Diamond Transistors Research Articles

Research on Techniques for Enhancing the Speed of Low-Power Operational Amplifier

In the context of low utilization, this paper explores various techniques for enhancing the speed of operational amplifier (OA). The main text delves into three methods to gain equilibrium: fully differential operational amplifier (FDA), twostage operational transconductance amplifier (OTA), and a novel high-speed calculation system architecture employing a “rhombus crystal tube.” The FDA improves speed by minimizing noise and enhancing bandwidth and output voltage swing, without increasing power consumption. The design of the two-stage OTA combines the characteristics of a differential amplifier and a telescopic amplifier, optimizing gain and slew rate through Miller compensation and noise gain manipulation, thereby achieving high-speed performance. A novel high-speed operational amplifier structure employs diamond transistors to supply substantial current for capacitor charging, enhancing the slew rate and overall speed. Throughout the text, these methods are presented as a means to jointly promote high speed, low power consumption, and the handling of a significant number of transistors. To further increase the speed, the size of the microcrystalline tube is crucial. The research direction is outlined, and while the design equipment is in the foreground, there remains room for progress, especially in applications for large-scale electrical models. Future research aims to enhance the power efficiency of circuits, making them more practical and efficient. This study provides valuable insights into balancing power consumption and speed in advanced electronic devices.

Vertical 3D diamond field effect transistors with nanoscale gate-all-around

Recent studies have shown that diamond field effect transistors (FETs) have great potential in high-power and high-frequency electronics, exhibiting the maximum leakage current of 1.35 A·mm−1 and the breakdown voltage of 2 kV. However, there is still a lack of preliminary exploration on diamond FETs with a three-dimensional (3D) architecture, which has been evidenced as a promising approach to increase device performance and density for silicon transistors. In this paper, we present a systematic investigation of 3D vertical gate-all-around (GAA) diamond transistors, combined with numerical simulation, device fabrication and characterization of device performance. The p-type conductivity of the diamond channel in a transistor is obtained by Boron-ion implantation. Our numberical simulations address the close correlation of transfer characteristics of the 3D vertical GAA diamond transistors with the ion implanted impurities. In the device fabrication, we introduce the focused ion and electron beam technology for the electrode interconnections in the 3D devices. Finally, we demonstrate that the 3D vertical GAA diamond transistor exhibits the on-off ratio of 2.5 × 103, subthreshold swing of 334.3 mV·dec−1, and threshold voltage of 0.64 V. And the temperature dependence of transfer characteristics of the 3D devices indicates that 3D vertical GAA diamond transistors have potential application prospects in extreme environments such as high temperature.

On possible new methods for δ-layer synthesis of the diamond transistor

An upper estimate of the activation barrier Ea, overcome by boron atom during its adsorption on singular face of unalloyed diamond, has been obtained using quantum chemistry methods. This process is connected with the formation of boron-doped δ-layer in the unalloyed diamond, which is an integral part of the diamond transistor. The estimate of Ea was found to be approximately 6–7 eV. It is correspondent by the order of magnitude with floating electrode potential drop relative to stationary low-temperature plasma at electronic temperature above ~ 1 eV. Meanwhile, in the existing high-frequency plasma-enhanced CVD (MWCVD) technology used to create boron-doped δ-layers, the source of the δ-layer growth is high-energy boron ions with energies of hundreds of eV (and even units of keV) which enter the substrate from MW plasma. This energy that the ions acquire at the near-electrode voltage drop is clearly excessive and leads both to excessively high depth of ion penetration into the diamond and to large dispersion of this depth. This reduces the quality of the δ-layer and the transistor efficiency as a whole. At the same time, specificity of high-frequency plasma, created in the MW-resonator for the diamond growth, allows plasma modes and diamond growth both with large (units of keV) and small (units of eV) near-electrode voltage drops. This paper considers the possibility of switching between these two modes. The high-drop mode realizes the rapid growth of unalloyed diamond and the low-drop mode – high quality δ-layer.

