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Design consideration of high voltage Ga2O3 vertical Schottky barrier diode with field plate
Results in Physics 9 (2018) 1170–1171
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Design consideration of high voltage Ga2O3 vertical Schottky barrier diode with field plate J.-H. Choia, C.-H. Chob, H.-Y. Chaa, a b
T
⁎
School of Electronic and Electrical Engineering, Hongik University, Seoul, Republic of Korea Department of Electronic and Electrical Engineering, Hongik University, Sejong-Si, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Gallium oxide Field plate Breakdown voltage High permittivity Schottky barrier diode
Gallium oxide (Ga2O3) based vertical Schottky barrier diodes (SBDs) were designed for high voltage switching applications. Since p-type Ga2O3 epitaxy growth or p-type ion implantation technique has not been developed yet, a field plate structure was employed in this study to maximize the breakdown voltage by suppressing the electric field at the anode edge. TCAD simulation was used for the physical analysis of Ga2O3 SBDs from which it was found that careful attention must be paid to the insulator under the field plate. Due to the extremely high breakdown field property of Ga2O3, an insulator with both high permittivity and high breakdown field must be used for the field plate formation.
Introduction Wide bandgap semiconductors, such as SiC and GaN, have been studied for high power, high temperature, and optoelectronic applications due to their unique material properties [1]. Recently, great attention has also been paid to an ultra-wide bandgap semiconductor, gallium oxide (Ga2O3). Properties of wide energy bandgap (∼4.9 eV) and high breakdown field (∼8 MV/cm) in conjunction with a potentially low price of bulk wafers make β-Ga2O3 suitable for high-power switching applications [2]. There have been promising developments in Ga2O3 SBDs recent years [2]. The biggest technical challenge of Ga2O3 is the absence of p-type epitaxy or p-type ion implantation technique, which is commonly used in Si and SiC technologies to form the edge termination, so-called ‘guard ring structure’ [3,4]. Therefore, the breakdown voltage of Ga2O3 vertical SBD will be limited by the localized high electric field at the anode edge. In this study, a field plate structure that has been commonly used in planar type devices was employed for Ga2O3 vertical SBD to suppress the electric field at the anode edge. We report important design consideration about how to select the insulator for the field plate formation for Ga2O3 vertical SBD. Methods The device simulation was performed using a commercial TCAD software, Silvaco Atlas. The SBD structure generated for simulation is shown in Fig. 1(a). The doping concentration and thickness of the n-
⁎
type drift region were 1 × 1017 cm−3 and 2 µm, respectively, which were decided to achieve a breakdown voltage of > 1000 V. A highly doped n-type substrate (1 × 1019 cm−3) was used with a thickness of 0.2 µm in order to save the simulation time. A Schottky anode contact with a work function of 5 eV and an ideal ohmic cathode contact were assumed. The structural variables were the field plate length and the insulator materials under the field plate. The insulators investigated in this study were SiO2 (εr = 3.9), Al2O3 (εr = 9.1), and HfO2 (εr = 25) with a fixed thickness of 100 nm. A constant electron mobility of 200 cm2/V s was assumed and the impact ionization coefficient model for Ga2O3 was given by [5]
α = 0.79 × 106 cm−1 exp ⎛− ⎝ ⎜
2.92 × 107 V/cm ⎞ E ⎠ ⎟
(1)
Results and discussion The peak electric field strengths at three locations (see A, B, and C in Fig. 1(a)) were investigated as a function of reverse bias voltage where the field plate length was 3 μm (see Fig. 1(b)–(d)). The insets are the zoomed-in electric field distribution plots with a reverse voltage of 1000 V. While no significant difference was observed at the location ‘B’ or ‘C’ inside Ga2O3 for different insulators, the electric field strength at the location ‘A’ inside the insulator exhibited strong dependency on the insulator material. According to the Gauss’s law, the relationship between electric field and permittivity for two adjacent materials ‘1’ and ‘2’ is given by ε1E1 = ε2E2 with an assumption of no interface charge.
Corresponding author. E-mail address: [email protected] (H.-Y. Cha).
https://doi.org/10.1016/j.rinp.2018.04.042 Received 28 March 2018; Received in revised form 13 April 2018; Accepted 13 April 2018 Available online 22 April 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 9 (2018) 1170–1171
J.-H. Choi et al.
Fig. 1. (a) Structure file of Ga2O3 SBD used for simulation and the zoomed-in area near the anode edge. Electric field strengths versus reverse voltage at locations A, B, and C with different insulators; (b) SiO2, (c) Al2O3, and (d) HfO2. (e) Reverse breakdown and (f) forward current-voltage characteristics with HfO2 insulator as a function of field plate length.
