# Fileset

[JSAPReview_MDR.zip](https://mdr.nims.go.jp/filesets/68152117-3799-4cc6-b34b-efa570d43ee5/download)

## Creator

[Yamaguchi Takahide](https://orcid.org/0000-0003-0208-7317)

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Diamond field-effect transistors](https://mdr.nims.go.jp/datasets/2984184c-4000-4aa7-86e9-aa6a720ef759)

## Fulltext

9403tutorialreview_yamaguchi_translated_revised_�Å�I�Å.docxTutorial ReviewDiamond field-effect transistorsYamaguchi TakahideNational Institute for Materials Science, Tsukuba, Ibaraki 305-0044, JapanE-mail: yamaguchi.takahide@nims.go.jpDiamond exhibits unique properties enabling the fabrication of high-performance p-channel field-effect transistors, an achievement that remains challenging for other wide-bandgap semiconductors. This distinct potential positions diamond as an exceptional semiconductor material for realizing complementary power inverter circuits and other advancements in power electronics. This paper provides a comprehensive review of the progress on diamond field-effect transistors, with particular emphasis on the diamond/gate insulator interface and its critical influence on device performance.Received November 1, 2024; Accepted November 25, 2024Translated from Oyo Buturi 94, 124 (2025) DOI:  https://doi.org/10.11470/oubutsu.94.3_1241. IntroductionDiamond semiconductor devices have frequently been in the news of late. Compared with the currently used silicon carbide (SiC) and gallium nitride (GaN), diamond has the potential to produce power and high-frequency devices with superior characteristics. For this reason, it has been the focus of attention for many years [1], with numerous studies having been conducted on diamond field-effect transistors (FETs) and diodes. The recent development of technology for synthesizing large single-crystal substrates [2,3] has shifted momentum towards practical applications, including the establishment of start-up companies from universities and national research institutes.The first report on fabricating a diamond FET dates back to 1989 [4]; it was a metal–semiconductor (MES) FET that consisted of an Al Schottky gate on a p-type boron-doped layer deposited by chemical vapor deposition (CVD). Metal–insulator–semiconductor (MIS) FETs were reported in 1991 [5]. In these early studies, a boron-doped layer was used as the channel. In 1994, MESFETs utilizing the surface conduction property of hydrogen-terminated diamond were reported and normally off operation was demonstrated for the first time [6]. Diamond FETs having various structures have subsequently been fabricated, including delta-doped FETs [7], junction FETs (JFETs) [8], solution-gate FETs [9,10], deep-depletion FETs [11], Fin-FETs [12], MIMS-FETs [13], solid-state electric double-layer FETs [14], and optically controlled FETs [15].In this paper, we provide a detailed explanation of p-channel MISFETs based on hydrogen-terminated diamond, which have been the most frequently reported, and introduce recently developed FETs based on silicon-bonded surfaces. Particular attention is given to the influence of the diamond/gate insulator interface on device performance.2. The allure of diamondThe physical properties of diamond and major semiconductors are listed in Table 1 [16]. Diamond has an ultra-wide bandgap of 5.5 eV and a high breakdown field. This allows diamond FETs to operate at high temperatures and high voltages, with the consequent simplifications to the cooling system also expected to reduce the size and weight of the device. Furthermore, by reducing the thickness of the drift region—an area that maintains the voltage between the drain and source when the FET is in the off state—and increasing the dopant concentration, it is possible to achieve low loss through a reduction in the specific on-resistance. Although all wide bandgap semiconductors have these advantages in common, diamond’s would provide the greatest benefits.Furthermore, diamond has high electron mobility and especially high hole mobility at room temperature. Besides providing higher drive current and faster switching, high mobility enables devices to have low loss. The high mobility is