Hydrogen-terminated Diamond Field Effect Research Articles

Germanium gate hydrogen-terminated diamond field effect transistor with Al2O3 dielectric layer

Investigation of germanium gate hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with Al2O3 dielectric layer has been successfully performed. The device demonstrates a normally-on characteristics, whose maximum drain-source current density, threshold voltage, maximum transconductance, on/off ratio, subthreshold swing, capacitance, carrier density, saturation carrier mobility, fixed charge density and interface state density are of −37.3 mA/mm, 0.22 V, 6.42 mS/mm, 108, 134 mV/dec, 0.33 μF/cm2, 9.83 × 1012 cm−2, 97.9 cm2/V·s, 7.63 × 1012 cm−2 and 2.56 × 1012 cm−2·eV−1, respectively. This work is significant to the development of H-terminated diamond FET.

Electrical characteristics of normally off hydrogen-terminated diamond field effect transistors with lanthanum oxide gate dielectric

A normally off hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field effect transistor (MOSFET) is realized by using lanthanum oxide (La2O3) gate dielectric. The threshold voltage is demonstrated to be −0.797 V, indicating that the La2O3-gated H-diamond MOSFET has normally off characteristics. The normally off mode could be greatly ascribed to the low work function of La2O3. Based on the capacitance–voltage (C–V) curves, the dielectric constant of La2O3 is calculated to be as high as 25.6. Moreover, the small hysteresis voltage extracted from the C–V curves exhibits low trapped charge density in the La2O3 layer. The maximum drain–source current, maximum transconductance, subthreshold swing, effective mobility, current on/off ratio, and sheet hole density of La2O3-gated MOSFET with a gate length of 2 μm are calculated to be −13.55 mA/mm, 4.37 mS/mm, 161 mV/dec, 202.2 cm2/V·s, 108, and 6.53 × 1012 cm−2, respectively. This work will significantly promote the development of normally off H-diamond MOSFET devices.

Electrical properties of cerium hexaboride gate hydrogen-terminated diamond field effect transistor with normally-off characteristics

Fabrication of hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with low work function cerium hexaboride (CeB6) gate material has been successfully performed. For the 8 μm gate length device, the threshold voltage (VTH) is extracted to be −0.46 V, demonstrating a normally-off characteristics. The maximum drain source current density (IDmax) is −83.8 mA/mm, and the competitive IDmax may be ascribed to the undamaged two-dimensional hole gas channel. The interface state density (Dit) is evaluated to be 1.93 × 1012 cm−2·eV−1, and the relatively low value may be attributed to the uncontaminated interface in gate material. This work provides a promising strategy to realize normally-off H-terminated diamond FET for fail-safety and energy-efficient power electronic devices.

Normally-off Hydrogen-Terminated Diamond Field-Effect Transistor with SnOx Dielectric Layer Formed by Thermal Oxidation of Sn

SnOx films were deposited on a hydrogen-terminated diamond by thermal oxidation of Sn. The X-ray photoelectron spectroscopy result implies partial oxidation of Sn film on the diamond surface. The leakage current and capacitance–voltage properties of Al/SnOx/H-diamond metal-oxide-semiconductor diodes were investigated. The maximum leakage current density value at −8.0 V is 1.6 × 10−4 A/cm2, and the maximum capacitance value is measured to be 0.207 μF/cm2. According to the C–V results, trapped charge density and fixed charge density are determined to be 2.39 × 1012 and 4.5 × 1011 cm−2, respectively. Finally, an enhancement-mode H-diamond field effect transistor was obtained with a VTH of −0.5 V. Its IDMAX is −21.9 mA/mm when VGS is −5, VDS is −10 V. The effective mobility and transconductance are 92.5 cm2V−1 s−1 and 5.6 mS/mm, respectively. We suspect that the normally-off characteristic is caused by unoxidized Sn, whose outermost electron could deplete the hole in the channel.

Leakage current reduction of normally off hydrogen-terminated diamond field effect transistor utilizing dual-barrier Schottky gate

Normally Off diamond field-effect transistor (FET) is demanded for energy saving and safety for practical application. Metal/diamond Schottky junction serving as the gate is a simple and effective approach to deplete holes under the gate, whereas low Schottky barrier height (SBH) is undesirable. In this work, a dual-barrier Schottky gate hydrogen,oxygen-terminated diamond (H,O-diamond) FET (DBG-FET) with Al gate was realized. Normally Off DBG-FET with enhanced SBH and reduced leakage was achieved. H,O-diamond, which was defined by x-ray photoelectron spectroscopy (XPS) technique, was realized by ultraviolet ozone (UV/O3) treatment with nanoparticle-Al mask. The enlarged SBH of 0.94 eV owing to the C–O bond minimized the diode reverse current and nicely shut down the DBG-FET at zero gate bias. Moreover, the forward current of diode can be well-reduced by hundred times ascribed to oxidized Al nanoparticles during the UV/O3 process. Based on this diode gate structure, the maximum drain current density, transconductance, on/off ratio, and subthreshold swing of the normally off DBG-FET are 21.8 mA/mm, 9.1 mS/mm, 109, and 96 mV/dec, respectively. The DBG-FET is expected to promote the development of normally off diamond FETs.