Unlike other conventional semiconductors, the breakdown field of Ga2O3 is so high that great care must be taken when selecting the insulator for Ga2O3 devices. Although SiO2 has a higher breakdown field than Ga2O3, it is not a good choice because of its low permittivity; the dielectric breakdown of SiO2 limits the SBD breakdown voltage in a range from ∼500 V to ∼700 V. Al2O3 is better than SiO2 but still limits the performance; the breakdown voltage is estimated in a range from ∼800 V to ∼1000 V (note the breakdown field of ∼8 MV/cm for Ga2O3). HfO2 is the best choice and can maximize the breakdown voltage of Ga2O3 SBD due to its very high permittivity as well as high breakdown field. In the following study, HfO2 was used for the insulator. The field plate length was varied from 0 (i.e., no field plate) to 5 µm. The breakdown voltage without a field plate was ∼700 V, which increased as increasing the field plate length. The highest breakdown voltage was ∼1090 V with a field plate length of 3 µm or longer. The reverse and forward current-voltage characteristics as a function of the field plate length are plotted in Fig. 1(e) and (f), respectively. No difference was observed in the forward characteristics as a function of the field plate length due to the same anode contact region. The specific onresistance derived with the area of the anode contact region was 0.082 mΩ·cm2.
vertical SBD, great care must be taken to the dielectric breakdown of the film that will limit the breakdown voltage of SBD. In this respect, HfO2 is a good candidate due to its high permittivity as well as high breakdown field. Acknowledgments The authors thank Mr. Jong-Ik Kang for his technical assistance. This work was supported by Material Component Development Program of MOTIE/KEIT (10080736) and Basic Science Research Programs (2015R1A6A1A03031833 and 2016R1D1A1B03935445) through NRF. References [1] Burk Jr AA, O’Loughlin MJ, Siergiej RR, Agarwal AK, Sriram S, Clarke RC, MacMillan MF, Balakrishna V, Brandt CD. SiC and GaN wide bandgap semiconductor materials and devices. Solid-State Electron 1999;43(8):1459–64. [2] Higashiwaki M, Sasaki K, Murakami H, Kumagai Y, Koukitu A, Kuramata A, Masui T, Yamakoshi S. Recent progress in Ga2O3 power devices. Semicond Sci Technol 2016;31(3):034001. [3] Varley JB, Janotti A, Franchini C, Van de Walle CG. Role of self-trapping in luminescence and p-type conductivity of wide-band-gap oxides. Phys Rev B 2012;85(8):081109. [4] Baliga BJ. Fundamentals of power semiconductor devices. New York, NY, USA: Spring-Verlag; 2008. p. 197–8. [5] Ghosh K, Singisett U.Impact ionization in monoclinic β-Ga2O3. arXiV:1705.09203v1 [cond-mat.mtrl-sci] 25 May 2017.
Conclusions When a dielectric film is deposited on top of the surface of Ga2O3
1171
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Design consideration of high voltage Ga2O3 vertical Schottky barrier diode with field plate J.-H. Choia, C.-H. Chob, H.-Y. Chaa, a b
T
⁎
School of Electronic and Electrical Engineering, Hongik University, Seoul, Republic of Korea Department of Electronic and Electrical Engineering, Hongik University, Sejong-Si, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Gallium oxide Field plate Breakdown voltage High permittivity Schottky barrier diode
Gallium oxide (Ga2O3) based vertical Schottky barrier diodes (SBDs) were designed for high voltage switching applications. Since p-type Ga2O3 epitaxy growth or p-type ion implantation technique has not been developed yet, a field plate structure was employed in this study to maximize the breakdown voltage by suppressing the electric field at the anode edge. TCAD simulation was used for the physical analysis of Ga2O3 SBDs from which it was found that careful attention must be paid to the insulator under the field plate. Due to the extremely high breakdown field property of Ga2O3, an insulator with both high permittivity and high breakdown field must be used for the field plate formation.
Introduction Wide bandgap semiconductors, such as SiC and GaN, have been studied for high power, high temperature, and optoelectronic applications due to their unique material properties [1]. Recently, great attention has also been paid to an ultra-wide bandgap semiconductor, gallium oxide (Ga2O3). Properties of wide energy bandgap (∼4.9 eV) and high breakdown field (∼8 MV/cm) in conjunction with a potentially low price of bulk wafers make β-Ga2O3 suitable for high-power switching applications [2]. There have been promising developments in Ga2O3 SBDs recent years [2]. The biggest technical challenge of Ga2O3 is the absence of p-type epitaxy or p-type ion implantation technique, which is commonly used in Si and SiC technologies to form the edge termination, so-called ‘guard ring structure’ [3,4]. Therefore, the breakdown voltage of Ga2O3 vertical SBD will be limited by the localized high electric field at the anode edge. In this study, a field plate structure that has been commonly used in planar type devices was employed for Ga2O3 vertical SBD to suppress the electric field at the anode edge. We report important design consideration about how to select the insulator for the field plate formation for Ga2O3 vertical SBD. Methods The device simulation was performed using a commercial TCAD software, Silvaco Atlas. The SBD structure generated for simulation is shown in Fig. 1(a). The doping concentration and thickness of the n-
⁎
type drift region were 1 × 1017 cm−3 and 2 µm, respectively, which were decided to achieve a breakdown voltage of > 1000 V. A highly doped n-type substrate (1 × 1019 cm−3) was used with a thickness of 0.2 µm in order to save the simulation time. A Schottky anode contact with a work function of 5 eV and an ideal ohmic cathode contact were assumed. The structural variables were the field plate length and the insulator materials under the field plate. The insulators investigated in this study were SiO2 (εr = 3.9), Al2O3 (εr = 9.1), and HfO2 (εr = 25) with a fixed thickness of 100 nm. A constant electron mobility of 200 cm2/V s was assumed and the impact ionization coefficient model for Ga2O3 was given by [5]
α = 0.79 × 106 cm−1 exp ⎛− ⎝ ⎜
2.92 × 107 V/cm ⎞ E ⎠ ⎟
(1)
Results and discussion The peak electric field strengths at three locations (see A, B, and C in Fig. 1(a)) were investigated as a function of reverse bias voltage where the field plate length was 3 μm (see Fig. 1(b)–(d)). The insets are the zoomed-in electric field distribution plots with a reverse voltage of 1000 V. While no significant difference was observed at the location ‘B’ or ‘C’ inside Ga2O3 for different insulators, the electric field strength at the location ‘A’ inside the insulator exhibited strong dependency on the insulator material. According to the Gauss’s law, the relationship between electric field and permittivity for two adjacent materials ‘1’ and ‘2’ is given by ε1E1 = ε2E2 with an assumption of no interface charge.
