Breakdown Enhancement and Current Collapse ... - IEEE Xplore

IEEE ELECTRON DEVICE LETTERS, VOL. 38, NO. 11, NOVEMBER 2017

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Breakdown Enhancement and Current Collapse Suppression by High-Resistivity GaN Cap Layer in Normally-Off AlGaN/GaN HEMTs Ronghui Hao , Weiyi Li, Kai Fu, Guohao Yu, Liang Song, Jie Yuan, Junshuai Li, Xuguang Deng, Xiaodong Zhang, Qi Zhou, Yaming Fan, Wenhua Shi, Yong Cai, Xinping Zhang, and Baoshun Zhang Abstract — In this letter, a device structure of highresistivity-cap-layer HEMT (HRCL-HEMT) is developed for normally-off p-GaN gate HEMT toward high breakdown voltage and low current collapse. It demonstrates that the breakdown capability and current collapse of the device were effectively improved due to the introduction of a thick HR-GaN cap layer. The fabricated HRCL-HEMT exhibits a high breakdown voltage of 1020 V at IDS = 10 µA/mm with the substrate grounded. Meanwhile, the dynamic Ron is only 2.4 times the static Ron after off-state VDS stress of 1000 V with the substrate grounded (the OFF to ON switching time interval is set to 200 µs). Index Terms — HR-GaN, cap layer, HEMT, p-GaN gate, normally-off, breakdown voltage, current collapse.

I. I NTRODUCTION aN-BASED power devices are emerging as promising candidates for the next generation power switching application due to their wide band gap, high mobility and low on-resistance [1], [2]. At present, most GaN power devices under development are based on the lateral AlGaN/GaN high electron mobility transistor (HEMT) due to

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Manuscript received August 28, 2017; accepted September 5, 2017. Date of publication September 7, 2017; date of current version October 23, 2017. This work was supported in part by the Key Research and Development Program of Jiangu Province under Grant BE2016084, in part by the National Natural Science Foundation of China under Grant 11404372, in part by the Youth Innovation Promotion Association CAS under Grant 2014277, in part by the National Key Scientific Instrument and Equipment Development Project of China under Grant 2013YQ470767, and in part by the National Key Research and Development Program of China under Grant 2016YFC0801203. The review of this letter was arranged by Editor T. Egawa. (Corresponding author: Baoshun Zhang.) R. Hao, J. Yuan, and X. Zhang are with the School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China, and also with the Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, CAS, Suzhou 215123, China W. Li, K. Fu, G. Yu, L. Song, J. Li, X. Deng, X. Zhang, Y. Fan, W. Shi, and Y. Cai are with the Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, CAS, Suzhou 215123, China. B. Zhang is with the Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, CAS, Suzhou 215123, China, and also with Suzhou Powerhouse Electronics Co., Ltd, Suzhou 215123, China (e-mail: [email protected]). Q. Zhou is with the State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2017.2749678

the high density 2-demensional electron gas (2DEG) formed at the interface of AlGaN/GaN heterojunction [3]. However, the lateral AlGaN/GaN HEMT suffers from electric field concentration at the gate edge in the drain side and current collapse, which limit the breakdown voltage and increase the on-resistance of devices. In general, surface passivation and field plates are adopted to improve surface properties and manage surface electric field distribution of the GaN HEMT, respectively [4], [5]. In addition, thick cap layer technology is also one of viable solutions for breakdown enhancement and current collapse suppression. Akira Nakajima et al. have demonstrated a high-voltage normally-on super HFET by introducing a thick i-GaN cap layer between gate and drain to obtain a flat electric field distribution [6], [7] and Powdec K.K. is heading towards commercialization in 1200 V GaN power devices using this technology [8]. Thick p-GaN cap layer technology has also proved to be effective in suppressing current collapse of normally-on AlGaN/GaN HEMTs by screening the channel from surface potential fluctuations [9]. Besides, it has also been used to realize GaN p-channel devices [10], [11]. We have recently realized a normally-off p-GaN gate HEMT by hydrogen plasma treatment, which features a thick high-resistivity (HR) GaN cap layer at the access region [12]. However, few researches about HR-GaN cap layer have been reported. In this work, the effect of HR-GaN cap layer on the performance of the GaN HEMT has been analyzed. It was demonstrated that the breakdown capability and current collapse of the p-GaN gate HEMT were improved effectively, by using a thick HR-GaN cap layer. The fabricated HRCL-HEMT shows a high breakdown voltage of 1020 V and the dynamic Ron is only 2.4 times the static Ron after an off-state VDS stress of 1000 V. II. D EVICE S TRCTURE AND FABRICATION As shown in Fig.1, the HRCL-HEMT features a p-GaN gate which raises the conduction band beyond the Fermi level ensuring normally-off operation and a HR-GaN cap layer at access region. The HR-GaN was formed by passivating p-GaN using hydrogen plasma treatment in our work, which has been discussed in detail in [12]. Please note that other process methods can also be attempted to realize the HRCL-HEMT, such as ion implantation, regrowth or regional activation of

