Hydrogen-terminated Diamond Field-effect Transistors Research Articles

Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator

Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator

Performance of normally off hydrogen-terminated diamond field-effect transistor with Al2O3/CeB6 gate materials

In this work, we demonstrate a hydrogen-terminated diamond (H-diamond) field-effect transistor (FET) with Al2O3/CeB6 gate materials. The CeB6 and Al2O3 films have been deposited by electron beam evaporation technique, sequentially. For the 4/8/12/15 μm gate length (LG) devices, the whole devices demonstrate distinct p-type normally off characteristics, and all the threshold voltage are negative; all the absolute values of leakage current density are 10−4 A/cm2 at a VGS of −11 V, exhibiting a relatively low leakage current density compared with CeB6 FETs, and this further demonstrates the feasibility of the introduction of Al2O3 to reduce the leakage current density; the maximum drain–source current density is −114.6, −96.0, −80.9, and −73.7 mA/mm, which may be benefited from the well-protected channel. For the 12 μm LG devices, the saturation carrier mobility is 593.6 cm2/V s, demonstrating a good channel transport characteristic. This work may provide a promising strategy for the application of normally off H-diamond FETs significantly.

Mobility-enhanced normally off hydrogen-terminated diamond FET with low interface state density using Al2O3/Nd gate stack

A mobility-enhanced normally off hydrogen-terminated diamond (H-diamond) field-effect transistor (FET) with low interface state density has been realized using Al2O3/Nd gate stack deposited by electron beam evaporator. The threshold voltage is extracted to be −1.48 V, indicating an obvious normally off characteristic. The enhancement mode could be ascribed to the large work function difference between Nd and H-diamond. The subthreshold swing is deduced as low as 86.3 mV/dec, revealing a high working speed in the subthreshold region. More importantly, the mobility (518.5 cm2/V·s) has been enhanced due to the fairly low interface state density (3.19 × 1011 cm−2·eV−1). In addition, the device exhibits a relatively low trapped charge density and fixed charge density from the C–V characteristic. This work provides a simple method to realize the normally off device and suggests the great potential of adopting the Al2O3/Nd gate stack for achieving high-performance H-diamond FET.

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.

Normally-off C-H Diamond FETs With Partial Al/C-O Diamond Junction Attaining Low off-State Current

Normally- OFF hydrogen-terminated diamond (C-H diamond) field effect transistor (FET) is desirable for safe and energy-saving power switching applications. Whereas, the OFF-state current driven by the power-supply voltage at zero gate bias is relatively high to date, which would generate large static power consumption. In this work, the partial Al/oxygen-terminated diamond (C-O diamond) Schottky gate was utilized to realize normally- OFF operation with a noise-level OFF-state current density (~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{9}}$ </tex-math></inline-formula> mA/mm). The partial oxygen-terminated diamond was realized by ultraviolent ozone process with Al2O3-nanoparticle mask. Compared with Al/C-H diamond, the Al/C-O diamond junction possesses a higher Schottky barrier height (SBH) against hole flow, such that the OFF-state current of diamond FETs can be well-suppressed. Also, the enhanced SBH contributes to the negatively shifted threshold voltage, which may avoid device failure from gate overdrive. Moreover, a competitive current density of −91 mA/mm was achieved. Besides, the threshold voltage, ON/OFF ratio, and subthreshold swing were −1.3 V, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${9} \times {10} ^{{9}}$ </tex-math></inline-formula> , and 110 mV/dec, respectively. This approach will promote the application of diamond FETs in power switching system significantly.

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.

Solution-processed tin oxide thin film for normally-off hydrogen terminated diamond field effect transistor

The solution processed method has been wildly used in the thin film fabrication because of the advantages of low cost, high efficiency, large scale for production, and long-term stability. In this paper, a normally-off hydrogen-terminated diamond field-effect transistor (FET) has been realized by using a solution processed SnO2 (sp-SnO2) film as an insulator layer. X-ray photoelectron spectroscopy results demonstrated the stoichiometry of the sp-SnO2 film, which shows good insulator properties with leakage current density less than 2.1 × 10−5 A·cm−2 at gate voltages from −6.0 to 6.0 V. The drain–source current maximum, threshold voltage, extrinsic transconductance maximum, and effective mobility of the FET with a gate length of 10 μm are −17.6 mA·mm−1, −0.5 V, 5.7 mS·mm−1, and 41.3 cm2/V s, respectively. According to the capacitance voltage characteristic, the enhancement mode could be ascribed to the high positive fixed charge density in the sp-SnO2 film, which will repel the hole in the channel. This paper provides a simple method and a low temperature process to fabricate an insulator layer.

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.

HfAlOx/Al2O3 Bilayer Dielectrics for a Field Effect Transistor on a Hydrogen-Terminated Diamond.

In this work, a hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with HfAlOx/Al2O3 bilayer dielectrics is fabricated and characterized. The HfAlOx/Al2O3 bilayer dielectrics are deposited by the atomic layer deposition (ALD) technique, which can protect the H-terminated diamond two-dimensional hole gas (2DHG) channel. The device demonstrates normally-on characteristics, whose threshold voltage (VTH) is 8.3 V. The maximum drain source current density (IDSmax), transconductance (Gm), capacitance (COX) and carrier density (ρ) are −6.3 mA/mm, 0.73 mS/mm, 0.22 μF/cm2 and 1.53 × 1013 cm−2, respectively.

