Diamond and Related Materials 12 (2003) 399–402 0925-9635/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635(03)00034-7 High performance diamond MISFETs using CaF gate insulator 2 S. Miyamoto *, H. Matsudaira , H. Ishizaka , K. Nakazawa , H. Taniuchi , H. Umezawa , a,b, a,b a,b a,b a,b a,b M. Tachiki , H. Kawarada a,b a,b School of Science and Engineering, Waseda University, Tokyo, Japan a CREST, Japan Science and Technology Corporation (JST), Tokyo, Japan b Abstract A cut-off frequency of 15 GHz and a maximum frequency of oscillation of 20 GHz are realized in a 0.4-mm gate diamond metal–insulator–semiconductor field-effect transistor (MISFET). The cut-off frequency is the highest value for diamond FETs ever reported. The RF characteristics of the MISFETs are higher than those of metal–semiconductor FETs at the same gate lengths. The CaF gate insulator improves the carrier mobility according to the Hall measurement system. The mobility increases 2 in the surface conductive layer result in high RF performance. The source–gate passivation of CaF results in the high DC 2 transconductance because of the reduction of series resistances. A cut-off frequency of more than 30 GHz is expected with the gate minimization and the CaF passivation of source–gate and gate–drain spacings. 2  2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond properties application; Hydrogen-terminated; FET; MIS; CaF 2 1. Introduction Modern wireless communication systems require high-speed, high-linearity and high-power amplifiers. Such amplifiers are also attractive for new radar systems in the next generation. Therefore, high-frequency oper- ation amplifiers are sought in wide band gap semicon- ductors such as SiC, GaN and diamond. The surface conductive layer of hydrogen-terminated (H-terminated) diamond has excellent properties, such as high carrier concentration (10 cm ) and shallow carrier profiles 13 y2 (90% carriers exist -10 nm from the surface) w1–3x. Until now, microwave H-terminated diamondfield-effect transistors (FETs) fabricated in a metal–semiconductor (MES) w4x structure and a metal–insulator–semi- conductor (MIS) w5x structure using CaF have been 2 reported. Based on our previous investigations, MIS- FETs have demonstrated better RF performances than MESFETs. A cut-off frequency ( f ) of 11 GHz and a T f of 18 GHz were realized in the 0.7-mm gate-length max MISFET, although an f of 5 GHz and an f of 9 T max *Corresponding author. Kawarada Lab, Department of Electronics, Information and Communication Engineering, School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan. Tel.: q81-3-5286-3391; fax: q81-3-5286-3391. E-mail address: miyamoto@kaw.comm.waseda.ac.jp (S. Miyamoto). GHz were obtained in the 0.7-mm gate-length MESFET. Generally, minimizing the gate length enables high transconductance (g ) and high f .Becausethesource– m T gate and gate–drain spacings cannot be ignored in the series resistance of FET, the optimization of the spacings is required. When the gate length is minimized to 0.1 mm, which is comparable to the source–gate and gate– drain spacings, the series resistances of the spacings relatively increase. Accordingly, the extrinsic g is m saturated because of the relatively high resistance of the surface conductive layer. The removal of parasitic resis- tance at the source–gate and gate–drain spacings should be taken into consideration. 2. Experimental Microwave diamond FETs were fabricated on as- grown homoepitaxial diamond films. The homoepitaxial diamond films were deposited on high-pressure-synthetic type Ib diamond (001) substrates by microwave-plas- ma-assisted chemical vapor deposition. The source gas was CH diluted with H (0.1%). The as-grown homo- 4 2 epitaxial diamond surface shows p-type conduction. The sheet resistance, Hall carrier mobility and carrier density of the diamond films were 10–15 kVyh, 50 cmyVs and 1=10 ycm , respectively, at room temperature as 13 2