Corresponding author. E-mail address: [email protected] (H.-Y. Cha).
https://doi.org/10.1016/j.rinp.2018.04.042 Received 28 March 2018; Received in revised form 13 April 2018; Accepted 13 April 2018 Available online 22 April 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 9 (2018) 1170–1171
J.-H. Choi et al.
Fig. 1. (a) Structure file of Ga2O3 SBD used for simulation and the zoomed-in area near the anode edge. Electric field strengths versus reverse voltage at locations A, B, and C with different insulators; (b) SiO2, (c) Al2O3, and (d) HfO2. (e) Reverse breakdown and (f) forward current-voltage characteristics with HfO2 insulator as a function of field plate length.
Unlike other conventional semiconductors, the breakdown field of Ga2O3 is so high that great care must be taken when selecting the insulator for Ga2O3 devices. Although SiO2 has a higher breakdown field than Ga2O3, it is not a good choice because of its low permittivity; the dielectric breakdown of SiO2 limits the SBD breakdown voltage in a range from ∼500 V to ∼700 V. Al2O3 is better than SiO2 but still limits the performance; the breakdown voltage is estimated in a range from ∼800 V to ∼1000 V (note the breakdown field of ∼8 MV/cm for Ga2O3). HfO2 is the best choice and can maximize the breakdown voltage of Ga2O3 SBD due to its very high permittivity as well as high breakdown field. In the following study, HfO2 was used for the insulator. The field plate length was varied from 0 (i.e., no field plate) to 5 µm. The breakdown voltage without a field plate was ∼700 V, which increased as increasing the field plate length. The highest breakdown voltage was ∼1090 V with a field plate length of 3 µm or longer. The reverse and forward current-voltage characteristics as a function of the field plate length are plotted in Fig. 1(e) and (f), respectively. No difference was observed in the forward characteristics as a function of the field plate length due to the same anode contact region. The specific onresistance derived with the area of the anode contact region was 0.082 mΩ·cm2.
vertical SBD, great care must be taken to the dielectric breakdown of the film that will limit the breakdown voltage of SBD. In this respect, HfO2 is a good candidate due to its high permittivity as well as high breakdown field. Acknowledgments The authors thank Mr. Jong-Ik Kang for his technical assistance. This work was supported by Material Component Development Program of MOTIE/KEIT (10080736) and Basic Science Research Programs (2015R1A6A1A03031833 and 2016R1D1A1B03935445) through NRF. References [1] Burk Jr AA, O’Loughlin MJ, Siergiej RR, Agarwal AK, Sriram S, Clarke RC, MacMillan MF, Balakrishna V, Brandt CD. SiC and GaN wide bandgap semiconductor materials and devices. Solid-State Electron 1999;43(8):1459–64. [2] Higashiwaki M, Sasaki K, Murakami H, Kumagai Y, Koukitu A, Kuramata A, Masui T, Yamakoshi S. Recent progress in Ga2O3 power devices. Semicond Sci Technol 2016;31(3):034001. [3] Varley JB, Janotti A, Franchini C, Van de Walle CG. Role of self-trapping in luminescence and p-type conductivity of wide-band-gap oxides. Phys Rev B 2012;85(8):081109. [4] Baliga BJ. Fundamentals of power semiconductor devices. New York, NY, USA: Spring-Verlag; 2008. p. 197–8. [5] Ghosh K, Singisett U.Impact ionization in monoclinic β-Ga2O3. arXiV:1705.09203v1 [cond-mat.mtrl-sci] 25 May 2017.
Conclusions When a dielectric film is deposited on top of the surface of Ga2O3
1171