thought to be due to the large optical phonon energy and high acoustic phonon velocity, which minimize the effects of phonon scattering [17]. Another major characteristic of diamond is that its thermal conductivity is the highest among the semiconductors, thereby making it especially suitable for high-power and high-frequency operation due to its efficient heat dissipation. Its low relative permittivity, 5.7, is also suitable for high-speed operation. Additionally, diamond has high radiation resistance [18] due to the high energy required to displace carbon atoms and generate electron-hole pairs, making it promising for use in high-radiation environments such as space and nuclear reactors.Meanwhile, a problematic physical property of diamond is the low ionization rate of dopants [16,19]. P-type and n-type diamonds can be synthesized by doping with boron and phosphorus, respectively. However, the ionization energies of the acceptors and donors are large (0.36 eV for boron and 0.57 eV for phosphorus). Therefore, even when diamond is doped with 1017 cm−3 boron, for example, the hole density at room temperature is low, less than 1015 cm−3 [16]. In order to achieve significant advantages from using a boron-doped layer in the drift region, it is necessary to operate the device at high temperatures [16]. However, as shown below, diamond has a unique feature that enables substantial p-type conduction at room temperature without employing boron doping.3. Diamond surface treatmentThe most crucial step in making diamond FETs is the surface treatment. Specifically, it is important to determine which elemental atoms to bond with the dangling bonds of the carbon atoms on the outermost surface of the diamond. In many previous studies, hydrogen-terminated surfaces—formed by bonding hydrogen to terminate the dangling bonds—(Fig. 1(a)) have been used for the channel region.3.1 Hydrogen terminationA hydrogen-terminated surface can be formed through processes such as hydrogen plasma treatment, and it remains stable in air near room temperature. The surface of diamond films deposited via CVD, using a gas mixture of hydrogen and carbon sources like methane, is hydrogen-terminated. A hydrogen-terminated surface is suitable for the channel region of FETs because of its lower surface defect (state) density compared with other terminations [20]. Diamond also has the unique property of exhibiting p-type surface conduction when it is exposed to air, even when it is not doped with boron. This property is considered useful for avoiding the low ionization rate problem of boron [19]. The sheet resistance, hole density, and mobility are approximately 5–50 kΩ, 1×1012–5×1013 cm−2, and 10–200 cm2 V−1 s−1, respectively [21,22].The factors causing surface conduction can be generally understood through the concept of surface transfer doping [22–26]. When hydrogen-terminated diamond is exposed to air, atmospheric gas molecules adsorb on the surface and act as acceptors (referred to as surface acceptors). These surface acceptors withdraw electrons from the valence band of the diamond, thus generating holes (Fig. 1(b)). Substances such as weakly acidic water containing atmospheric CO2 [24], water containing O2 [25], NO2 [23,26], or O3 [25,26] are thought to act as surface acceptors. Moreover, holes are considered to be confined to a depth of a few nanometers from the surface by an electric field, as in the inversion layer of Si MOSFETs [27,28]. A more fundamental reason for the occurrence of p-type conduction through such transfer doping is the exceptionally high valence band maximum of hydrogen-terminated diamond [29,30] (Fig. 2). Since the valence band maximum is positioned higher than the H+/H2 redox level [24] and the SOMO (singly occupied molecular orbital) level of NO2 [26], electrons are transferred and holes are generated. High hole densities of 1013–1014 cm-2 can be obtained by depositing oxides with high electron affinity (a low conduction band minimum), such as MoO3 (Fig. 2) and V2O5 [22]. Surface conduction also occurs when Al2O3 is deposited. Although Al2O3 has a high conduction band minimum, defects in the film are thought to act as