1 W/mm Output Power Density for H-Terminated Diamond MOSFETs With Al2O3/SiO2Bi-Layer Passivation at 2 GHz

We have demonstrated a novel method of depositing 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> /PECVD-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bi-layer dielectric to passive the surface channels of the hydrogen-terminated diamond (H-diamond). After 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> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> passivation, the surface current increased with time and then tended to be saturated. Afterwards, it became much more stable and showed a larger current than an unpassivated counterpart. The H-diamond MOSFETs were fabricated by using this bi-layer passivation structure and an extremely low Ohmic contact resistance of 0.87 Ω·mm was obtained. The H-diamond RF MOSFET with gate length of 0.45 μm achieved a high current density of -549 mA/mm and an extrinsic fT/fmax of 15/36 GHz. By load-pull measurement, a high output power density of 1.04 W/mm was obtained at frequency of 2 GHz. The results reveal that it is a promising solution for high-stable and high-power diamond transistors by using the 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> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bi-layer passivation.

A Valleytronic Diamond Transistor: Electrostatic Control of Valley Currents and Charge-State Manipulation of NV Centers.

The valley degree of freedom in many-valley semiconductors provides a new paradigm for storing and processing information in valleytronic and quantum-computing applications. Achieving practical devices requires all-electric control of long-lived valley-polarized states, without the use of strong external magnetic fields. Because of the extreme strength of the carbon–carbon bond, diamond possesses exceptionally stable valley states that provide a useful platform for valleytronic devices. Using ultrapure single-crystalline diamond, we demonstrate electrostatic control of valley currents in a dual-gate field-effect transistor, where the electrons are generated with a short ultraviolet pulse. The charge current and the valley current measured at the receiving electrodes are controlled separately by varying the gate voltages. We propose a model to interpret experimental data, based on drift-diffusion equations coupled through rate terms, with the rates computed by microscopic Monte Carlo simulations. As an application, we demonstrate valley-current charge-state modulation of nitrogen-vacancy centers.

H-terminated diamond RF MOSFETs with AlOx/SiNx bi-layer passivation and selectively etched T-shaped gates

This paper reports a novel ALD-AlOx/PECVD-SiNx bi-layer structure to passivate the surface channels of the hydrogen terminated diamond transistors. The upper SiNx was selectively etched to fabricate the robust T-shaped gates, while the lower thin AlOx with high quality was retained as the gate insulator. The fabricated device with gate length of 0.3 μm demonstrated a high current density of 715 mA/mm, a low on-resistance of 8 Ω·mm, and an extrinsic fT/fmax of 10/20 GHz. RF power output characteristics were investigated at 2 GHz and a maximum output power density of 703 mW/mm was obtained at a low drain voltage of −15 V. It reveals that it is a promising solution for high stable and high power diamond RF transistors and microwave integrated circuits by using the AlOx/SiNx passivation structure.

Diamond lateral FinFET with triode-like behavior

In this letter we report a diamond lateral FinFET fabricated using an ohmic regrowth technique. The use of ohmic regrowth separates the source/drain and gate fabrication, providing a viable means to improve ohmic contact resistance while protecting the top surface of the diamond channel from dry etch damage. Enabled by high channel quality, the diamond transistor behavior was shown to transit from a pentode-like to a triode-like characteristic when channel length decreased. For the first time, space charge limited transport in diamond FinFETs with a short channel length was demonstrated. We have analyzed the space charge limited transport from room temperature to 150 °C. This space charge limited transport, in combination with improved ohmic contacts, will enable diamond FinFETs for various high-power applications.

