Vertical Diamond p-FETs with Normally-Off Operation for Complementary High Power and High Speed Inverters

Since the inverter circuit is composed of upper and lower arms of n-channel field effect transistors (n-FETs), dead time and delay in the bootstrap circuit are inevitable. In a complementary inverter with upper n-FET and lower p-channel FET (p-FET), however, the gate and source potentials are the same for both FETs [1]. There is only one gate drive circuit, and no need for synchronization between the upper and lower gate drivers. The dead time is eliminated and MHz operation can be achieved to reduce heavy inductive part such as filters and transformers. An ideal compact power system can be realized. However, high-voltage C-FET has not been developed yet. The reason is that there was no high-voltage p-FET with performance close to that of n-FET. In GaN or SiC, the performance of p-FET is about 1/100 of that of n-FET. On the other hand, diamond lateral p-FETs that use 2 Dimensional Hole Gas (2DHG) for the channel have high current drive characteristics with high breakdown voltages [2,3]. They are far superior to SiC and GaN p-FETs, and can be compared with those lateral n-FETs in normally-on type. In this study vertical diamond FETs and normally off diamond FETs with high performance have been investigated.<br/><b>Vertical FETs: </b>C-H surface has been studied for the channel of FETs since 1994 [4]. Not only the 2DHG formed beneath the C-H surface, but also the low surface state density is advantageous for MOSFET application. The regrown homo epitaxial layer in a trench structure of vertical MOSFET has no distinct interfacial defects at the side walls observed by TEM. All most the same hole conductance as that in horizontally grown layer is obtained in trench walls even though interfaces are inclined to be far from low index planes such as (001), (111), or (110) etc.. Holes from a source on the top are controlled by a gate near the edge of a trench, pass through drift layer on the side walls and reach the p⁺ substrate at the bottom of trench [5,6]. The vertical MOSFETs exhibit high breakdown voltage of 580V [6]. The current density exceeds 10,000 Acm^-2[4] and the real current exceeds 3 A at large channel width &gt;10 mm at present. The minimum specific on resistance decreased to be 2 mΩcm² [4].<br/><b>Normally-off high performance diamond p-FET with C-Si MOS interface:</b>For high voltage switching, normally-off with large threshold voltage (Vth) more than 3V is desired in the power device application. Alternative interface for C-H is C-Si which appears as oxidized Si termination. From its negative χ [7], the valence band maximum position is near that of C-H indicating p-type tendency. C-Si connects diamond to SiO₂, which is a traditional insulator, but the most reliable gate oxide in MOSFETs. The most advantageous function of C-Si interface is normally-off operation with large Vth &gt;5V without deteriorating MOSFET performance such as current density and transconductance [8]. C-O-Si can also connect diamond to SiO₂, but C-O interface bonding is not suitable for current modulation. C-Si can replace some of the functions of C-H such as channel for normally-off FET.<br/>Combined with vertical device structure and C-Si channel for normally-off, diamond FETs can contribute to the complementary high power circuit topology which will open a new era of power electronics.<br/>[1] K. Okuda, T. Isobe, H. Tadano, N. Iwamuro <i>Proc. Eur. Conf. Power Electron. Appl.</i>, Sep. 2016, pp. 1–10.<br/>[2] H. Kawarada et al.<i> Sci. Rep. </i><b>7</b>, 42368, (2017)<br/>[3] N. C. Saha, M. Kasu <i>et al </i><i>., IEEE Elec Dev Lett </i><b>42</b>, 903 (2021)<br/>[4] H. Kawarada et al. <i>Appl. Phys. Let</i>t. <b>65</b>, 1563 (1994). H. Kawarada <i>Surf. Sci. Rep</i>.<b> 26</b>, 205 (1996).<br/>[5] N. Oi, H. Kawarada et al. <i>Scientific Reports</i>,<b> 8</b>, 10660 (2018), M. Iwataki, H. Kawarada et al., Elec. Dev. Lett. <b>41</b>, 111 (2020).<br/>[6] J. Tsunoda, H. Kawarada et al.,(submitted).<br/>[7] A. Schenk, A. Tadich, M. Sear, K. M. O’Donnell, L. Ley, A. Stacey, C. Pakes, <i>Appl. Phys. Lett. </i><b>106</b>, 191603 (2015).<br/>[8] W. Fei, H. Kawarada et al. <i>Appl. Phys. Lett.</i> <b>116</b> 212103 (2020).