Diamond Field Effect Transistors Research Articles

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.

Phonon-Dominated Mobilities for Carriers in a Diamond Field Effect Transistor With a cBN Overlayer

In this letter we investigate phonon-dominated mobilities for carriers in a diamond field effect transistor with a cubic Boron Nitride (cBN) overlayer. We investigate the intra-subband scattering due to interaction of electrons with acoustic phonons, treated properly as quantized surface acoustic Rayleigh waves, and include, for the first time, the interaction with remote polar phonons originating in the cBN overlayer. We conclude that the surface acoustic phonon scattering is the dominant mechanism limiting the mobility of electrons for temperatures below 375 K.

Over 1 A/mm drain current density and 3.6 W/mm output power density in 2DHG diamond MOSFETs with highly doped regrown source/drain

This paper reports the direct current and radio frequency characteristics of two-dimensional hole gas (2DHG) diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with microwave-plasma-chemical-vapor-deposition regrown P++-diamond (B ≈ 1 × 1022 cm−3) layers as their source and drain regions. To evaluate the ohmic contact resistance, two patterns of transmission line method were fabricated—one with P++-diamond/2DHG diamond interfaces and the other with only the P++-diamond layer. Three resistance components (metal/P++-diamond contact resistance (0.39 Ωmm), P++-diamond access resistance, and P++-diamond/2DHG contact resistance (∼0.65 Ω mm) were evaluated for the first time. The introduction of the P++-diamond ohmic contacts generated a drain current density of 1 A/mm and an output power density of 3.6 W/mm at a quiescent drain voltage (VDS, Q) of −50 V at 1 GHz; an output power of 1.7 W was achieved at a VDS, Q of −40 V at 1 GHz with a 1 mm gate width. These values were obtained in 2DHG diamond MOSFETs with a regrown P++-diamond layer with 1 μ m gate length. These results indicate that regrown P++-diamond has the potential to improve the drain current density and output power density of 2DHG diamond field-effect transistors.

Thermal performance of diamond field-effect transistors

In this report, the thermal performance of a hydrogen (H)-terminated diamond field-effect transistor (FET) is investigated using Raman spectroscopy and electrothermal device modeling. First, the thermal conductivity (κdiamond) of the active diamond channel was determined by measuring the temperature rise of transmission line measurement structures under various heat flux conditions using nanoparticle-assisted Raman thermometry. Using this approach, κdiamond was estimated to be 1860 W/m K with a 95% confidence interval ranging from 1610 to 2120 W/m K. In conjunction with measured electrical output characteristics, this κ was used as an input parameter for an electrothermal device model of an H-terminated diamond FET. The simulated thermal response showed good agreement with surface temperature measurements acquired using nanoparticle-assisted Raman thermometry. These diamond-based structures were highly efficient at dissipating heat from the active device channel with measured device thermal resistances as low as ∼1 mm K/W. Using the calibrated electrothermal device model, the diamond FET was able to operate at a very high power density of 40 W/mm with a simulated temperature rise of ∼33 K. Finally, the thermal resistance of these lateral diamond FETs was compared to lateral transistor structures based on other ultrawide bandgap materials (Al0.70Ga0.30N, β-Ga2O3) and wide bandgap GaN for benchmarking. These results indicate that the thermal resistance of diamond-based lateral transistors can be up to ∼10× lower than GaN-based devices and ∼50× lower than other UWBG devices.

A monolithic, back-gated diamond field-effect transistor for tunable color centers

We present an innovative design for a monolithic field effect transistor, where all components consist of the wide-bandgap material diamond. The back gate-electrode is realized by a buried, degenerately boron-doped diamond (resistivity < 10−2 Ω cm), while the dielectric material is made of lightly nitrogen-doped diamond. The 2DHG on the hydrogen-terminated surface serves as the conductive channel of the transistor. We discuss the band structure of this device, the function of each individual component and show the sample preparation routine. Furthermore, we investigate the electrical tunability of the 2DHG and the optical tunability of NV-centers in a first proof-of-principle sample. Additionally, we use the field effect to manipulate the charge state of color centers in the nitrogen-doped film. This vertical and monolithic device structure opens up a range of applications, not only in the diamond semiconductor and quantum information technology, but also for sensing applications where the back-gating is advantageous or where an all-diamond layer sequence is beneficial.