Transport Properties of the Two-Dimensional Hole Gas for H-Terminated Diamond with an Al2O3 Passivation Layer

Diamonds are thought to be excellent candidates of next-generation semiconductor materials for high power and high frequency devices. A two-dimensional hole gas in a hydrogen-terminated diamond shows promising properties for microwave power devices. However, high sheet resistance and low carrier mobility are still limiting factors for the performance improvement of hydrogen-terminated diamond field effect transistors. In this work, the carrier scattering mechanisms of a two-dimensional hole gas in a hydrogen-terminated diamond are studied. Surface roughness scattering and ionic impurity scattering are found to be the dominant scattering sources. Impurity scattering enhancement was found for the samples after a high-temperature Al2O3 deposition process. This work gives some insight into the carrier transport of hydrogen-terminated diamonds and should be helpful for the development of diamond field effect transistors.

Normally-off hydrogen-terminated diamond field effect transistor with a bilayer dielectric of Er2O3/Al2O3

A normally-off hydrogen-terminated diamond field effect transistor with an Er2O3/Al2O3 bilayer dielectric was fabricated. Dielectric materials Er2O3 and Al2O3 were deposited on H-diamond surface using radio frequency sputtering and atomic layer deposition, respectively. The threshold voltage of the device was −0.49 V at VDS of −20 V, which indicated a normally off characteristic, that could be attributed to the low work function of Er2O3 and the polar character of amorphous Er2O3. The maximum drain-source current for the hydrogen-terminated diamond field effect transistor with 6 μm gate length was −16.1 mA/mm and the sheet carrier density integrated was 1.08 × 1013 cm−2. The maximum effective hole mobility was 110.47 cm2/V·s and the interfacial trapped charge density was 3.16 × 1012 eV−1 cm−2. This bilayer dielectric of Er2O3/Al2O3 provides an alternative plan for the fabrication of normally-off diamond device.

Normally-off hydrogen-terminated diamond field effect transistor with yttrium gate

The normally-off Yttrium (Y) gate hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with 5 nm Al2O3 dielectric layer has been successfully fabricated and evaluated in this work. The threshold voltage is extracted to be - 0.21 V at drain source voltage of −10 V, and the normally-off mode should be caused by the low work function of Y. The saturated drain current (IDSsat) is - 16.14 mA/mm obtained at gate voltage (VGS) of - 2.5 V with 4 μm gate length and 100 μm gate width. The value of IDSsatIDSmax is relatively high compared with other normally-off H-terminated diamond FETs at the same gate voltage, and this is ascribed to the well-preserved two-dimensional hole gas channel. The maximum transconductance of the fabricated device is 7.97 mS/mm. The current on/off ratio is about 7 orders of magnitude with a subthreshold swing of 150 mV/dec. The maximum effective mobility is 233.08 cm2/V. The results contribute to the development of normally-off H-terminated diamond FETs significantly.

Characteristics of hydrogen-terminated single crystalline diamond field effect transistors with different surface orientations

Diamond has great potential applications in high-power, high-frequency semiconductor devices because of its wide band gap (5.5 eV), high thermal conductivity (22W/(cm·K)), and high carrier mobility (4500 cm<sup>2</sup>/(V·s) for electron, and 3800 cm<sup>2</sup>/(V·s) for hole). It has been widely considered as an ultimate semiconductor. From the analysis of our previous work, we find that the output current of field effect transistor based on hydrogen-terminated polycrystalline diamond is usually larger than that based on single crystal diamond, and that the preferential orientations of the polycrystalline diamond are mainly <inline-formula><tex-math id="Z-20200108113531-1">\begin{document}$ \langle 110\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20191013_Z-20200108113531-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20191013_Z-20200108113531-1.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20200108113531-2">\begin{document}$ \langle 111\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20191013_Z-20200108113531-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20191013_Z-20200108113531-2.png"/></alternatives></inline-formula> shown by XRD results. Therefore, in order to further analyze the effect of surface orientation on the device performance of hydrogen-terminated diamond field effect transistor (FET), we study the devices fabricated respectively on the (110) plane and (111) plane single crystal diamond plates obtained from a single 3.5-mm-thick single crystal diamond grown by the microwave plasma chemical vapor deposition on the high-pressure high-temperature synthesized diamond substrate. Prior to processing the device, these diamond plates are characterized by atomic force microscope, Raman spectra and photoluminescence (PL) spectra. The results of Raman and PL spectra show that (110) plane and (111) plane plates originating from the same CVD single crystal diamond have no significant difference in optical property. Then the normally-on hydrogen-terminated diamond FET with a gate length of 6 μm is achieved. The device on (111) plane delivers a saturation drain current of 80.41 mA/mm at a gate voltage <i>V</i><sub>GS</sub> = –4 V, which is approximately 1.4 times that of the device on (110) plane. Meanwhile, the on-resistance of the device on (111) plane is 48.51 Ω·mm, and it is only 67% of the device on (110) plane. Analyses of the capacitance-voltage show that the hole concentration of the gated device on (110) plane and (111) plane are 1.34 × 10<sup>13</sup> cm<sup>–2</sup> and 1.45 × 10<sup>13</sup> cm<sup>–2</sup>, respectively, approximately at the same level. In addition, the hole density of the device on both (110) and (111) plane increase near-linearly with the increase of gate voltage from the threshold voltage to – 4 V, indicating that the control effect of the gate on the carrier in the channel is uniform. The possible reason for the higher saturation drain current as well as the lower on-resistance of the device on (111) plane is that its sheet resistance is lower.