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Fig. 1. Simulated band diagrams of access region (black line) and gate region (red line) in the HRCL-HEMT. Inset depicts schematic crosssection of the HRCL-HEMT. Fig. 3. Schematic cross-sections of the conventional p-GaN gate HEMT and the HRCL-HEMT at the OFF-state are shown in (a) and (b), respectively. The 2-D simulated electric field distribution of the conventional p-GaN gate HEMT and the HRCL-HEMT at VGS = 0 V and VDS = 600 V are shown in (c) and (d), respectively.

Fig. 2. I-V characteristics of the proposed HRCL-HEMT with LGS /LG /LGD /WG = 4/4/15/100 µm. (a) Output characteristics. (b) Transfer characteristics in the log scale.

Mg-doped GaN as the gate region by low-energy electron beam irradiation (LEEBI) [13], [14]. The p-GaN/AlGaN/GaN heterostructure was grown by metal organic chemical vapor deposition (MOCVD) on a 6-inch Si (111) substrate, consisting of a 70 nm p-GaN layer with Mg doping concentration of 2 ∼ 3 × 1019 cm−3 , a 18 nm undoped Al0.2 Ga0.8 N barrier, a 1 nm AlN spacer layer, a 150 nm GaN channel layer, and a 4.8 μm carbondoped GaN buffer layer. The device fabrication started from removing the p-GaN cap layer on the source and drain contacting area by inductively coupled plasma (ICP) etch, followed by Ti/Al/Ni/Au (20/130/50/50 nm) evaporation by e-beam evaporator and annealing at 850 °C for 30 s in N2 ambient by rapid thermal annealing (RTA). Next, the devices were isolated by fluorine ion implantation. Then, Ni/Au (50/150 nm) metal stack was evaporated on the p-GaN as the gate. Finally, the HR-GaN cap layer was formed by a self-alignment hydrogen plasma treatment. The high density hydrogen plasma was produced in the Oxford Plasmalab System100 ICP 180, with an ICP power of 300 W and a low pressure of 8 mTorr. The RF power has a great effect on the depth of the implanted hydrogen. Therefore, an optimized RF power of 2 W was used to passivate the p-GaN and reduce the effect of hydrogen on the 2DEG channel. RTA at 350 °C for 5 min was also adopted to repair the plasma damage. III. R ESULTS AND D ISCUSSIONS Fig. 2(a) and (b) show the output characteristic and transfer characteristic of the HRCL-HEMT with L GS /L G /L GD / WG = 4/4/15/100μm, respectively. The threshold

voltage (Vth ) is determined to be + 1 V based on the linear extrapolation of the transfer curve. The ON/OFF drain current ratio (ION /IOFF ) is about 1.6 × 109 . The maximum saturation drain current (IDS−max ) is 352 mA/mm at VGS = 6 V. The ON-resistance (Ron ) is 12.3 ·mm, which corresponds to a specific on-resistance (Ron ·A) of 3.06 m· cm2 , with 0.95 μm transfer length of each Ohmic contact taken into account for calculation of the area. Fig. 3(a) shows the schematic cross-sections of the conventional p-GaN gate HEMT at the OFF-state and the electric field is concentrated at the gate edge in the drain side due to the lateral electric field crowd, which limits the breakdown voltage of the lateral GaN HEMT. In the HRCL-HEMT, negative charges can appear at the interface of HR-GaN/AlGaN due to negative polarization [15], which increase the vertical electric field in AlGaN and reduce the lateral electric field crowd near the gate edge in the drain side (Fig. 3 (b)). The 2D simulated electric field distribution of the conventional pGaN gate HEMT and the HRCL-HEMT at VGS = 0 V and VDS = 600 V are shown in Fig. 3(c) and (d), respectively. It can be seen that the HRCL-HEMT exhibits a lower peak electric field at the gate edge in the drain side than that in the conventional p-GaN gate HEMT. OFF-state breakdown characteristics of the conventional p-GaN gate HEMT and the HRCL-HEMT are shown in Fig. 4. The conventional p-GaN gate HEMT was fabricated by etching technology and RTA at 450 °C for 10 min has been done to repair the etch damage. The HRCL-HEMT with L GD = 15μm exhibits a breakdown voltage of 1020 V at a drain current criterion of 10 μ A/mm, which is about 400 V higher than that of the conventional p-GaN gate HEMT. The higher leakage current in HRCL-HEMT than that in conventional p-GaN HEMT could be caused by the direct contact between Ohmic metal and HR-GaN, which can be improved by process optimization for higher resistivity or structure optimization. The HRCL-HEMT with a passivation layer of 150 nm SiNx deposited by PECVD at 350° is also shown in the Fig. 4. After passivation, the HRCL-HEMT exhibits a higher breakdown

