Filament-free bulk RRAM (resistive-switching random access memory) devices have been proposed to offer multi-level conductance states with less variations and noise and forming-free operation for neuromorphic computing applications. Understanding conduction mechanism and switching dynamics of filament-free bulk RRAM devices is crucial to optimize device characteristics and to build large-scale arrays for compute in memory and neuromorphic computing applications. Here, we first analyze switching characteristics of bulk RRAM by temperature dependent I-V measurements. We then present a quantitative physical model describing the conduction across trilayer stack by a series combination of multiple conduction mechanisms across each layer. Using this model and fitting it to the experimental characteristics of filament-free bulk RRAM devices, we investigate the origin of bulk switching in trilayer stacks. We demonstrate that our model can be used as a guide to design bulk switching RRAM devices from multilayer stacks of metal oxides.

Dual-layer detectors provide a low-cost solution to improved material decomposition and lesion differentiation in X-ray imaging, while eliminating motion artifacts from multiple exposures. Most designs utilize two indirect detectors with scintillators designed for low-energy and higher-energy detection and separated by a copper filter to harden the beam for high energy detection. To improve the performance of the bottom detector and lower dose requirements, we have previously proposed an alloyed amorphous selenium photodetector to achieve improved resolution and absorption at green wavelengths, better suited to high-performance scintillators such as CsI:Tl. In this work, we demonstrate a baseline prototype for the bottom layer-a continuous, large area 83 m pixel pitch flat panel indirect detector with well-established amorphous selenium as the photodetector-and verify the architecture's performance and detector design. We characterize lag, noise-power spectrum, detective quantum efficiency, and modular transfer function of the detector, and show resolution up to 6 lp/mm when operated at an applied bias of 150 V. This provides a starting point for evaluating the alloyed selenium materials, and shows promise for this detector in the future dual-layer design.

In this letter, a 263 GHz traveling wave tube for electron paramagnetic resonance spectroscopy is designed, fabricated and tested. A periodic permanent magnet focused pencil beam electron optical system is adopted. A folded waveguide slow-wave structure with modified serpentine bends is optimized to provide high-power wideband performance and stable operation. An experiment has been performed to verify the analysis results and confirm the amplifier stability. The device provides a maximum 11.9 W saturation output power and 25.5 dB saturation gain. Although the available solid-state signal source is unable to drive the amplifier to saturation beyond 260 - 264 GHz, 10 W output power over 5.6 GHz bandwidth has been measured.

We report a new physics-based model for dual-gate amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) which we developed and fine-tuned through experimental implementation and benchtop characterization. We fabricated and characterized a variety of test patterns, including a-IGZO TFTs with varying gate widths (100-1000 m) and channel lengths (5-50 m), transmission-line-measurement patterns and ground-signal-ground (GSG) radio frequency (RF) patterns. We modeled the contact resistance as a function of bias, channel area, and temperature, and captured all operating regimes, used physics-based modeling adjusted for empirical data to capture the TFT characteristics including ambipolar subthreshold currents, graded interbias-regime current changes, threshold and flat-band voltages, the interface trap density, the gate leakage currents, the noise, and the relevant small signal parameters. To design high-precision circuits for biosensing, we validated the dc, small signal, and noise characteristics of the model. We simulated and fabricated a two-stage common source amplifier circuit with a common drain output buffer and compared the measured and simulated gain and phase performance, finding an excellent fit over a frequency range spanning 10 kHz-10 MHz.

The application of radio frequency (RF) vacuum electronics for the betterment of the human condition began soon after the invention of the first vacuum tubes in the 1920s and has not stopped since. Today, microwave vacuum devices are powering important applications in health treatment, material and biological science, wireless communication-terrestrial and space, Earth environment remote sensing, and the promise of safe, reliable, and inexhaustible energy. This article highlights some of the exciting application frontiers of vacuum electronics.

In this work, a novel sensing structure based on Au nanoparticles/HfO2/fully depleted silicon-on-insulator (AuNPs/HfO2/FDSOI) MOSFET is fabricated. Using such a planar double gate MOSFET, the electrostatic enrichment (ESE) process is proposed for the ultrasensitive and rapid detection of the coronavirus disease 2019 (COVID-19) ORF1ab gene. The back-gate (BG) bias can induce the required electric field that enables the ESE process in the testing liquid analyte with indirect contact with the top-Si layer. It is revealed that the ESE process can rapidly and effectively accumulate ORF1ab genes close to the HfO2 surface, which can significantly change the MOSFET threshold voltage ([Formula: see text]). The proposed MOSFET successfully demonstrates the detection of zeptomole (zM) COVID-19 ORF1ab gene with an ultralow detection limit down to 67 zM (~0.04 copy/[Formula: see text]) for a test time of less than 15 min even in a high ionic-strength solution. Besides, the quantitative dependence of [Formula: see text] variation on COVID-19 ORF1ab gene concentration from 200 zM to 100 femtomole is also revealed, which is further confirmed by TCAD simulation.

