 0 0 4 1 9 - 7 590 S. Yoshida et al. / Solid-State Electronics 47 (2003) 589–592 2. Experimental An AlGaN/GaN heterostructure was grown using gas source molecular beam epitaxy (GSMBE) [8]. A 50 nm-thick GaN buffer layer was first formed on a sapphire substrate at 700 °C. A 2 lm-thick layer of GaN and a 30 nm-thick Al0:2 Ga0:8 N heterostructure were grown using Ga and ammonia gas on the GaN buffer layer at 850 °C. The Hall mobility of the AlGaN/GaN layer was 1200 cm2 /Vs, and the sheet carrier density was 1:0
1013 cm2 at room temperature. We next carried out the fabrication of an HFET having the schematic structure shown in Fig. 1. Etching of the GaN and Al0:2 Ga0:8 N was carried out by a dry-etching technique using an electron cyclotron resonance (ECR) plasma to fabricate the FET. The etching gas was a mixture of CH4 (5 sccm), Ar (7 sccm) and H2 (15 sccm). The etching rate of the GaN layers was 14 nm/min. The etching depth of GaN for isolation was about 400 nm. Furthermore, a highly Si-doped GaN contact layer with a carrier concentration of 5
1019 cm3 was selectively grown in the region of the source and drain. Si-doped GaN for the contact layer was successfully grown in the opening area of the mask. That is, there was no growth on the region for gate electrode formation. Therefore, Fig. 1. Schematic drawing of a one-unit AlGaN/GaN HFET for large-current operation. Fig. 2. Breakdown voltage of the FET versus the distance of the gate and drain (Lgd ). GSMBE was effective for GaN selective growth. After patterning using a photoresist combined with a SiO2 mask, we formed a source and a drain using Al/Ti/Au, and a Schottky gate as Pt/Au on the patterned surface by an ECR sputter evaporation method, respectively. 1000-unit FETs were connected using a multi-electrode structure. SiO2 was used to isolate the source, drain, and gate electrodes. The gate length was 2 lm and the total gate width was enlarged to 20 cm. The distance of a source and drain was 13 lm. The chip size of the HFET was 0.25 cm2 . 3. Results and discussions In order to improve the breakdown voltage and to reduce the specific on-state resistance of the FET, we experimentally investigated the optimum value of the length of the gate-source ðLgs Þ and the gate-drain ðLgd Þ. As a result, it was found that the breakdown voltage was increased until above 370 V with an increase of Lgd as a constant of Lgs (3 lm), as shown in Fig. 2. To maintain the low on-state resistance, it was decided that Lgs and Lgd should be 3 and 8 lm, respectively. We investigated the relation between the contact resistance and the carrier concentration of a selectively grown contact layer. The contact resistance was reduced with an increase in the carrier concentration of the contact layer, as shown in Fig. 3. The lowest value was 3:5
108 X cm2 . We also investigated the breakdown voltage of a highly resistive undoped GaN layer by removing the AlGaN layer, since a highly resistive layer was important for a high breakdown voltage of the FET. Electrodes with a 10 lm gap were fabricated on the undoped GaN buffer layer. It was also confirmed that an undoped GaN layer Fig. 3. Contact resistance versus the carrier concentration of a selective grown contact layer. S. Yoshida et al. / Solid-State Electronics 47 (2003) 589–592 591 Table 1 Results of the Schottky breakdown voltage and leakage current of different Schottky electrode materials Schottky electrode Breakdown voltage (V) Leak current (lA) Pt/Au Ni/Au Pt–Ni/Au 600 350 250 10 10 30 had a high breakdown voltage of about 2000 V; the breakdown field was estimated to be above 2 MV/cm. We also investigated the Schottky breakdown voltage and leakage current of different Schottky electrode materials (Pt/Au, Ni/Au, Pt–Ni/Au). Table 1 gives the results of the breakdown voltages and the leakage currents. Using a Pt/Au, a high Schottky breakdown voltage (600 V) and a lower leakage current (10 lA) were obtained. Fig. 4 shows the Schottky property between the gate and the source of the HFET. The breakdown voltage was over 400 V. We next investigated the large-current operation of the AlGaN/GaN HFET. Fig. 5 shows the Ids  Vds property of the HFET, which was operated above 10 A. The pinch-off voltage was 2 V. The breakdown voltage of the HFET was about 370 V. The breakdown voltage will be increased by improving the FET fabrication process. The specific on-state resistance ðRon Þ is an important factor for power devices. That is, Ron of the HFET is estimated to be about 8 mX cm2 from Ids –Vds property of Fig. 5. The maximum Ids was increased to 20 A. This was the highest of the previously reported values [14–18]. The on-state resistance for our AlGaN/GaN HFET was about 8 mX cm2 at 370 V of the breakdown voltage and 2 mX cm2 at 100 V [19]. These values are smaller Fig. 4. Schottky property of the gate and source of an AlGaN/ GaN HFET. Fig. 5. Current–voltage property of an AlGaN/GaN HFET under large-current operation. The