High Output Power Density of 2DHG Diamond MOSFETs With Thick ALD-Al2O3

This article reports on the high operation voltage large-signal performance of two-dimensional hole gas diamond metal-oxide semiconductor field-effect transistors (MOSFETs) with thick atomic-layer-deposition (ALD)-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> formed on high purity polycrystalline diamond with a (110) preferential orientation. MOSFETs with a 1- μm gate-length having a gate oxide layer of 200-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , formed by ALD and asymmetric structures, to withstand high-voltage operations. The large-signal performances were evaluated at a quiescent drain voltage of greater than -60 V for the first time in diamond field-effect transistor (FET). As a result, an output power density of 2.5 W/mm under class-A operation at 1 GHz, which is higher than that of diamond FETs fabricated by a self-aligned gate process, was obtained. Moreover, an output power density of 1.5 W/mm was exhibited by the MOSFET when biased at a quiescent drain voltage of -40 V under class-AB operation at 3.6 GHz using an active load-pull system. This is the highest recorded value for diamond FETs at a frequency greater than 2 GHz, owing to the high-voltage operation. These results indicate that diamond p-FETs under high-voltage operations are the most suitable for high-power amplifiers with complementary circuits.

Microwave power performance analysis of hydrogen terminated diamond MOSFET

The hydrogen terminated diamond (H-diamond) field effect transistors (FETs) have been the mainstream structure of diamond microwave power devices, but to improve their performance is still very challenging. A detailed analysis of the microwave power performance at 1GHz of a 0.5-μm H-diamond FET with 25-nm-thick Al2O3 gate insulator is carried out by numerical simulation of the large signal transients, and a GaN high electron mobility transistors (HEMT) in similar structure is used as a reference. Compared with the output power (Pout) of 10.76 W/mm demonstrated at a quiescent drain bias (VdQ = 48 V) by the GaN HEMT, the H-diamond FET achieved about one third of Pout (3.69 W/mm) at VdQ = -57 V based on the transconductance with much lower peak intensity (less than one fifth) but better linearity. According to the location of the dynamic load line on the direct current output characteristics, the available drain current swing has been fully utilized in the H-diamond FET. Further Pout improvement need higher bias point and larger dynamic swing of the drain voltage.

Fabrication of diamond modulation-doped FETs by NO2 delta doping in an Al2O3 gate layer

We demonstrated modulation doping in diamond and fabricated diamond field-effect transistors (FETs) by NO2 p-type delta doping in an Al2O3 gate layer. We confirmed modulation doping effects: a spatial separation between the NO2 acceptors in the Al2O3 gate insulator and the hole channel on the diamond surface increased the hole mobility, and the high hole sheet concentration was maintained till high temperatures. The diamond FETs showed maximum drain current density of −627 mA mm−1, and transconductance of 131 mS mm−1. The mobility increased to 2465 cm2 V−1 · s−1 near the threshold voltage, and the Baliga’s figure-of-merit was 179 MW cm−2.

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.

High performance ionic-liquid-gated air doped diamond field-effect transistors

We report successful fabrication of high performance ion-gated field-effect transistors (FETs) on hydrogenated diamond surface. Investigations on the hydrogen (H)-terminated diamond by Hall effect measurements shows Hall mobility as high as ∼200 cm2 V−1 s−1. In addition we demonstrate a rapid fabrication scheme for achieving stable high performance devices useful for determining optimal growth and fabrication conditions. We achieved H-termination using hydrogen plasma treatment with a sheet resistivity as low as ∼1.3 kΩ/sq. Conductivity through the FET channel is studied as a function of bias voltage on the liquid ion-gated electrode from −3.0 to 1.5 V. Stability of the H-terminated diamond surface was studied by varying the substrate temperature up to 350 °C. It was demonstrated that the sheet resistance and carrier densities remain stable over 3 weeks in ambient air atmosphere even at substrate temperatures up to 350 °C, whereas increasing temperature beyond this limit has effected hydrogenation. This study opens new avenues for carrying out fundamental research on diamond FET devices with ease of fabrication and high throughput.