Real-time spike sorting and processing are crucial for closed-loop brain-machine interfaces and neural prosthetics. Recent developments in high-density multi-electrode arrays with hundreds of electrodes have enabled simultaneous recordings of spikes from a large number of neurons. However, the high channel count imposes stringent demands on real-time spike sorting hardware regarding data transmission bandwidth and computation complexity. Thus, it is necessary to develop a specialized real-time hardware that can sort neural spikes on the fly with high throughputs while consuming minimal power. Here, we present a real-time, low latency spike sorting processor that utilizes high-density CuO resistive crossbars to implement in-memory spike sorting in a massively parallel manner. We developed a fabrication process which is compatible with CMOS BEOL integration. We extensively characterized switching characteristics and statistical variations of the CuO memory devices. In order to implement spike sorting with crossbar arrays, we developed a template matching-based spike sorting algorithm that can be directly mapped onto RRAM crossbars. By using synthetic and recordings of extracellular spikes, we experimentally demonstrated energy efficient spike sorting with high accuracy. Our neuromorphic interface offers substantial improvements in area (~1000× less area), power (~200× less power), and latency (4.8s latency for sorting 100 channels) for real-time spike sorting compared to other hardware implementations based on FPGAs and microcontrollers.

A new type of graphene-based quantum Hall standards is tested for electrical quantum metrology applications at alternating current (ac) and direct current (dc). The devices are functionalized with Cr(CO) to control the charge carrier density and have branched Hall contacts based on NbTiN superconducting material. The work is an in-depth study about the characteristic capacitances and related losses in the ac regime of the devices and about their performance during precision resistance measurements at dc and ac.

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800 MHz Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic ( = 2 ) of the electron cyclotron frequency. It produces up to 9.3 W continuous microwave (CW) power at 527.2 GHz frequency using a diode type electron gun operating at V = 16.65 kV, I = 110 mA in a TE mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~ 0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE mode of an internal 12 mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12 mm to 16 mm i.d. waveguide followed by 3 m of the 16 mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~ 84% HE mode content.

To combat the large variability problem in RRAM, current compliance elements are commonly used to limit the in-rush current during the forming operation. Regardless of the compliance element (1R-1R or 1T-1R), some degree of current overshoot is unavoidable. The peak value of the overshoot current is often used as a predictive metric of the filament characteristics and is linked to the parasitic capacitance of the test structure. The reported detrimental effects of higher parasitic capacitance seem to support this concept. However, this understanding is inconsistent with the recent successes of compliance-free ultra-short pulse forming which guarantees a maximum peak overshoot current. We use detailed circuit analysis and experimental measurements of 1R-1R and 1T-1R structures to show that the peak overshoot is of the parasitic capacitance while the overshoot duration is strongly dependent on the parasitic capacitance. Forming control can be achieved, in ultra-short pulse forming, since the overshoot duration is always less than the applied pulse duration. The demonstrated success of ultra-short pulse forming becomes easier to reconcile after identifying the importance of overshoot duration.

Charge-capture/emission is ubiquitous in electron devices. Its dynamics often play critical roles in device operation and reliability. Treatment of this basic process is found in many text books and is considered well understood. As in many electron device models, the individuality of immobile charge is commonly replaced with the average quantity of charge density. This has worked remarkably well when large numbers of individual charges (ensemble) are involved. As device geometries become very small, the ensemble "averaging" becomes far less accurate. In this work, the charge-capture/emission dynamic of Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) is re-examined with full consideration of individual charges and the local field in their immediate vicinity. A dramatic modification of the local band diagram resulted, forcing a drastic change in emission mechanism. The implication is that many well-understood phenomena involving charge capture/emission will need to be reconsidered. As an example, this new picture is applied to the random telegraph noise (RTN) phenomenon. When the screening of a trapped charge by a polar medium such as SiO is quantitatively accounted for in this local field picture, a new physically sound RTN emission mechanism emerges. Similarly, the dynamics of post-stress recovery of Negative-Bias-Instability of p-channel MOSFET can be more rationally explained.

The design and characterization of mm-scale GaAs photovoltaic cells are presented and demonstrate highly efficient energy harvesting in the near infrared. Device performance is improved dramatically by optimization of the device structure for the near-infrared spectral region and improving surface and sidewall passivation with ammonium sulfide treatment and subsequent silicon nitride deposition. The power conversion efficiency of a 6.4 mm cell under 660 nW/mm NIR illumination at 850 nm is greater than 30 %, which is higher than commercial crystalline silicon solar cells under similar illumination conditions. Critical performance limiting factors of sub-mm scale GaAs photovoltaic cells are addressed and compared to theoretical calculations.

