Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L 1222–L 1224 Part 2, No. 11A, 1 November 1999 c 1999 Publication Board, Japanese Journal of Applied Physics High-Performance Diamond Metal-Semiconductor Field-Effect Transistor with 1 μm Gate Length Hitoshi UMEZAWA 1,3 , Kazuo TSUGAWA 1,3 , Sadanori YAMANAKA 2,3 , Daisuke TAKEUCHI 2,3 , Hideyo OKUSHI 2,3 and Hiroshi KAWARADA 1,3 1 School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjyuku, Tokyo 169-0072, Japan 2 ETL (Electrotechnical Laboratory), 1-1-4 Umezono, Tukuba, Ibaraki 305-8568, Japan 3 CREST, JST (Japan Science and Technology Corporation), 5-3 Yonban, Chiyoda, Tokyo 102-0081, Japan (Received May 17, 1999; accepted for publication August 16, 1999) High-performance metal-semiconductor field-effect transistors (MESFETs) using the p-type surface conductive layer on homoepitaxial diamond are demonstrated. The maximum transconductance is 110 mS/mm, which is the highest value ever reported in diamond FETs. This value exceeds the normal transconductance of a Si–metal-oxide semiconductor field-effect transistors (MOSFET) with equivalent gate length. The transconductance of the present diamond FETs is proportional to the reciprocal of gate length. Accordingly, the characteristics can be improved by the refinement of gate length. By using an appropriate FET fabrication process, it is expected that the transconductance of a diamond MESFET exceeds 500 mS/mm at gate lengths less than 0.2 μm. KEYWORDS: diamond, hydrogen termination, MESFET, transconductance L 1222 1. Introduction Semiconducting diamond exhibits superior properties for electron devices, such as wide gap (5.5 eV), maxi- mum thermal conductivity in materials (20 W·cm -1 ·K -1 ), low dielectric constant (5.7), and high carrier mobility (1800 cm 2 ·V -1 ·s -1 (hole)). p-Type and n-type semiconductor diamond have been realized by boron and phosphorus doping, respectively. Since n-type diamond displays semi-insulating property at room temperature, due to the deep donor level (0.43 eV), 1) research on diamond electron devices has con- centrated on p-type unipolar devices, such as the field-effect transistor (FET). 2, 3) However, a FET using a boron-doped active layer operates poorly at room temperature, due to its deep acceptor level of 0.37 eV. In addition, the high contact resistance and the high surface state density of the oxygen- terminated surface, which is used conventionally in the case of boron-doped diamond, limit the FET performance. A thin p-type conduction layer exists on the hydrogen- terminated diamond surface prepared by chemical vapor de- position (CVD) without doping impurities. 4) This conduction layer possesses suitable properties for device fabrication, such as low acceptor level (less than 50 meV), high carrier con- centration (10 13 cm -2 ), 5) and low density of surface states be- cause of hydrogen termination of surface dangling bonds. In the device simulation for metal-semiconductor (MES) FETs on a hydrogen-terminated diamond surface 6) the thickness of the surface conduction layer has been estimated to be less than 10 nm. This surface conduction layer has been applied for the source, the drain and the channel of MESFETs and metal- insulator-semiconductor (MIS) FETs. 7–9) Using the p–type surface conduction layer, the highest transconductance in di- amond FETs has been reported in an Al-gate MESFET with a gate length of 3 μm. 10) Although the gate length and the space between source and drain of the FETs are long. The series re- sistance between the source and the gate limits the effective transconductance. In order to reduce the parasitic series resis- tance, the source-gate spacing is required to be minimized by a reduction of the gate length. The sheet resistance of the surface conduction layer is around 10 k˜/sq. The resistance is basically determined by the surface hydrogen-termination, and hardly lower by dop- ing. On the other hand, the contact resistance of the ohmic contact between Au and the hydrogen-terminated surface is less than ∼10 -6 ˜cm 2 . 11) Accordingly, the source-gate and the gate-drain spacings almost determine the parasitic series resistance. Consequently, the device performance is improved by reduction of the electrode spacings. From the device sim- ulation based on the drift-diffusion model, 6) the transcon- ductance exceeds 100 mS/mm in a 1 μm gate MESFET with 0.1 μm electrode spacings. In the present study, we fabri- cated diamond FETs with 1 μm gate length and submicron electrode spacings using the recess-type self-alignment gate fabrication process. 2. Experimental MESFETs were fabricated on homoepitaxial diamond films. The diamond homoepitaxial films are deposited on high-pressure synthetic Ib (001) substrates by microwave plasma CVD. The source gas is methane diluted with hy- drogen (CH 4 /(H 2 + CH 4 ) = 0.025%). The deposition time is 48 h. The thickness of the homoepitaxial layer is 0.3 μm. The sheet resistance of this substrate is 7.5–10 k˜/sq. MESFETs were fabricated on these substrates using the re- cess self-aligned gate fabrication process. The sequence of the entire process is described elsewhere. 8) In order to reduce the source-gate distance, the previous re- cess self-aligned gate fabrication process 8) is improved in this study. The problem in the previous process is the inaccuracy in etching of Au by KI solution. In that process, due to the incomplete adhesion of the electron beam resist used as the etching mask and the substrate, KI solution penetrates the in- terface between the Au electrode and the etching mask. As a result, overetching occurs. In addition, the edge of the Au electrode is not sharply etched. In order to improve the ad- hesion between the Au electrode and the resist, we baked the electron beam resist for 45–60 min. As a result, the overetch- ing is also minimized. Consequently, this improved process realizes finer patterning than the previous process.