HAO et al.: BREAKDOWN ENHANCEMENT AND CURRENT COLLAPSE SUPPRESSION

Fig. 4. OFF-state breakdown characteristics of the conventional p-GaN gate HEMT, the HRCL-HEMT and the passivated HRCL-HEMT with substrate grounded.

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Fig. 6. Normalized dynamic Ron with various values of OFF-state VDS stress from 100 to 1000 V of the HRCL-HEMT and the conventional p-GaN gate HEMT.

that in [18]. The ON-state was chosen to feature VGS = 6 V and VDS = 0.5 V. At the OFF-state, VGS was fixed at 0 V and the off-state stress time was 10 ms. Dynamic Ron was measured at 200 μs after the switching from OFF-state to ON-state. As shown in Fig. 6, a sharp increase of the dynamic Ron is observed at 400 V for the conventional p-GaN gate HEMT. By comparison, the dynamic Ron of the HRCL-HEMT is only 1.5 and 2.4 times the static Ron after off-state VDS stress of 600 V and 1000 V, respectively. This result suggests that effective current collapse suppression has been achieved in the HRCL-HEMT, owing to the introduction of the HR-GaN cap layer. Fig. 5. Specific ON-resistance versus breakdown voltage for normallyoff GaN HEMTs. The red star represents the HRCL-HEMT.

voltage of 1210 V, indicating that the HR-GaN cap layer technology and passivation technology are compatible in the HRCL-HEMT. As shown in Fig. 5, the HRCL-HEMT with L GD = 15μm is benchmarked in the specific ON-resistance (Ron ·A) versus breakdown voltage (VBR ) against other reported normally-off GaN HEMTs. The HRCL-HEMT with a specific ON-resistance of 3.06 m· cm2 can sustain an OFF-state VDS of 1020 V at IDS = 10μA/mm. Current collapse in the GaN HEMT is related to the bulk and surface trap states [16], [17]. In the proposed HRCL-HEMT, a thick GaN buffer layer of 4.8 μm has been adopted to suppress the trapping effect of bulk trap states. It has been demonstrated that the peak electric field at the gate edge in the drain side in AlGaN can be effectively reduced during the OFF-state with the existence of HR-GaN cap layer, which decreases the trapped electrons due to the surface trap states. Besides, the thick HR-GaN cap layer could have other beneficial effects on the current collapse suppression for the possible screening effect [9] and the increased distance between surface and the 2DEG channel. The current collapse of the HRCL-HEMT and the conventional p-GaN gate HEMT has been evaluated by the Agilent dynamic measurement system. The measurement setup is the same as

IV. C ONCLUSION The adoption of a high-resistivity GaN cap layer has been shown to improve electric field distribution and suppress current collapse on a normally-off p-GaN gate HEMT. A high-performance normally-off GaN high-resistivity-caplayer HEMT (HRCL-HEMT) has been demonstrated. The fabricated HRCL-HEMT exhibits a high breakdown voltage of 1020 V at a drain current criterion of 10 μA/mm with the substrate grounded. Switching after an off-state VDS stress of 1000 V, the dynamic on-resistance Ron is only 2.4 times compared with the static Ron . ACKNOWLEDGMENTS We thank the Suzhou nanofabrication facility of Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), CAS, for the fabrication, characterization and testing of the HRCL-HEMT and Vacuum Interconnected Nanotech Workstation of SINANO, CAS, for the technical support. R EFERENCES [1] Y. Dora, A. Chakraborty, L. McCarthy, S. Keller, S. P. DenBaars, and U. K. Mishra, “High breakdown voltage achieved on AlGaN/GaN HEMTs with integrated slant field plates,” IEEE Electron Device Lett., vol. 27, no. 9, pp. 713–715, Sep. 2006, doi: 10.1109/LED.2006.881020. [2] T. Xin, L. Yuanjie, G. Guodong, W. Li, D. Shaobo, S. Xubo, G. Hongyu, Y. Jiayun, C. Shujun, and F. Zhihong, “High performance AlGaN/GaN HEMTs with AlN/SiNx passivation,” J. Semicond., vol. 36, no. 7, p. 074008, 2015, doi: 10.1088/1674-4926/36/7/074008.