Electrical properties of yttrium gate hydrogen-terminated diamond field effect transistor with Al2O3 dielectric layer

This is the attempt to apply yttrium (Y) gate material to hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with a 10 nm Al2O3 dielectric layer. The maximum drain source current of Y/Al2O3/H-terminated diamond FET with a gate length of 8 μm is −53.9 mA/mm obtained at a gate voltage of −7 V and a drain source voltage of −20 V, and its current on–off ratio exceeds 109, which is large enough for practical applications. Based on the relationship between gate voltage and drain source current, the subthreshold swing is extracted to be 198 mV/dec. The electrical performance reveals that Y/Al2O3/H-terminated diamond FET would pave the way for the development of H-terminated diamond FET.

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.

Performance of hydrogenated diamond field-effect transistors on single and polycrystalline diamond

In this work, we investigate the influence of defect concentration of the diamond substrates on the performance of hydrogen-terminated diamond field-effect transistors by Raman spectra, pulsed I–V characteristics analysis, and radio frequency performances measurements. It is found that a sample with higher defect concentration shows larger drain-lag effect and lower large-signal output power density. Defects in the diamond act as traps in the carrier transport and have a considerable influence on the large-signal output power density of diamond field-effect transistors. This work should be helpful for further performance improvement of the microwave power diamond devices.

Normally Off Hydrogen-Terminated Diamond Field-Effect Transistor With Ti/TiOx Gate Materials

A normally OFF hydrogen-terminated diamond (H-terminated diamond) field-effect transistor (FET) has been successfully achieved with Ti/TiO_<i>x</i> gate materials. A 5-nm Ti film was deposited on H-terminated diamond by electron beam evaporation technique and then it was thermally oxidized in air at 120 &#x00B0;C for 10 h to form Ti/TiO_<i>x</i>, which was confirmed by X-ray photoelectron spectroscopy. The threshold voltage of H-terminated diamond FET is &#x2212;0.14 V at <inline-formula> <tex-math notation="LaTeX">${V}_{DS}$ </tex-math></inline-formula> of &#x2212;8 V, indicating a normally OFF operation. The normally OFF property could be attributed to the difference of work difference between Ti and H-terminated diamond, which may deplete the hole carriers in H-terminated diamond 2-D hole gas (2DHG) conduction channel. The maximum mobility of H-terminated diamond FET is 313 cm²/Vs at <inline-formula> <tex-math notation="LaTeX">${V}_{GS}$ </tex-math></inline-formula> of &#x2212;0.2 V. And, the fixed negative charge density is <inline-formula> <tex-math notation="LaTeX">$3.37 \times 10 _{11}$ </tex-math></inline-formula> cm^&#x2212;2. The results demonstrate that Ti/TiO_<i>x</i> H-terminated diamond FET is a good solution to fabricate normally OFF device with a simple fabrication process, undamaged 2DHG channel, and uncontaminated interface between Ti and TiO_<i>x</i> gate materials, which may promote the development of normally OFF H-terminated diamond FET.

Diamond Field-Effect Transistors With V2O5-Induced Transfer Doping: Scaling to 50-nm Gate Length

Wereport on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5.

Charge-carrier mobility in hydrogen-terminated diamond field-effect transistors

Diamond field-effect transistors (FETs) have potential applications in power electronics and high-output high-frequency amplifications. In such applications, high charge-carrier mobility is desirable for a reduced loss and high-speed operation. We recently fabricated diamond FETs with a hexagonal-boron-nitride gate dielectric and observed a high mobility above 300cm2V−1s−1. In this study, we identify the scattering mechanism that limits the mobility of our FETs through theoretical calculations. Our calculations reveal that dominant carrier scattering is caused by surface charged impurities with a density of ≈1×1012cm−2 and suggest that an increase in mobility over 1000cm2V−1s−1 is possible by reducing these impurities.

An Enhancement-Mode Hydrogen-Terminated Diamond Field-Effect Transistor With Lanthanum Hexaboride Gate Material

An enhancement-mode hydrogen-terminated diamond field-effect transistor (FET) is realized by using a low work function gate material, namely, lanthanum hexaboride (LaB6). The reason for the enhancement mode should be that the electrons in the LaB6 layer flow into the two-dimensional hole gas (2DHG) channel and compensate the holes, such that the channel is shut down. The threshold voltages ( ${\mathrm {V}}_{\text {TH}}$ ) range from − 0.29 V to − 0.72 V with different gate lengths. The device with 2 $\mu \text{m}$ gate length shows a − 57.9 mA/mm maximum drain current density ( ${\mathrm {I}}_{\text {DSmax}}$ ) at ${\mathrm {V}}_{\text {GS}} =-$ 5 V. The on/off ratio is around 9 orders of magnitude, with a subthreshold swing of 130 mV. Effective mobility ( $\mu _{\text {eff}}$ ) as high as 195.4 cm2/ $\text{V}\cdot \text{s}$ is obtained from the device. This technique reveals undamaged 2 DHG characteristics, uncontaminated interface between LaB6 and aluminum gate metal, and a simple fabrication process, which will promote the development of enhancement diamond FETs.