surface acceptors [19,22,31].Transfer doping can be viewed as the presence of acceptors with a low (or even negative) ionization energy on the surface [25] or as a modulation doping (such as is used in making GaAs HEMTs) with no or very thin spacer layers. The ohmic contacts for surface conduction of hydrogen-terminated diamond are typically obtained by depositing high work function metals such as Au and Pd, forming TiC after Ti deposition and subsequent heating, or using boron-doped layers [22,32]. Schottky contacts can be obtained by depositing low work function metals such as Al [32].3.2 Oxygen terminationOxygen-terminated surfaces can be formed with oxygen plasma, UV ozone, and acid treatments [33]. These surfaces do not exhibit conduction even when they are exposed to air. The reason is thought to be that, as shown in Fig. 2, the valence band maximum is positioned at too low a level for transfer doping. Pauling’s electronegativity is 2.1 for hydrogen, 2.5 for carbon, and 3.5 for oxygen. Hydrogen and oxygen termination generate electric dipoles with opposite orientations (Cδ−–Hδ+ and Cδ+–Oδ−), which is responsible for the difference in electron affinity between the two [29,30]. While oxygen-terminated surfaces are often used for device isolation for hydrogen-terminated diamond FETs, their high density of surface states [30] make it difficult to use them in the channel regions of FETs.4. Hydrogen-terminated diamond FETsSince the first report on MESFETs in 1994, hydrogen-terminated diamond MISFETs using various types of gate insulators have been developed alongside MESFETs. P-channel FETs have been made with various heterojunctions containing different oxides and fluorides giving large valence band offsets, as shown in Fig. 2. However, it is necessary to deposit an insulator without degrading the hydrogen termination. Vapor deposition and atomic layer deposition (ALD) methods are considered to have a relatively low impact on the surface, but there have been concerns that sputtering might cause plasma-induced damage. A method was devised that deposits a thin layer of Al2O3 by ALD and then sputters another insulator to avoid the damage [34,35]. Various oxides (Al2O3, SiO2, HfO2, Y2O3, TiO2, ZrO2, Ta2O5, LaAlO3, MoO3) [34–38] and fluorides (CaF2, MgF2, BaF2) [39–41] have been used as gate insulators in diamond FETs.Normally on characteristics, indicating conduction even without applying a gate voltage, have been obtained in many cases. Surface adsorbates or defects in the insulator are thought to act as surface acceptors and generate holes [19,22,31,35]. A method was devised that actively increases the hole density by exposing the hydrogen-terminated surface to NO2 gas before the insulator is deposited [42]. This method produced the highest maximum drain current density IDmax, 1.3 A mm-1, reported so far for diamond FETs [43]. MoO3 and V2O5 depositions in the channel and gate–drain regions have also been used to increase the hole density [22]. Meanwhile, normally off operation, which is necessary for fail-safe operation of power electronics, has been realized using various methods, such as partial oxidation of hydrogen terminations and nitrogen ion implantation in the channel region [35,38,44].The following is a review of the performance of hydrogen-terminated diamond FETs reported to date. All of them are p-channel FETs.High temperature operation: FETs operating at 400 °C have been reported [45]. The gate insulator Al2O3 was deposited by ALD at 450 ºC.Breakdown voltage: A breakdown voltage VB of 365 V has been reported for a lateral FET with a gate-drain distance LGD of 1 μm (VB/LGD = 3.6 MV cm-1) [31]. An FET with an LGD of 11 μm and VB of 2568 V has also been reported [46]. The latter FET exhibited a maximum drain current of 0.68 A mm−1 and an on-resistance of 50 Ω mm. The hydrogen-terminated surface was exposed to N2-diluted NO2 gas, and an Al2O3 insulator was formed on it by using two-step ALD at 120 °C and 230 °C (Fig. 3(a)). It is also noteworthy that the FETs were fabricated using heteroepitaxially grown single-crystal diamond substrates. A similar technique has been used to fabricate FETs that