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[3] K. J. Chen, O. Häberlen, A. Lidow, C. L. Tsai, T. Ueda, Y. Uemoto, and Y. Wu, “GaN-on-Si power technology: Devices and applications,” IEEE Trans. Electron Devices, vol. 64, no. 3, pp. 779–795, Mar. 2017, doi: 10.1109/TED.2017.2657579. [4] B. M. Green, K. K. Chu, E. M. Chumbes, J. A. Smart, J. R. Shealy, and L. F. Eastman, “The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs,” IEEE Electron Device Lett., vol. 21, no. 6, pp. 268–270, Jun. 2000, doi: 10.1109/55.843146. [5] Y.-F. Wu, A. Saxler, M. Moore, R. P. Smith, S. Sheppard, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh, “30-W/mm GaN HEMTs by field plate optimization,” IEEE Electron Device Lett., vol. 25, no. 3, pp. 117–119, Mar. 2004, doi: 10.1109/LED.2003.822667. [6] A. Nakajima, Y. Sumida, M. H. Dhyani, H. Kawai, and E. M. S. Narayanan, “High density two-dimensional hole gas induced by negative polarization at GaN/AlGaN heterointerface,” Appl. Phys. Exp., vol. 3, no. 12, Dec. 2010, Art. no. 121004, doi: 10.1143/APEX.3. 121004. [7] A. Nakajima, Y. Sumida, M. H. Dhyani, H. Kawai, and E. M. S. Narayanan, “GaN-based super heterojunction field effect transistors using the polarization junction concept,” IEEE Electron Device Lett., vol. 32, no. 4, pp. 542–544, Apr. 2011, doi: 10.1109/LED.2011. 2105242. [8] E. A. Jones, F. F. Wang, and D. Costinett, “Review of commercial GaN power devices and GaN-based converter design challenges,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 4, no. 3, pp. 707–719, Sep. 2016, doi: 10.1109/JESTPE.2016.2582685. [9] R. Coffie, D. Buttari, S. Heikman, S. Keller, A. Chini, L. Shen, and U. K. Mishra, “P-capped GaN-AlGaN-GaN high-electron mobility transistors (HEMTs),” IEEE Electron Device Lett., vol. 23, no. 10, pp. 588–590, Oct. 2002, doi: 10.1109/LED.2002.803764. [10] A. Nakajima, S. Nishizawa, H. Ohashi, R. Kayanuma, K. Tsutsui, S. Kubota, K. Kakushima, H. Wakabayashi, and H. Iwai, “GaN-based monolithic power integrated circuit technology with wide operating temperature on polarization-junction platform,” in Proc. ISPSD, Hong Kong, May 2015, pp. 357–360, doi: 10.1109/ISPSD.2015.7123463. [11] R. Chu, Y. Cao, M. Chen, R. Li, and D. Zehnder, “An experimental demonstration of GaN CMOS technology,” IEEE Electron Device Lett., vol. 37, no. 3, pp. 269–271, Mar. 2016, doi: 10.1109/LED. 2016.2515103. [12] R. Hao, K. Fu, G. Yu, W. Li, J. Yuan, L. Song, Z. Zhang, S. Sun, X. Li, Y. Cai, X. Zhang, and B. Zhang, “Normally-off p-GaN/AlGaN/GaN high electron mobility transistors using hydrogen plasma treatment,” Appl. Phys. Lett., vol. 109, p. 152106, Oct. 2016, doi: 10.1063/1.4964518. [13] H. Okita, M. Hikita, A. Nishio, T. Sato, K. Matsunaga, H. Matsuo, M. Mannoh, and Y. Uemoto, “Through recessed and regrowth gate technology for realizing process stability of GaN-GITs,” in Proc. 28th Int. Symp. Power Semiconductor Devices ICs (ISPSD), Jun. 2016, pp. 23–26, doi: 10.1109/ISPSD.2016.7520768.