gate voltage ðVgs Þ was changed from 0 to )7 V. than those of a conventional Si metal oxide semiconductor (MOS) FET. The specific on-state resistance of a GaN FET is, moreover, expected to become lower by improving the quality of the epitaxial wafer and the accuracy of the fabrication process. Based on these results, it was confirmed that a GaN HFET with the above-mentioned structure is effective for large-current devices. Fig. 6 shows the input capacitance ðCgs ; Cgd Þ of the HFET. The maximum Cgs and Cgd be capacitances were about 1000 PF, respectively. These values will, moreover, reduced by optimizing the FET structure. The operation frequency, fT ¼ gm =2pðCgs þ Cgd Þ, is estimated to be about 100 MHz using these values and gm ¼ 100 mS/mm. Switching properties of HFET were also measured. A turn-on time was about 40 ns and a turn-off time was about 30 ns. When this AlGaN/GaN Fig. 6. Input capacitance of Cgs and Cgd versus Vgs and Vgd . 592 S. Yoshida et al. / Solid-State Electronics 47 (2003) 589–592 HFET is applied for an inverter or converter, higher speed and lower loss switching is expected. [7] 4. Summary A high-power AlGaN/GaN HFET for large-current operation was fabricated. An AlGaN/GaN heterostructure was grown by GSMBE. The total gate width of the HFET was 20 cm and the gate length was 2 lm. The gate, source and the drain were isolated using SiO2 . The maximum breakdown voltage of the gate and the source was 600 V. An undoped GaN layer had a high breakdown voltage about 2000 V and the breakdown field of the undoped GaN layer was estimated to be about 2 MV/cm. The on-state resistance of a HFET was about 8 mX cm2 at a breakdown voltage of 370 V. The maximum operation current of the HFET was 20 A. A higher switching speed (turn-on time 40 ns and turn-off time 30 ns) was obtained. The HFET for large-current operation was thus demonstrated. [8] [9] [10] [11] [12] [13] [14] References [1] Mohammad SN, Salvador AA, Morkoc H. Emerging gallium nitride based devices. Proceedings of the IEEE 1995;83:1306–56. [2] Chow TP, Tyagi R. Wide bandgap compound semiconductors for superior high-voltage unipolar power devices. IEEE Trans Electron Devices 1994;41:1481–3. [3] Khan MA, Kuznia TN, Bhattaraia AR, Olson T. Metal semiconductor field-effect transistor based on single-crystal GaN. Appl Phys Lett 1993;62:1786–7. [4] Pankove J, Chang SS, Chang HC, Lee HC, Molnar RJ, Moustakas TD, et al. High-temperature GaN/SiC heterojunction bipolar transistor with high gain. IEDM Tech Dig 1994:389–92. [5] Khan MA, Shur MS, Kuzunia JN, Chin Q, Burm J, Schaff W. GaN/AlGaN heterostructure field effect transistors operating at temperatures up to 300 °C. Appl Phys Lett 1995;6:1083–5. [6] Akutas O, Fan ZF, Mohammad SN, Botchkarev AE, Morkocß H. High temperature characteristics of AlGaN/ [15] [16] [17] [18] [19] GaN modulation doped field-effect transistors. Appl Phys Lett 1996;69:3872–4. Yang W, Lu J, Asifkhan M, Adesida I. AlGaN/GaN HEMTs on SiC with over 100 GHz fT and low microwave noise. IEEE Trans Electron Devices 2001;48:581–5. Yoshida S, Suzuki J. Reliability of GaN metal semiconduvtor field-effect transistor at high temperature. Jpn J Appl Phys Lett 1998;37:482–4. Yoshida S, Suzuki J. Reliability of metal semiconductor field effect transistor using GaN at high temperature. J Appl Phys 1998;84:2940–3. Yoshida S, Suzuki J. Characterization of GaN bipolar transistor after operation at 300 °C for 300 h. Jpn J Appl Phys Lett 1999;38:851–3. Yoshida S, Suzuki J. High temperature reliability of GaN electronic devices. Mater Res Soc Proc 1999;595:W.4.8.1– 7. Khan MA, Kuznia JN, Oloson DT, Schaff WJ, Burm JW, Shur MS. Microwave performance of 0.25 lm gate AlGaN/GaN heterostructure field effect transistors. Appl Phys Lett 1994;65:1121–3. Yoshida S, Ishii H. A high power GaN-based field effect transistor for large current operation. Phys Status Solidi (a) 2001;188:243–6. Zhang N-Q, Moran B, Denbaars SP, Mishara UK, Wang XW, Ma TP. Kilovolt AlGaN/GaN HEMTs as switching devices. Phys Stat Sol 2001;188:213–7. Rumyantsev SL, Pala N, Shur MS, Levinshtein ME, Gaska R, Hu X, Yang J, Simin G, Khan MA. Effect of gate leakage current on noise in AlGaN/GaN HFET. In: Proceedings of the international workshop on nitride semiconductors, IPAP conference, vol. 1; 2000. p. 938– 41. Hu X, Tarakji A, Kumar A, Simin G, Yang JW, Kahn MA, Gaska R, Shur MS. Large periphery AlGaN/ GaN metal-oxide-semiconductor heterostructure field effect transistors on SiC substrates. In: Proceedings of the international workshop on nitride semiconductors, IPAP conference, vol. 1; 2000. p. 946–8. Weitzel CE. Wide bandgap semiconductor power electronics. Tech Dig Int Electron Devices Meet 1998:51–4. Yoshida S, Ishii H, Li J. AlGaN/GaN hetero field-effect transistor for a large current operation. Mater Sci Forum 2002;389–393:1527–30. Yoshida S, Ishii H. A high power AlGaN/GaN hetero field effect transistor. Proc Mater Res Soc 2002;693:805–10.