exhibit a VB in excess of 3000 V [47]. Among the FETs that exhibit normally off operation, an FET with an LGD of 21 μm, VB of 2000 V, and IDmax of 8.3 mA mm-1 has been reported [44].High-frequency operation: High-frequency characteristics have also been measured in FETs with shorter gate lengths [32]. A MISFET with a gate length of 100 nm showed a current-gain cutoff frequency fT of 70 GHz [48]. Similarly, a MESFET with the same gate length had a maximum oscillation frequency fmax of 120 GHz [49]. Additionally, a MISFET with a gate length of 0.9 μm demonstrated an output power density of 4.2 W mm−1 (2 GHz) [50].Radiation tolerance: In a study on X-ray irradiation of MISFETs, increases in drain current density and transconductance, as well as a positive shift in threshold voltage, were observed at total doses up to 10 kGy. However, it was shown that these parameters do not change significantly thereafter until 1 MGy [51].Heat dissipation: The surface temperature of MESFETs has been measured by Raman spectroscopy of TiO2 nanoparticles, yielding a device thermal resistance as low as 1 mm K W−1 [52]. This is approximately 1/10th and 1/50th of the values measured for GaN and β-Ga2O3 n-channel FETs.Vertical FETs: Development of vertical FETs for high-voltage and high-current applications is also underway. Vertical FETs that use hydrogen-terminated surfaces for conduction in the voltage-sustaining region, as in lateral FETs, have demonstrated a specific on-resistance of 3.2 mΩ cm2, IDmax = 12800 A cm−2, and VB = 315 V [53]. Moreover, a vertical FET with a boron-doped drift layer had a specific on-resistance of 23 mΩ cm2 and VB = 580 V [54]. The structure of this FET is shown in Fig. 3(b). Here, the trench structure is formed by plasma etching with oxygen gas after depositing a boron-doped drift layer, undoped layer, and nitrogen-doped layer on a heavily boron-doped diamond substrate. Another undoped layer is re-grown to cover the surface of the trench, and Al2O3 gate insulator is deposited on the hydrogen-terminated surface by ALD. This structure takes advantage of the property that conduction occurs on the hydrogen-terminated surface regardless of the crystallographic orientation. The nitrogen-doped layer is used to suppress leakage current.Channel mobility: While it is generally believed that transfer doping is required to generate the holes for hydrogen-terminated diamond FETs to operate, the authors have worked on improving channel mobility by eliminating as many surface acceptors as possible [21,28,55]. The presence of ionized surface acceptors just above the channel causes Coulomb scattering of holes [56] (and normally on operation), so it was thought to be preferable to remove them. To do so, the diamond surface was hydrogen-terminated, and then, cleaved single-crystal hexagonal boron nitride (h-BN) was bonded to it as a gate insulator without exposing it to the atmosphere (Fig. 4). FETs fabricated with this method exhibited high room-temperature channel mobility (680 cm2 V−1 s−1) as well as normally off operation. The mobility was more than three times higher than in other studies that used oxide or fluoride gate insulators at the same hole density. It was also much higher than the reported values for SiC and GaN p-channel FETs (less than 28 cm2 V−1 s−1 and 30 cm2 V−1 s−1, respectively [57,58]).5. Bonding with insulatorsAs mentioned above, ALD has been used in many studies to deposit Al2O3 on hydrogen-terminated surfaces. However, the bonding state at the interface is not certain. A recent study using photoelectron holography showed that some carbon atoms CD on the diamond surface bond to O instead of H and to Al2O3 in a CD–O–Al–O–CD bridge structure [59]. Additionally, the signal intensity from CD–O bonds correlates with the density of interfacial defects, suggesting that a reduction in the density of CD–O bonds would improve the FET characteristics. These results suggest that, in hydrogen-terminated diamond FETs with Al2O3 used as the gate insulator, there is a trade-off between the conduction characteristics and the bonding strength of the insulator.It has been reported that an FET can be fabricated by