Diamond metal-oxide-semiconductor field-effect transistors (FETs) have been fabricated on IIa-type large-grain diamond substrates with a (110) preferential surface. The drain current and cutoff frequency are −790mA∕mm and 45GHz, respectively, which are higher than those of single-crystal diamond FETs fabricated on (001) homoepitaxial diamond films. The hole carrier density of the hole accumulation layer depends on the orientation of the hydrogen-terminated diamond surface, for which (110) preferentially oriented films show 50%–70% lower sheet resistance than a (001) substrate. We propose that the hole density of the surface accumulation layer is proportional to the C–H bond density on the surface.

AlGaN/GaN heterostructures with a wurtzite structure were epitaxially grown on single-crystal diamond (111) with a diamond structure by metalorganic vapor phase epitaxy. In the AlGaN/GaN heterostructure, two-dimensional electron gas with sheet carrier density of 1.0×1013 cm−2 and mobility of 730 cm2/V s was obtained. The 3-μm-gate-length AlGaN/GaN high-electron mobility transistors (HEMTs) show maximum drain current of 220 mA/mm, cut-off frequency of 3 GHz, and maximum frequency of oscillation of 7 GHz. The thermal resistance of the AlGaN/GaN HEMTs on diamond substrates is 4.1 K mm/W, the lowest ever reported for AlGaN/GaN HEMTs, due to the high thermal conductivity of single-crystal diamond.

The effects of surface charge density on DNA hybridization have been investigated on a mixture of hydrogen-, oxygen-, and amine-terminated diamond surfaces. A difference in the hybridization efficiencies of complementary and mismatched DNA was clearly observed by fluorescence and potentiometric observations at a particular coverage of oxygen. In the fluorescence observation, singly mismatched DNA was detected with high contrast after appropriate hybridization on the surface with 10-20% oxygen coverage. The amount of oxygen in the form of C-O(-) (deprotonated C-OH) producing the surface negative-charge density was estimated by X-ray photoelectron spectroscopy. Electrolyte solution gate field-effect transistors (SGFETs) were used for potentiometric observations. The signal difference (change in gate potential) on the SGFET, which was as large as approximately 20 mV, was caused by the difference in the hybridization efficiencies of complementary target DNA (cDNA) and singly mismatched (1MM) target DNA with a common probe DNA immobilized on the same SGFET. The reversible change in gate potential caused by the hybridization and denaturation cycles and discriminating between the complementary and 1MM DNA targets was very stable throughout the cyclical detections. Moreover, the ratio of signals caused by hybridization of the cDNA and 1MM DNA targets with the probe DNA immobilized on the SGFET was determined to be 3:1 when hybridization had occurred (after 15 min on SGFET), as determined by real-time measurements. From the viewpoint of hybridization kinetics, the rate constant for hybridization of singly mismatched DNA was a factor of approximately 3 smaller than that of cDNA on this functionalized (oxidized and aminated) diamond surface.

Using nitrogen-dioxide (NO2) adsorption treatment and Al2O3 passivation technique, we improved drain current (I DS) of hydrogen-terminated (H-terminated) diamond field-effect transistors (FETs). The Al2O3 passivation layer also serves as a gate-insulator in a gate region. Maximum I DS (I DSmax) of -1.35 A/mm was obtained for the diamond FETs with NO2 adsorption and the Al2O3 passivation layer. This I DSmax is the highest ever reported for diamond FETs and indicates that the Al2O3 passivation layer can stabilize adsorbed NO2, which increases the hole carrier concentration on the H-terminated diamond surface. In RF small-signal characteristics, the diamond FETs with NO2 adsorption and the Al2O3 passivation layer showed high cutoff-frequency (f T) and maximum frequency of oscillation (f max) in a wide gate–source voltage (V GS) range (>10 V). This is because the Al2O3 gate insulator with a high potential barrier against hole carriers can confine and control the high concentration of hole carriers and then high forward-bias voltage can be applied without noticeable gate leakage current.

We have established an atomic-layer-deposited Al2O3 overlayer deposition method, which makes the H-surface-terminated p-type channel diamond surface thermally stable and completely keeps the concentration and mobility high even at 150 °C. In a range from 230 to 500 K, the mobility is proportional to the inverse of temperature showing a property characteristic for degenerate hole gas. The ionization energy is estimated to be 6.1 meV, indicating that holes are not generated mainly by thermal activation. This thermal stabilization technology enables us to measure hole properties up to 230 °C and to realize H-terminated diamond field-effect transistors with a reproducible high drain current.

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Through the enhancement of hole accumulated density near hydrogen-terminated (111) diamond surfaces, low sheet resistance ($5 k/sq) has been obtained compared with widely used (001) diamond surfaces ($10 k/sq). Using the hole accumulation layer channel, a high drain current density of À850 mA/mm was obtained in p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). This drain current density is the highest value for diamond FETs. The high drain current on the (111) surface is attributed to two factors: The low source and drain resistances owing to the high hole carrier density and the high channel mobility at a high gate-source voltage on the (111) surface.