Radiation Effects in Diamond pHEMTs

Field effect transistors (FET) based on diamond have 2-dimensional hole gas (2DHG) as conduction channel. In terms of usability in harsh environment, an important aspect of diamond-based FETs is its promise of radiation hardness. Because of high binding energy of diamond, diamond transistors are predicted to be radiation hard. However, the way radiation affects the 2DHG conduction layer is unknown. This may be a problem as radiation may disrupt the 2D hole concentrations in the 2DHG channel. In this work, radiation effects in diamond with 2DHG layer is explored in two ways: (1) Using Hall effect measurements to monitor changes in transport properties, and (2) measuring current-voltage characteristics for the three-terminal devices to analyze changes in their device properties. Two different types of radiation were used. A cyclotron was used for proton irradiation on diamond substrates, and a high capacity dry cell, panoramic gamma irradiator with 60Co as source material was used for gamma irradiation on diamond-based FETs. Diamond substrates were irradiated with protons at 152 keV and fluence of 1012 particles/cm2. Likewise, diamond-based FETs were irradiated with gammas from a total dose of 100kRad up to 10 MRads and exhibited an annealing effect. ACKNOWLEDGEMENTS The authors thank the University of Maryland NanoCenter, the University of Maryland Radiation Facilities and Argonne National Laboratory for equipment and staff support. The effort depicted is sponsored by the Department of Defense, Defense Threat Reduction Agency under Grant No. HDTRA1-17-1-0007. The content of this information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. The authors acknowledge the assistance in material preparation from Jim Butler of EuclidTech Labs.

Radiofrequency performance of hydrogenated diamond MOSFETs with alumina

Hydrogenated diamond MOSFETs with self-oxidized alumina as a gate dielectric are fabricated. The diamond MOSFETs show a high maximum drain current density of 466 mA/mm at VGS = −6 V, a transconductance of 58 mS/mm, and an off-state breakdown voltage of −53 V. The maximum output power density reaches 745 mW/mm at 2 GHz continuous wave, which is the highest reported value for diamond transistors measured at 2 GHz. The output power value measured is lower than that estimated. Pulse I-V analysis shows that the main factor that affects the output power of the diamond MOSFETs is the traps in the channel.

A Planar Scanning Probe Microscope

Scanning probe microscopy (SPM) is traditionally based on very sharp tips, where the small size of the apex is critical for resolution. This paradigm is about to shift, since a novel generation of planar probes (color centers in diamond, superconducting sensors and single electron transistors) promises to image small electric and magnetic fields with hitherto inaccessible sensitivity. To date, much effort has been put into fabricating these planar sensors on tip-like structures. This compromises performance and poses a considerable engineering challenge, which is mastered by only a few laboratories. Here we present a radically simplified, tipless, approach - a technique for scanning an extended planar sensor parallel to a planar sample at a distance of few tens of nanometers. It is based on a combination of far-field optical techniques to measure both tilt and distance between probe and sample with sub-mrad and sub-nm precision, respectively. Employing these measurements as a feedback signal, we demonstrate near-field optical imaging of plasmonic modes in silver nanowires by a single NV center. Our scheme simultaneously improves the sensor quality and enlarges the range of available sensors beyond the limitations of existing tip-based schemes.

Methods for Extension of Tunability Range in Synthetic Inductance Simulators

Presented work focuses on methods for the extended electronic control in the designs of inductance simulators based on single-loop feedback topology. Implementation is based on diamond transistors as voltage-to-current converters and two voltage-mode multipliers connected to different positions in topology (providing amplification or control of equivalent resistance). Presented solutions achieve significantly extended adjustability of equivalent inductance value in comparison to standard approaches based on CMOS differential pair-based operational transconductances amplifiers. Proposed designs of synthetic inductors are based on utilization of integer-order as well as fractional-order (constant phase element) capacitor. Their features are verified both via PSpice simulations and experimental measurements. Results confirmed intended purposes of designs implementing discussed methods.DOI: http://dx.doi.org/10.5755/j01.eie.24.3.20976

Diamond FinFET without Hydrogen Termination

In this letter we report the first diamond fin field-effect transistor (diamond FinFET) without a hydrogen-terminated channel. The device operates with hole accumulation by metal-oxide-semiconductor (MOS) structures built on fins to maintain effective control of the channel conduction. Devices with 100-nm—wide fins were designed and fabricated to ensure that the channel pinched off at zero gate bias. The transfer characteristic of FinFET showed a greater than 3000 on/off ratio, successfully demonstrating the transistor behavior. Devices were characterized at room temperature and at 150 °C, showing 30 mA/mm current density at 150 °C, 35 times more than current density at room temperature. The diamond FinFET, which leverages the fin concept from the silicon industry and the material advance of diamond, enables a new class of diamond transistors for applications from digital to power and radio frequency (RF) electronics.