first terminating a diamond surface with hydroxy (OH) groups through heating in nitrogen gas containing water vapor and subsequently depositing Al2O3 on the surface by ALD [60]. Ideally, this method would form CD–O–Al– bonds on the entire diamond surface. It has been used to fabricate inversion p-channel FETs with boron-doped p-type source and drain regions on phosphorus- or nitrogen-doped n-type diamond. Although the high interface state density currently limits the mobility to around 20 cm2V-1s-1, this approach would be preferable in terms of bonding strength compared to performing ALD on hydrogen-terminated surfaces.6.  FETs using silicon-bonded surfacesRecently, p-channel FETs using CD–Si bonded diamond surfaces were fabricated that exhibited normally off operation and had excellent conduction properties [61]. CD–Si–O– bonds were reportedly formed by depositing SiO2 on an oxygen-terminated diamond surface and placing the surface in a hydrogen plasma environment at 800 °C or by depositing a few atomic layers of Si on a hydrogen-terminated surface, heating it to about 900 °C, and then oxidizing it. It was also found that the CD–Si–O– bonds remained even after the SiO2 layer was etched in hydrofluoric acid. FETs with this Si-bonded surface show normally off operation with a threshold voltage of −5 to −16 V, which is higher than that of hydrogen-terminated diamond FETs. Their mobility, from 120 to 200 cm2 V−1 s−1, is comparable to that of typical hydrogen-terminated diamond FETs [62,63]. Si-bonded diamond surfaces have also been utilized in the channel region of vertical FETs. The large threshold voltage is thought to be due to the valence band maximum being lower than in the case of hydrogen termination and the positive charge in SiO2 [63]. (An electron affinity of −0.25 eV [64] has been reported experimentally for oxidized Si-terminated diamond, while first-principles calculations indicate a stable structure with an electron affinity of +1.11 eV [65].)7. ConclusionWide bandgap semiconductor FETs generally have p-channel characteristics that are inferior to their n-channel characteristics. However, diamond has a unique feature that can produce excellent p-channel FETs. A complementary (CMOS) inverter combining a diamond p-channel FET and a GaN n-channel FET has been proposed [31], and a monolithic integration of these devices has been demonstrated [66]. (It is worth mentioning that n-channel diamond FETs were recently reported in 2023. [67].)Many studies have used hydrogen-terminated surfaces. However, for practical applications, there is room for research on which element to use for termination or bonding to the insulator in the channel and gate-drain regions. It will be important to find junction structures that combine low interface state density and proper band alignment in a reliable manner. The recent realization of FETs using Si-bonded surfaces has opened up new possibilities for future development. There is room for improving the channel mobility. Here, hole mobilities of 2190 cm2 V−1 s−1 at room temperature and 1.03 × 106 cm2 V−1 s−1 at 13 K have recently been reported for high-purity bulk diamond [68]. The mobility may be even higher in local micro regions [69]. It will be interesting to see the extent to which channel mobility can be improved.Practical application of diamond FETs is hampered by the high cost of diamond single-crystal substrates and the lack of production methods for large-diameter wafers. However, large-area synthesis technology has made progress in recent years, with the development of mass production technology for heteroepitaxially grown 2-inch wafers [70] and single-crystal wafers with a diameter of 100 mm [71]. Regarding the high dislocation density, which is an issue for heteroepitaxially grown substrates, methods have been developed to suppress the propagation of threading dislocations during CVD growth [72,73]. We hope that the steady progress in research and development of crystal synthesis and device fabrication technologies will soon yield practical applications of diamond FETs.AcknowledgmentsThe work of the authors described in this paper was done in collaboration with Dr. Yosuke Sasama of the National Institute for Materials Science and many others. We would like to express our deepest gratitude to them. Part of this work was supported by a Grant-in-Aid for Scientific Research (KAKENHI) (19H02605) from the Japan Society for the Promotion of Science.References[1] R. F. Davis, Z. Sitar, B. E. Williams, H. S. Kong, H. J. Kim, J. W. Palmour, J. A. Edmond, J. Ryu, J. T. Glass, and C. H. Carter Jr., Mater. Sci. Eng. B 1, 77 (1988).