[14] H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, “P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI),” Jpn. J. Appl. Phys., vol. 28, pp. L2112–L2114, Dec. 1989. [15] H. Ishida, D. Shibata, M. Yanagihara, Y. Uemoto, H. Matsuo, T. Ueda, T. Tanaka, and D. Ueda, “Unlimited high breakdown voltage by natural super junction of polarized semiconductor,” IEEE Electron Device Lett., vol. 29, no. 10, pp. 1087–1089, Oct. 2008, doi: 10.1109/ LED.2008.2002753. [16] R. Vetury, N. Q. Zhang, S. Keller, and U. K. Mishra, “The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs,” IEEE Trans. Electron Devices, vol. 48, no. 3, pp. 560–566, Mar. 2001. [17] S. C. Binari, K. Ikossi, J. A. Roussos, W. Kruppa, D. Park, H. B. Dietrich, D. D. Koleske, A. E. Wickenden, and R. L. Henry, “Trapping effects and microwave power performance in AlGaN/GaN HEMTs,” IEEE Trans. Electron Devices, vol. 48, no. 3, pp. 465–471, Mar. 2001. [18] Z. Zhang, G. Yu, X. Zhang, X. Deng, S. Li, Y. Fan, S. Sun, L. Song, S. Tan, D. Wu, W. Li, W. Huang, K. Fu, Y. Cai, Q. Sun, and B. Zhang, “Studies on high-voltage GaN-on-Si MIS-HEMTs using LPCVD Si3 N4 as gate dielectric and passivation layer,” IEEE Trans. Electron Devices, vol. 63, no. 2, pp. 731–738, Feb. 2016, doi: 10.1109/TED.2015.2510445. [19] O. Hilt, A. Knauer, F. Brunner, E. Bahat-Treidel, and J. Wurfl, “Normally-off AlGaN/GaN HFET with p-type GaN gate and AlGaN buffer,” in Proc. ISPSD, Jun. 2010, pp. 347–350. [20] R. Chu, A. Corrion, M. Chen, R. Li, D. Wong, D. Zehnder, B. Hughes, and K. Boutros, “1200-V normally off GaN-on-Si field-effect transistors with low dynamic ON-resistance,” IEEE Electron Device Lett., vol. 32, no. 5, pp. 632–634, May 2011, doi: 10.1109/LED.2011.2118190. [21] B. Lu, E. Matioli, and T. Palacios, “Tri-gate normally-off GaN power MISFET,” IEEE Electron Device Lett., vol. 33, no. 3, pp. 360–362, Mar. 2012, doi: 10.1109/LED.2011.2179971. [22] I. Hwang, H. Choi, J. Lee, H. S. Choi, J. Kim, J. Ha, C.-H. Um, S.-K. Hwang, J. Oh, J.-Y. Kim, J. K. Shin, Y. Park, U.-I. Chung, I.-K. Yoo, and K. Kim, “1.6 kV, 2.9 m cm2 normally-off p-GaN HEMT device,” in Proc. 24th Int. Symp. Power Semiconductor Devices ICs, Jun. 2012, pp. 41–44, doi: 10.1109/ISPSD.2012.6229018. [23] M. Ishida, T. Ueda, T. Tanaka, and D. Ueda, “GaN on Si technologies for power switching devices,” IEEE Trans. Electron Devices, vol. 60, no. 10, pp. 3053–3059, Oct. 2013, doi: 10.1109/TED.2013.2268577. [24] Z. Tang, Q. Jiang, Y. Lu, S. Huang, S. Yang, X. Tang, and K. J. Chen, “600-V normally off SiN x /AlGaN/GaN MIS-HEMT with large gate swing and low current collapse,” IEEE Electron Device Lett., vol. 34, no. 11, pp. 1373–1375, Nov. 2013, doi: 10.1109/LED.2013.2279846. [25] J. Wei, S. Liu, B. Li, X. Tang, Y. Lu, C. Liu, M. Hua, Z. Zhang, G. Tang, and K. J. Chen, “Low on-resistance normally-off GaN doublechannel metal–oxide–semiconductor high-electron-mobility transistor,” IEEE Electron Device Lett., vol. 36, no. 12, pp. 1287–1290, Dec. 2015, doi: 10.1109/LED.2015.2489228.