(Invited) High Power Diamond Devices with 2-D Transport Channels

Diamond transistors with surface 2D conduction channels are projected to be radiation hard with respect to neutrons. However, ionizing radiation hardness may be a problem due to the disruption of the 2D hole concentration. In this paper, the processing of three terminal devices with 2D surface conducting channels is presented as well as analysis of device transfer characteristics. This paper addresses the fabrication, radiation induced defects and device characteristics of microwave power field effect transistors (FET) built on 1) high quality diamond surfaces exploiting the 2 dimensional surface conductivity, and 2) the surface two dimensional “hydrogen terminated” conduction layer in diamond single crystals grown by CVD. Diamond is an ultra-wide bandgap semiconductor material that exhibits a number of ideal properties for future power and RF electronics applications beyond the current generation of SiC- and GaN-based technology. These properties include a bandgap of 5.5 eV, a high breakdown field (>10 MV/cm), high theoretical room-temperature carrier mobilities (>3800 cm2/V*sec), extremely high thermal conductivity (>22 W/cm*K), and excellent mechanical and thermal stability in vacuum. However, diamond doping is difficult, as the activation energies for typical dopants (B, P) exceed 0.37 eV and lead to very low dopant activation levels.[3] This drawback of dopant activation efficiency has been, in part, mitigated by two-dimensional conductivity found in hydrogen-terminated diamond surfaces. Hydrogen-termination (usually accomplished via exposure to hydrogen plasma) has been found to create a C-H surface dipole layer that reduces the carrier ionization energy by nearly 1.5 eV. Adsorbed species, such as atmospheric H2O, NO2, or intentionally-introduced passivation species contact this dipole layer, and together generate a two-dimensional hole gas (2DHG) with 1013-1014 holes/cm2 carrier density several nanometers below the diamond surface. Hydrogen-terminated diamond has been used as the basis for metal-semiconductor field effect transistors (MESFETs) and metal-oxide-semiconductor field effect transistors (MOSFETs) hydrogen-terminated diamond. Both unpassivated and passivated surface conduction devices have been fabricated. In the case of unpassivated devices, exposure to air (and thus atmospheric H2O) causes the devices to exhibit sheet charge densities in the 1013 holes/cm2 range.[1] MESFETs typically rely on Au metal for Ohmic contacts and Al metal for Schottky contacts to the hydrogen-terminated diamond. These devices have been normally-on devices, with threshold voltages around between -1V to +1V, depending on device dimensions.[1-4] MOSFETs fabricated using atomic layer deposited (ALD) oxides such as Al2O3 have been fabricated with either normally-off or normally-on behavior, depending on thermal history. Annealing of these devices at 180°C caused the devices to switch from normally-on to normally-off, with threshold voltages again in the range of ±1V range; the change was likely due to the removal of adsorbed species in the unpassivated diamond surfaces between the source, gate, and drain regions in the device, causing a reduction in the 2DHG sheet carrier density.[4] The diamond materials and surface preparation supplied by Euclid Techlabs using proprietary processes to produce defect free, ultra-smooth (Sa ~ 0.2 nm) diamond surfaces and buried boron ‘delta doped’ CVD layers. Euclid techlabs has demonstrated the delta doped transport channels in Diamond have mobilities in excess of 700 cm2/V sec. Device design and process science development was carried out within the facilities of the UMD NanoCenter and the NanoFab Lab. Radiation testing has been carried out at the radiation facilities of the University of Maryland. Device electrical performance and characterization has been carried out. We perform baseband noise, as well transconductance/output resistance dispersion characterization of 2D Surface Channel Diamond FETs fabricated under different processes and having different geometry and designs in order to identify device defects and their reliability and their susceptibility to radiation effects. The impact of the various types of noise on device defect and radiation hardness will be reported. Materials characterization techniques including Raman spectroscopy (strain, sp2 vs sp3 bonding), FTIR, and SIMS. Common diamond point defects are characterized by photoluminescence and FTIR.