[2] M. Schreck, S. Gsell, R. Brescia, and M. Fischer, Sci. Rep. 7, 44462 (2017).[3] S.-W. Kim, R. Takaya, S. Hirano, and M. Kasu, Appl. Phys. Express 14, 115501 (2021).[4] H. Shiomi, Y. Nishibayashi, and N. Fujimori, Jpn. J. Appl. Phys. 28, L2153 (1989).[5] G. S. Gildenblat, S. A. Grot, C. W. Hatfield, and A. R. Badzian, IEEE Electron Device Lett. 12, 37 (1991).[6] H. Kawarada, M. Aoki, and M. Ito, Appl. Phys. Lett. 65, 1563 (1994).[7] H. El-Hajj, A. Denisenko, A. Kaiser, R. S. Balmer, and E. Kohn, Diam. Relat. Mater. 17, 1259 (2008).[8] T. Iwasaki, J. Yaita, H. Kato, T. Makino, M. Ogura, D. Takeuchi, H. Okushi, S. Yamasaki, and M. Hatano, IEEE Electron Device Lett. 35, 241 (2014).[9] H. Kawarada, Y. Araki, T. Sakai, T. Ogawa, and H. Umezawa, Phys. Status Solidi A 185, 79 (2001).[10] F. Zou, Z. Wang, Z. Lin, C. Wang, and Z. Yuan, Electronics 13, 3881 (2024).[11] C. Masante, N. Rouger, and J. Pernot, J. Phys. D: Appl. Phys. 54, 233002 (2021).[12] B. Huang, X. Bai, S. K. Lam, and K. K. Tsang, Sci. Rep. 8, 3063 (2018).[13] M. Liao, L. Sang, T. Shimaoka, M. Imura, S. Koizumi, and Y. Koide, Adv. Electron. Mater. 5, 1800832 (2019).[14] M. Takayanagi, D. Nishioka, T. Tsuchiya, M. Imura, Y. Koide, T. Higuchi, and K. Terabe, Mater. Today Adv. 18, 100393 (2023).[15] C. Masante, M. Kah, C. Hébert, N. Rouger, and J. Pernot, Adv. Electron. Mater. 8, 2100542 (2021).[16] N. Donato, N. Rouger, J. Pernot, G. Longobardi, and F. Udrea, J. Phys. D: Appl. Phys. 53, 093001 (2019).[17] J. Pernot, P. N. Volpe, F. Omnès, P. Muret, V. Mortet, K. Haenen, and T. Teraji, Phys. Rev. B 81, 205203 (2010).[18] T. Shimaoka, S. Koizumi, and H. J. Kaneko, Funct. Diam. 1, 205 (2021).[19] M. W. Geis, T. C. Wade, C. H. Wuorio, T. H. Fedynyshyn, B. Duncan, M. E. Plaut, J. O. Varghese, S. M. Warnock, S. A. Vitale, and M. A. Hollis, Phys. Status Solidi A 215, 1800681 (2018).[20] A. Stacey, N. Dontschuk, J. P. Chou, D. A. Broadway, A. K. Schenk, M. J. Sear, J. P. Tetienne, A. Hoffman, S. Prawer, C. I. Pakes, A. Tadich, N. P. de Leon, A. Gali, and L. C. L. Hollenberg, Adv. Mater. Interfaces 6, 1801449 (2018).[21] Y. Sasama, K. Komatsu, S. Moriyama, M. Imura, T. Teraji, K. Watanabe, T. Taniguchi, T. Uchihashi, and T. Yamaguchi, APL Mater. 6, 111105 (2018).[22] K. G. Crawford, I. Maini, D. A. Macdonald, and D. A. J. Moran, Prog. Surf. Sci. 96, 100613 (2021).[23] R. Gi, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, and M. Iida, Jpn. J. Appl. Phys. 38, 3492 (1999).[24] F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85, 3472 (2000).[25] J. Ristein, J. Phys. D: Appl. Phys. 39, R71 (2006).[26] Y. Takagi, K. Shiraishi, M. Kasu, and H. Sato, Surf. Sci. 609, 203 (2013).[27] C. E. Nebel, B. Rezek, and A. Zrenner, Diam. Relat. Mater. 13, 2031 (2004).[28] Y. Sasama, K. Komatsu, S. Moriyama, M. Imura, S. Sugiura, T. Terashima, S. Uji, K. Watanabe, T. Taniguchi, T. Uchihashi, and T. Yamaguchi, Phys. Rev. Mater. 3, 121601(R) (2019).[29] F. Maier, J. Ristein, and L. Ley, Phys. Rev. B 64, 165411 (2001).[30] S. J. Sque, R. Jones, and P. R. Briddon, Phys. Rev. B 73, 085313 (2006).[31] H. Kawarada, T. Yamada, D. Xu, H. Tsuboi, Y. Kitabayashi, D. Matsumura, M. Shibata, T. Kudo, M. Inaba, and A. Hiraiwa, Sci. Rep. 7, 42368 (2017).[32] H. Kawarada, J. Phys. D: Appl. Phys. 56, 053001 (2023).[33] R. Zulkharnay, G. Zulpukarova, and P. W. May, Appl. Surf. Sci. 658, 159776 (2024).[34] J. Liu, M. Liao, M. Imura, A. Tanaka, H. Iwai, and Y. Koide, Sci. Rep. 4, 6395 (2014).[35] J.W. Liu, M.Y. Liao, M. Imura, R.G. Banal, and Y. Koide, J. Appl. Phys. 121, 224502 (2017).[36] F. Wang, W. Wang, G. Chen, P. Yang, Y. Wang, M. Zhang, R. Wang, W. Hu, and H. Wang, Diam. Relat. Mater. 143, 110905 (2024).