Wireless biomedical implantable devices on the mm-scale enable a wide range of applications for human health, safety, and identification, though energy harvesting and power generation are still looming challenges that impede their widespread application. Energy scavenging approaches to power biomedical implants have included thermal [1-3], kinetic [4-6], radio-frequency [7-11] and radiative sources [12-14]. However, the achievement of efficient energy scavenging for biomedical implants at the mm-scale has been elusive. Here we show that photovoltaic cells at the mm-scale can achieve a power conversion efficiency of more than 17 % for silicon and 31 % for GaAs under 1.06 μW/mm infrared irradiation at 850 nm. Finally, these photovoltaic cells demonstrate highly efficient energy harvesting through biological tissue from ambient sunlight, or irradiation from infrared sources such as used in present-day surveillance systems, by utilizing the near infrared (NIR) transparency window between the 650 nm and 950 nm wavelength range [15-17].

Silicon photovoltaics are prospective candidates to power mm-scale implantable devices. These applications require excellent performance for small-area cells under low-flux illumination condition, which is not commonly achieved for silicon cells due to shunt leakage and recombination losses. Small area (1-10 mm) silicon photovoltaic cells are studied in this work, where performance improvements are demonstrated using a surface n-type doped emitter and SiN passivation. A power conversion efficiency of more than 17% is achieved for 660 nW/mm illumination at 850 nm. The silicon cells demonstrate improved power conversion efficiency and reduced degradation under low illumination conditions in comparison to conventional crystalline silicon photovoltaic cells available commercially.

We report a new technique for the rapid measurement of full capacitance-voltage (C-V) characteristic curves. The displacement current from a 100 MHz applied sine-wave, which swings from accumulation to strong inversion, is digitized directly using an oscilloscope from the metal-oxide-semiconductor (MOS) capacitor under test. A C-V curve can be constructed directly from this data but is severely distorted due to non-ideal behavior of real measurement systems. The key advance of this work is to extract the system response function using the same measurement set-up and a known MOS capacitor. The system response correction to the measured C-V curve of the unknown MOS capacitor can then be done by simple deconvolution. No de-skewing and/or leakage current correction is necessary, making it a very simple and quick measurement. Excellent agreement between the new fast C-V method and C-V measured conventionally by an LCR meter is achieved. The total time required for measurement and analysis is approximately 2 seconds, which is limited by our equipment.

GaAs photovoltaics are promising candidates for indoor energy harvesting to power small-scale (≈1 mm) electronics. This application has stringent requirements on dark current, recombination, and shunt leakage paths due to low-light conditions and small device dimensions. The power conversion efficiency and the limiting mechanisms in GaAs photovoltaic cells under indoor lighting conditions are studied experimentally. Voltage is limited by generation-recombination dark current attributed to perimeter sidewall surface recombination based on the measurements of variable cell area. Bulk and perimeter recombination coefficients of 1.464 pA/mm and 0.2816 pA/mm, respectively, were extracted from dark current measurements. Resulting power conversion efficiency is strongly dependent on cell area, where current GaAs of 1-mm indoor photovoltaic cells demonstrates power conversion efficiency of approximately 19% at 580 lx of white LED illumination. Reductions in both bulk and perimeter sidewall recombination are required to increase maximum efficiency (while maintaining small cell area near 1 mm) to approach the theoretical power conversion efficiency of 40% for GaAs cells under typical indoor lighting conditions.

We present a novel structure for the back-side readout silicon photomultipler (SiPM). Current SiPMs are front-illuminated structures with front-side readout, which have relatively small geometric fill factor leading to degradation in their photon detection efficiency (PDE). Back-side readout devices will provide an advantageous solution to achieve high PDE. We designed and investigated a novel structure that would allow back-side readout while creating a region of high electric field optimized for avalanche breakdown. In addition, this structure has relatively high fill factor and also allow direct coupling of individual micro-cell of the SiPM to application-specific integrated circuits. We will discuss the performance that can be attained with this structure through device simulation and the process flow that can be used to fabricate this structure through process simulation.

The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE. The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.

Fabricating powerful neuromorphic chips the size of a thumb requires miniaturizing their basic units: synapses and neurons. The challenge for neurons is to scale them down to submicrometer diameters while maintaining the properties that allow for reliable information processing: high signal to noise ratio, endurance, stability, reproducibility. In this work, we show that compact spin-torque nano-oscillators can naturally implement such neurons, and quantify their ability to realize an actual cognitive task. In particular, we show that they can naturally implement reservoir computing with high performance and detail the recipes for this capability.

Single-crystalline MoSe and MoTe platelets were grown by Chemical Vapor Transport (CVT), followed by exfoliation, device fabrication, optical and electrical characterization. We observed that for the field-effect-transistor (FET) channel thickness in range of 5.5 nm to 8.5 nm, MoTe shows p-type, whereas MoSe with channel thickness range of 1.6 nm to 10.5 nm, shows n-type conductivity behavior. At room temperature, both MoSe and MoTe FETs have high ON/OFF current ratio and low contact resistance. Controlling charge carrier type and mobility in MoSe and MoTe layers can pave a way for utilizing these materials for heterojunction nanoelctronic devices with superior performance.

In this paper, we show that quantum Hall resistance measurements using two terminals may be as precise as four-terminal measurements when applying superconducting split contacts. The described sample designs eliminate resistance contributions of terminals and contacts such that the size and complexity of next-generation quantized Hall resistance devices can be significantly improved.