CFOA-Based Fractional Order PIλDδ Controller

Conventional Current Feedback Operational Amplifier (CFOA) is not current controllable or not electronically controllable. It is thus of interest to add a current mirror into the CFOA in order to make it current controllable. This modification can be achieved by using Diamond Transistor (DT) instead of going through complicated IC fabrication process. This work applies the modified CFOA in fractional-order proportional integral derivative (PIλDδ) controller. Both simulation and experimental results confirm that the modified CFOA is electronically controllable.

Comparison of field-effect transistors on polycrystalline and single-crystal diamonds

High quality diamond epitaxial layers are prepared on polycrystalline and single-crystal diamonds by MPCVD. Field-effect transistors with gate length of 1μm are fabricated by self-aligned process on them. The surface p-type channel of the diamond films were obtained by microwave hydrogen plasma treatment. The DC and RF characteristics of the field-effect transistors are compared. The polycrystalline diamond FETs show better DC and small-signal performance. Single-crystal diamond FETs show higher breakdown voltage and output power density due to its high crystal quality. The maximum output power density reaches 320mW/mm and 450mW/mm@1GHz for the polycrystalline diamond and single crystal diamond transistors, respectively.

Simple Oscillator with Enlarged Tunability Range Based on ECCII and VGA Utilizing Commercially Available Analog Multiplier

Abstract This work presents an example of implementation of electronically controllable features to an originally unsuitable circuit structure of oscillator. Basic structure does not allow any electronic control and has mutually dependent condition of oscillation (CO) and frequency of oscillation (FO) if only values of passive elements are considered as the only way of control. Utilization of electronically controllable current conveyor of second generation (ECCII) brings control of CO independent of FO. Additional application of voltage amplifier with variable gain in both polarities (voltage-mode multiplier) to feedback loop allows also important enlargement of the range of the independent FO control. Moreover, our proposal was tested and confirmed experimentally with commercially available active elements (“Diamond transistor”, current-mode multiplier, voltage-mode multiplier) in working range of tens of MHz.

Real-time Monitoring of Cell Activities by Diamond Solution-gated Field Effect Transistors

In this study, we present basic aspects for real-time monitoring of cell growth of human cell lines. We successfully fabricated fully optically transparent diamond-based solution-gated field effect transistor (SG-FET) on glass substrates. The diamond transistor functionalities are demonstrated by the sensitivity of electrical characteristics to solutions with different Na+ and K+ ion concentrations, and pH. Moreover, the diamond SG-FETs exhibited sensitivities to the adhesion of proteins and cells, to the cell delamination, or even to the cell death.

Mathematical simulation for estimating the potential of the hydrogen-terminated diamond FET in the mm-wave range

The estimation is based on the intrinsic transistor large-signal lumped-element equivalent circuit with the parameters and spline characteristics calculated by means of a two-dimensional model of the transistor active region. This model includes the quasi-hydrodynamic model of hole transport (with very low mobility on the surface), equations of the electric field, and dipole model of the hydrogen-terminated diamond surface. The object of simulation is the structure of a real 100-nm-long gate single-crystal diamond transistor, which is recovered by fitting the simulated output current---voltage characteristics to the experimental. Using the frequency-domain simulation of a power amplifier with such a transistor, a number of RF parameters (power-added efficiency, output power, and others) are calculated as functions of the maximum available power of the RF generator at frequencies of 30 and 60 GHz.

Reconnection-less OTA-based Biquad Filter with Electronically Reconfigurable Transfers

This paper deals with operational transconductance amplifiers (OTAs) -based active voltage-mode biquad filter with electronically reconfigurable transfer functions. Due to utilization of the very favourable active devices, this design is ready for immediate CMOS design. Presented filtering solution contains four active elements where each of them is directly used for reconnection-less change of transfer function or modification including electronic control of quality factor and tuning. The filter offers availability of all-pass, high-pass, band-pass, band-reject transfer response and special transfers as high-pass with zero and low-pass with zero. Measurement results based on utilization of diamond transistors confirmed expected behaviour of the circuit. DOI: http://dx.doi.org/10.5755/j01.eee.21.3.10205

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.