Drain Current Density Over 1.1 A/mm in 2D Hole Gas Diamond MOSFETs With Regrown p++-Diamond Ohmic Contacts

We report two-dimensional hole gas (2DHG) diamond field-effect transistors (FETs) with microwave plasma chemical vapor deposition (MPCVD)-regrown p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> diamond (B concentration ~ 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) ohmic contacts. The heavily doped p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -diamond shows low ohmic contact resistance of 1.1 Q·mm, which is the lowest value reported in diamond to date. In addition, the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -diamond with a TiC also offers much stronger metal adhesion when compared with previous Au/hydrogen-terminated diamond surfaces and is suitable for industrial use. Benefiting from the low contact resistance of the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -diamond layer, a maximum drain current density of 1170 mA/mm and an ON-resistance of 8.9 Q·mm were demonstrated in a 2DHG diamond metaloxide-semiconductor FET with a 1 μm gate length. These results indicate that the regrown p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> -diamond ohmic contacts will make it possible to realize further improvements in the maximum drain current density of 2DHG diamond FETs.

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.

Structural and electronic properties of 2D (graphene, hBN)/H-terminated diamond (100) heterostructures

We report a first-principles study of the structural and electronic properties of two-dimensional (2D) layer/hydrogen-terminated diamond (100) heterostructures. Both the 2D layers exhibit weak van-der-Waals (vdW) interactions and develop rippled configurations with the H-diamond (100) substrate to compensate for the induced strain. The adhesion energy of the hexagonal boron nitride (hBN) layer is slightly higher, and it exhibits a higher degree of rippling compared to the graphene layer. A charge transfer analysis reveals a small amount of charge transfer from the H-diamond (100) surface to the 2D layers, and most of the transferred charge was found to be confined within the vdW gap. In the graphene/H-diamond (100) heterostructure, the semi-metallic characteristic of the graphene layer is preserved. On the other hand, the hBN/H-diamond (100) heterostructure shows semiconducting characteristics with an indirect bandgap of 3.55 eV, where the hBN layer forms a Type-II band alignment with the H-diamond (100) surface. The resultant conduction band offset and valence band offset are 0.10 eV and 1.38 eV, respectively. A thin layer of hBN offers a defect-free interface with the H-diamond (100) surface and provides a layer-dependent tunability of electronic properties and band alignment for surface-doped diamond field effect transistors.

Microwave diamond devices technology: Field‐effect transistors and modeling

AbstractThis paper provides an overview of the developments in microwave diamond field‐effect transistor (D‐FET) technologies. Due to the ultrawide‐bandgap and high carrier velocity and thermal conductivity of diamond, it is a potential candidate for microwave power devices with high output power and operating frequency. Here, the properties of semiconductor materials for microwave applications are described. Then, the mechanisms of various diamond FETs are detailed. In recent years, hydrogen‐terminated diamond metal‐oxide‐semiconductor field‐effect transistors (HD MOSFETs) have been widely studied for their potential use in microwave power devices. Therefore, the structures and developments of HD MOSFETs are mainly discussed. Finally, in the prospective application of the HD FET microwave circuit, the state‐of‐the‐art large‐signal modeling of HD MOSFETs is presented.

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.

2D hole gas mobility at diamond/insulator interface

The hole mobility of two-dimensional (2D) gas at (001) and (111) diamond/insulator interfaces is investigated theoretically and compared with experimental data from the literature. It is shown that the surface impurity scattering is the limiting mechanism at room temperature in most of the H-terminated diamond field effect transistors, where the negative charges created by transfer doping are in the vicinity of the 2D gas. By repelling the negative charges at the metal/insulator interface, as recently reported for the (111) h-BN/diamond interface, we demonstrate that it is possible to achieve high mobility values of the order of 3000 cm2/V s when a pure phonon scattering occurs. This work confirms the potential of two-dimensional hole gas diamond field effect transistors for high power and high frequency applications.