[37] F. Zhao, Y. He, B. Huang, T. Zhang, and H. Zhu, Materials (Basel) 17, 3437 (2024).[38] M. Li, X. Zhang, S. Jiao, Y. Wang, S. Wei, J. Yan, J. Zhang, and X. Zhang, Funct. Diam. 4, 2357654 (2024).[39] Y. Yun, T. Maki, H. Tanaka, and T. Kobayashi, Jpn. J. Appl. Phys. 38, 2640 (1999).[40] Q. He, J. Zhang, Z. Ren, J. Zhang, K. Su, Y. Lei, D. Lv, T. Mi, and Y. Hao, Diam. Relat. Mater. 119, 108547 (2021).[41] Q. He, K. Su, J. Zhang, Z. Ren, Y. Xing, J. Zhang, Y. Lei, and Y. Hao, IEEE Trans. Electron Devices 69, 1206 (2022).[42] M. Kasu, H. Sato, and K. Hirama, Appl. Phys. Express 5, 025701 (2012).[43] K. Hirama, H. Sato, Y. Harada, H. Yamamoto, and M. Kasu, Jpn. J. Appl. Phys. 51, 090112 (2012).[44] Y. Kitabayashi, T. Kudo, H. Tsuboi, T. Yamada, D. Xu, M. Shibata, D. Matsumura, Y. Hayashi, M. Syamsul, M. Inaba, A. Hiraiwa, and H. Kawarada, IEEE Electron Device Lett. 38, 363 (2017).[45] H. Kawarada, H. Tsuboi, T. Naruo, T. Yamada, D. Xu, A. Daicho, T. Saito, and A. Hiraiwa, Appl. Phys. Lett. 105, 013510 (2014).[46] N. C. Saha, S.-W. Kim, T. Oishi, and M. Kasu, IEEE Electron Device Lett. 43, 777 (2022).[47] N. C. Saha, S.-W. Kim, K. Koyama, T. Oishi, and M. Kasu, IEEE Electron Device Lett. 44, 112 (2023).[48] X. Yu, J. Zhou, C. Qi, Z. Cao, Y. Kong, and T. Chen, IEEE Electron Device Lett. 39, 1373 (2018).[49] K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D. J. Twitchen, G. A. Scarsbrook, and S. E. Coe, IEEE Electron Device Lett. 27, 570 (2006).[50] C. Yu, C. Zhou, J. Guo, Z. He, M. Ma, H. Yu, X. Song, A. Bu, and Z. Feng, Funct. Diam. 2, 64 (2022).[51] T. Yamaguchi, H. Umezawa, S. Ohmagari, H. Koizumi, and J. H. Kaneko, Appl. Phys. Lett. 118, 162105 (2021).[52] J. S. Lundh, D. Shoemaker, A. G. Birdwell, J. D. Weil, L. M. De La Cruz, P. B. Shah, K. G. Crawford, T. G. Ivanov, H. Y. Wong, and S. Choi, Appl. Phys. Lett. 119, 143502 (2021).[53] M. Iwataki, N. Oi, K. Horikawa, S. Amano, J. Nishimura, T. Kageura, M. Inaba, A. Hiraiwa, and H. Kawarada, IEEE Electron Device Lett. 41, 111 (2020).[54] J. Tsunoda, N. Niikura, K. Ota, A. Morishita, A. Hiraiwa, and H. Kawarada, IEEE Electron Device Lett. 43, 88 (2022).[55] Y. Sasama, T. Kageura, M. Imura, K. Watanabe, T. Taniguchi, T. Uchihashi, and T. Yamaguchi, Nat. Electron. 5, 37 (2022).[56] Y. Sasama, T. Kageura, K. Komatsu, S. Moriyama, J.-i. Inoue, M. Imura, K. Watanabe, T. Taniguchi, T. Uchihashi, and T. Yamaguchi, J. Appl. Phys. 127, 185707 (2020).[57] K. Mikami, M. Kaneko, and T. Kimoto, IEEE Electron Device Lett. 45, 1113 (2024).[58] B. Reuters, H. Hahn, A. Pooth, B. Holländer, U. Breuer, M. Heuken, H. Kalisch, and A. Vescan, J. Phys. D: Appl. Phys. 47, 175103 (2014).[59] M. N. Fujii, M. Tanaka, T. Tsuno, Y. Hashimoto, H. Tomita, S. Takeuchi, S. Koga, Z. Sun, J. I. Enriquez, Y. Morikawa, J. Mizuno, M. Uenuma, Y. Uraoka, and T. Matsushita, Nano Lett. 23, 1189 (2023).[60] T. Matsumoto, H. Kato, T. Makino, M. Ogura, D. Takeuchi, S. Yamasaki, T. Inokuma, and N. Tokuda, Appl. Phys. Lett. 114, 242101 (2019).[61] H. Kawarada, K. Ota, Y. Fu, A. Narita, X. Zhu, A. Hiraiwa, and T. Fujishima, 2023 Int. Electron Devices Meet. (IEDM) (2023) [DOI: 10.1109/IEDM45741.2023.10413761].[62] Y. Fu, Y. Chang, X. Zhu, R. Xu, Y. Xu, and H. Kawarada, IEEE Trans. Electron Devices 69, 4144 (2022).[63] X. Zhu, T. Bi, X. Yuan, Y. Chang, R. Zhang, Y. Fu, J. Tu, Y. Huang, J. Liu, C. Li, and H. Kawarada, Appl. Surf. Sci. 593, 153368 (2022).[64] A. K. Schenk, M. J. Sear, A. Tadich, A. Stacey, and C. I. Pakes, J. Phys.: Condens. Matter 29, 025003 (2017).[65] H. Gomez, Jr., J. Cruz, C. Milne, T. Debnath, A. G. Birdwell, E. J. Garratt, B. B. Pate, S. Rudin, D. A. Ruzmetov, J. D. Weil, P. B. Shah, T. G. Ivanov, R. K. Lake, M. N. Groves, and M. R. Neupane, J. Chem. Phys. 161, 064702 (2024).[66] C. Ren, M. Malakoutian, S. Li, B. Ercan, and S. Chowdhury, ACS Appl. Electron. Mater. 3, 4418 (2021).[67] M. Liao, H. Sun, and S. Koizumi, Adv. Sci. 11, e2306013 (2024).[68] A. Portier, F. Donatini, D. Dauvergne, M. L. Gallin-Martel, and J. Pernot, Phys. Rev. Appl. 20, 024037 (2023).[69] K. Konishi, I. Akimoto, H. Matsuoka, J. Isberg, and N. Naka, Phys. Rev. Appl. 17, L031001 (2022).[70] https://www.ad-na.com/magazine/archives/3022 (2024/10/30).