Characteristics of H-terminated single crystalline diamond field effect transistors

Diamond has been considered as an ultimate semiconductor, which has great potential applications in high power, high frequency semiconductor devices. Up to now, the twodimensional hole gas (2DHG) induced on the hydrogenterminated diamond surface is used most popularly to form electric conduction in diamond semiconductor at room temperature, due to the obstacle caused by lacking of easily-ionized dopants. A 200-nm-thick single crystalline diamond is grown by microwave plasma chemical vapor deposition on the type-Ib high-pressure high-temperature synthesized diamond substrate. Then the sample is treated in hydrogen plasma atmosphere to achieve hydrogen terminated diamond surface. The sample is characterized by X-ray photoelectron spectroscopy and atomic force microscope. After that, the normally-on hydrogen-terminated diamond field effect transistors are achieved. The device with a gate length of 2 μup m delivers a saturation leakage current of 96 mA/mm at gate voltage VGS=-6 V, at which, however, the gate leakage current is too large. The saturation current reaches 77 mA/mm at VGS=-3.5 V with safety. The device shows typical long-channel behavior. The gate voltage varies almost linearly. In the saturation region of the device, the transconductance (gm) increases near-linearly to 30 mS/mm with the increase of the gate voltage in a range of 5.9 V. Analyses of the on-resistance and capacitance-voltage (C-V) data show that the 2DHG under the gate achieves a density as high as 1.99×1013 cm-2, and the extracted channel carrier density and mobility are always kept increasing with VGS negatively shifting to -2.5 V. The nearlinearly increasing of gm in a large VGS range is attributed to high 2DHG density, quite a large gate capacitance (good gate control), and increased mobility. The relevant researches of improving the carrier mobility in the channel and of finding proper gate dielectrics to improve the forward gate breakdown voltage are underway.

RF Operation of Hydrogen-Terminated Diamond Field Effect Transistors: A Comparative Study

Three sets of different gate-length field-effect transistors (250, 120, and 50 nm) have been defined on homoepitaxial hydrogen-terminated diamond with the 50-nm device being the smallest gate length diamond transistor fabricated to date. DC- and small-signal RF measurements were undertaken to compare the operation of these gate nodes. RF small-signal equivalent circuits were generated to contrast individual components and better understand the operation at various gate dimensions. Scaling the gate length to smaller dimensions leads to an increase in the cutoff frequency of these devices although parasitic elements are found to dominate at the shortest gate length of 50 nm, limiting the outstanding potential of these devices.

Hydrogen-Terminated Diamond Field-Effect Transistors With Cutoff Frequency of 53 GHz

Homoepitaxial diamond has been used to demonstrate the RF performance of 50-nm gate length hydrogen-terminated diamond field-effect transistors. An extrinsic cutoff frequency of 53 GHz is achieved which we believe to be the highest value reported for a diamond-based transistor. The generation of an RF small signal equivalent circuit is used to extract device elements to better understand variation between intrinsic and extrinsic operation. An intrinsic cutoff frequency of 90 GHz is extracted through this process, verifying the requirement to minimize access resistance to maximize the potential high-frequency performance of this technology.

50nm Gate-Length Hydrogen Terminated Diamond Field Effect Transistors – Characterization and Inspection of Operation.

ABSTRACTHydrogen terminated diamond field effect transistors (FET) of 50nm gate length have been fabricated, their DC operation characterised and their physical and chemical structure inspected by Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS). DC characterisation of devices demonstrated pinch-off of the source-drain current can be maintained by the 50nm gate under low bias conditions. At larger bias, off-state output conductance increases, demonstrating most likely the onset of short-channel effects at this reduced gate length.

Processing of 50 nm gate-length hydrogen terminated diamond FETs for high frequency and high power applications

To better understand the intrinsic scaling limitations of sub-100nm gate-length hydrogen-terminated diamond Field Effect Transistors (FETs), the fabrication and DC characterisation of 50nm gate length devices have been investigated. Physical processing of the hydrogenated diamond surface presents a challenge for the realisation of short gate length, high performance devices due to the inherent sensitivity of the surface to device processing. Through the development of a finely optimised device fabrication process, we have demonstrated transistor operation at a reduced gate length of 50nm. This is believed to be the shortest gate length operational diamond transistor yet reported. Inspection of device operation at larger voltages indicates the onset of potential short channel effects, attributed to the reduced dimensions of the gate. This is an important result in the investigation of the high frequency and high power potential of hydrogen terminated diamond transistors.