Normally-off AlGaN/GaN MIS-HFET using stacked NiO/Al2O3 Gate Structure Formed by Atomic Layer Deposition Yasuhiro Yamada1, Asamira Suzuki1, Nobuyuki Otsuka1 and Daisuke Ueda1, 2 1
R&D Division Device Solutions Center, Panasonic Corporation 3-1-1 Yagumo-naka-machi, Moriguchi City, Osaka 570-8501, Japan Phone: +81-6-6906-4850, FAX: +81-6-6906-4850, e-mail:
[email protected] 2 R&D Advanced Technology Research Laboratories, Panasonic Corporation 3-4 Hikari-dai, Seika-cho, Soraku-gun, Kyoto 619-0237, JAPAN Abstract In this paper, we report a novel normally-off GaN-based transistor fabricated using stacked p-type oxide semiconductor layer and insulator layer gate structure. The device exhibits a good normally-off operation without gate recess structure. The fabricated device shows extremely low off-state leakage current, low channel resistance of 3Ω Ωmm, and high threshold voltage of 1.6V. 1. Introduction GaN-based heterojunction field effect transistors (HFET) are very promising for power switching devices [1]. The demand to increase the operating frequency for the reduction of passive components is pushing the use of GaN overcoming the limitation by the existing Si devices. So far, p-type AlGaN gate AlGaN/GaN HFET has been demonstrated for power switching applications as the one choice of the highly reliable normally-off device [2]. Moreover, the p-type NiO has been employed for the gate of normally-off GaN transistor [3, 4]. However, the threshold voltages (Vth) of these HFETs are less than 1 V. Then, it will be difficult to increase the Vth and decrease the gate leakage current much further due to the limitation of the p–n junction barrier height. We propose the decrease in gate leakage current owing to high potential barrier for electron and hole with introducing metal-insulator-semiconductor (MIS) structure. In this paper we present normally-off AlGaN/GaN MIS-HFET with a newly stacked p-type NiO layer and insulator Al2O3 layer gate structure, which control maximum gate voltage and decrease leakage current. 2. Experiment Fig. 1(a) shows the schematic cross-sectional view of the AlGaN/GaN MIS-HFET with stacked gate structure of NiO/Al2O3 laysers. The GaN-based epilayers were grown on Si substrate using a metal organic chemical vapor deposition (MOCVD) system. A 10 nm-thick unintentional doped Al0.3Ga0.7N barrier layer was grown on an undoped GaN channel layer. NiO/Al2O3 gate materials were formed by the ALD process, over the narrow opening of the dielectric (SiN) film. Fig. 1(b) shows the cross sectional SEM image of the fabricated MIS-HFET structure. In Fig. 2, higher barrier height of Al2O3 layer suppresses leakage current as shown in the schematic band diagram of AlGaN/GaN MIS-HFET (NiO/Al2O3 gate) structure (a) compared to conventional HFET (NiO gate) structure (b).
3. Results and Discussion Igs-Vgs characteristics of the MIS-HFET and HFET are shown in Fig.3. The forward gate voltage range in the MIS-HFET was biased up to a value greater than 3V with less than 100mA of gate current. The MIS-HFET showed a two order of magnitude reduction in gate leakage current under reverse-bias as compared to HFET. It is confirmed that NiO gate material operates as p-type conductor because HFET shows diode characteristics. Fig. 4 shows the DC current-voltage (transfer) characteristics of the MIS-HFET. The fabricated MIS-HFET exhibits good normally-off operation. As for the fabricated transistor, it is experimentally found that the HFET exhibited the threshold voltage of 0.8V, while the voltage of the MIS-HFET was 1.6V. The shift of threshold voltage in MIS-HFET exhibits low capacitance as compared to that with HFET. The DC current-voltage (output) characteristics are shown in Fig. 5. The maximum drain current about 0.34 A/mm was obtained at the gate voltage of 6V. The device with 0.3µm-gate width shows Ron of 17Ωmm with the threshold voltage of 1.6V and the off-state breakdown voltage of 104V at the total film thickness of 1.2µm. The channel resistance is estimated as low as 3Ωmm. Note that the reduction of the contact resistance further decreases the Ron as well. 3. Conclusion In conclusion, we have demonstrated a novel normally-off operation of the AlGaN/GaN MIS-HFET with stacked NiO/Al2O3 gate structures using ALD method. The new device shows extremely low off-state leakage current as well as low channel resistance owing to the introduction of an insulator layer. The results indicate that these films are promising as gate structure for AlGaN/GaN transistors technology. Acknowledgements This work is partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, under the Strategic Development of Energy Conversion Technology Project. References [1] D. Ueda et al., IEDM Technical Digests, (2005) 377. [2] Y. Uemoto et al., IEEE Trans. Electron Device., 54 (2007) 3393. [3] N. Kaneko et al.,ISPSD Technical Digests, pp. 25-28, 2009. [4] A. Suzuki et al., Extended Abstracts of the WOCSDICE, (2013) 77. [5] M Kuraguchi et al., phys. stat. sol. (a) 204, (2007) 2010.
10-1 10-2 (b) conventional HFET (NiO gate) 10-3 10-4 10-5 10-6 10-7 (a) MIS-HFET (NiO/Al2O3) 10-8 Lgs = 1 µm Lg = 0.3 µm 10-9 Lgd =3 µm -10 10 10-11 10-12 -5 -4 -3 -2 -1 0 1 2 3 Vg (V)
(a) G Al2O3 (b) SiN
S AlGaN
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Fig.1 Schematic cross section of the fabricated GaN-based HFET with stacked p-type semiconductor layer (NiO) and insulator layer (Al2O3) gate structure. Cross sectional SEM image of the fabricated NiO/Al2O3 stacked gate MISHFET structure.
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Fig.2 Schematic conduction band diagram of (a) AlGaN/GaN MIS-HFET (NiO/Al2O3 gate) structure and (b) conventional HFET (NiO gate) structure.
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Fig. 4 Transfer characteristics of the fabricated gate transistors. Normally off characteristics with Vth of 1.6V is achieved in stacked gate NiO/Al2O3 gate structure. 0.5
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Fig. 3 The gate current characteristics of AlGaN/GaN MIS-HFETs. (a) MIS-HFET with 100 nm/5nm thick NiO/Al2O3, (b) conventional HFET (NiO).
Al2O3
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Vds (V) Fig. 5 Output characteristics of AlGaN/GaN MIS-HFET with NiO/Al2O3 gate structure. The drain to gate length, gate length and gate to source length were 3mm, 0.3mm and 1mm, respectively.