[71] https://www.prnewswire.com/news-releases/diamond-foundry-creates-worlds-first-diamond-wafer--largest-diamond-on-earth-301974595.html (2024/10/30).[72] S. Ohmagari, H. Yamada, N. Tsubouchi, H. Umezawa, A. Chayahara, Y. Mokuno, and D. Takeuchi, Phys. Status Solidi A 216, 1900498 (2019).[73] R. Wang, F. Lin, G. Niu, J. Su, X. Yan, Q. Wei, W. Wang, K. Wang, C. Yu, and H. X. Wang, Materials (Basel) 15, 444 (2022).[74] J. Robertson, J. Vac. Sci. Technol. A 31, 050821 (2013).[75] T. Knobloch, Y.Y. Illarionov, F. Ducry, C. Schleich, S. Wachter, K. Watanabe, T. Taniguchi, T. Mueller, M. Waltl, M. Lanza, M.I. Vexler, M. Luisier and T. Grasser, Nature Electron. 4, 98 (2021).[76] J. Chen, Z. Zhang, Y. Guo, J. Robertson, J. Appl. Phys. 131, 215302 (2022).ProfileYamaguchi TakahidePrincipal Researcher at the National Institute for Materials Science (NIMS). Professor, Institute of Pure and Applied Sciences, University of Tsukuba (University of Tsukuba-NIMS Joint Graduate School). He graduated from the University of Tokyo in 2000.Figure captionsFig. 1. (a) Hydrogen-terminated diamond. (b) Surface transfer doping model [24]. Electrons are transferred from the valence band maximum of the diamond to the unoccupied levels of surface adsorbates or deposited materials, generating holes on the diamond surface.Fig. 2. Band lineup of various semiconductors and insulators [24,26,29,74,75,76]. Due to the high valence band maximum of hydrogen-terminated diamond (H-diamond), electrons are considered to be transferred to materials with unoccupied levels lower than that, such as weakly acidic water, NO2, and MoO3, generating holes responsible for surface conduction. Surface conduction does not occur in oxygen-terminated diamond (O-diamond). The H+/H2 redox level is for pH 6 and a hydrogen partial pressure of 10-3 mbar [24]. For NO2, the SOMO and HOMO (highest occupied molecular orbital) levels are indicated [26].Fig. 3. (a) Lateral high-voltage hydrogen-terminated diamond FET [46]. After exposing the hydrogen-terminated surface to NO2 gas, Al2O3 insulating film is deposited by ALD. (b) Vertical high-voltage hydrogen-terminated diamond FET [54]. This FET employs the conductivity of the hydrogen-terminated surface of undoped layer regrown on the trench surface. This figure is based on references 46 and 54.Fig. 4. (a) High-mobility hydrogen-terminated diamond FET with single-crystal hexagonal boron nitride (h-BN) as a gate insulator [21]. Single-crystal h-BN was utilized to reduce the defect (acceptor) density within the gate insulator. (b) The acceptor density from the atmosphere was reduced by laminating the hydrogen-terminated surface with h-BN in a glove box without exposing it to the atmosphere. The FET fabricated using this method had a channel mobility of 680 cm² V⁻¹ s⁻¹, which is one of the highest among p-channel FETs based on wide bandgap semiconductors [55].Table 1. Physical properties of various semiconductors [16].  　  Si  4H-SiC  GaN  β-Ga2O3  Diamond                Bandgap (eV)  1.1  3.3  3.4  4.9  5.5    Breakdown electric field (MV cm-1)  0.3  2.8  3.3  8  10    Electron mobility (cm2 V-1 s-1)  1500  1000  ≥ 1200  200  ≥ 1060    Hole mobility (cm2 V-1 s-1)  450  120  < 200  —  ≥ 2100    Electron saturation velocity (107 cm s-1)  1.1  1.9  2.5  2  2.5    Hole saturation velocity (107 cm s-1)  0.8  1.2  —  —  1.4    Relative permittivity  11.8  9.7  9  10  5.7    Thermal conductivity (W cm-1 K-1)  1.5  3.7  2.5  0.27  22  19Fig1Fig3_Eng.pdfHoleConductionbandValencebandEFdiamonde-Unoccupiedlevel(a)(b)Figure 1HydrogenCarbonFigure 3DiamondAl  O2 3GateSource DrainH-terminated surface (exposed to 　    gas)NO2Source Gate Gate SourceDrainBoron-doped (p  ) diamond substrateBoron-doped (p ) drift layer-+UndopedN-dopedUndopedH-terminated surfaceAl  O2 3(a)(b)H-terminated Adsorbates(Deposits)Fig2_Eng.pdfEnergy (eV)02-2-4-6-8-10-12 Si4H-SiCGaNh-BNO - diamondH - diamondGa 23OAl 23O3MoO2NOHfO2 2SiO2CaFVacuum level2HH2+ +2e-↑↓Figure 2Valence bandConduction bandFig4_Eng.pdf(a)(b)Figure 4Hydrogen plasmaDiamondMicroscopeDiamondh-BNTransfer invacuumGlove boxCVD chamberGelsuitcaseAVVHVDSVGSDiamondh-BNAl2O3SDGBNHC