RC Time Constant Research Articles

Ultraslow relaxation of the structure at the ionic liquid|gold electrode interface to a potential step probed by electrochemical surface plasmon resonance measurements: asymmetry of the relaxation time to the potential-step direction

The relaxation of the structure in the electrical double layer at the ionic liquid|gold interface to the steps of the electrode potential has been studied using surface plasmon resonance (SPR) measurements. The relaxation of the SPR resonance angle occurs on the order of 100 s, which is distinctively slower than the RC time constant of the cell, about 0.1 s. A relaxation model considering the distribution of the time constants well reproduces the SPR curves. The potential steps to the positive direction lead to two orders of magnitude slower relaxation of the SPR resonance angle than those to the negative direction. The ultraslow relaxation and its asymmetry to the potential-step directions are likely to be caused by slow inter-layer dynamics of ions in the ionic multilayers and interaction of ions with gold, respectively.

Traveling wave electrode design of electro-optically modulated coupled-cavity surface-emitting lasers

We present a novel design of an electro-optically modulated coupled-cavity vertical-cavity surface-emitting laser (CC-VCSEL) with traveling wave electrodes of the modulator cavity, which allows to overcome the RC time constant of a traditional lumped electrode structures. The CC-VCSEL optical design is based on longitudinal mode switching which has recently experimentally demonstrated a record modulation speed. We carry out segmented transmission line electrical design of the modulator cavity in order to compensate for the low impedance of the modulator section and to match the 50 Ω electrical network. We have optimized two types of highly efficient modulator structures reaching -3 dB electrical cut-off frequency of f(cut-off) = 330 GHz with maximum reflection of -22 dB in the range from f(LF) = 100 MHz to f(cut-off) and 77 - 89% modulation efficiency.

A novel high performance ESD power clamp circuit with a small area

A MOSFET-based electrostatic discharge (ESD) power clamp circuit with only a 10 ns RC time constant for a 0.18-μm process is proposed. A diode-connected NMOSFET is used to maintain a long delay time and save area. The special structure overcomes other shortcomings in this clamp circuit. Under fast power-up events, the gate voltage of the clamp MOSFET does not rise as quickly as under ESD events, the special structure can keep the clamp MOSFET thoroughly off. Under a falsely triggered event, the special structure can turn off the clamp MOSFET in a short time. The clamp circuit can also reject the power supply noise effectively. Simulation results show that the clamp circuit avoids fast false triggering events such as a 30 ns/1.8 V power-up, maintains a 1.2 μs delay time and a 2.14 μs turn-off time, and reduces to about 70% of the RC time constant. It is believed that the proposed clamp circuit can be widely used in high-speed integrated circuits.

A new process for the fabrication of planar antenna coupled Ni–NiOx–Ni tunnel junction devices

Graphical abstractDisplay Omitted Highlights? Methods to fabricate planar, geometrically asymmetric Ni-NiOx-Ni tunnel diodes are developed. ? Planar structure decreased the parasitic capacitance enabling high cut-off frequency. ? Ni electrodes provided the surface oxidation control and low band edge offset with its native oxide. ? Asymmetric geometry of the electrodes resulted in electrical asymmetry across zero bias. ? Three Ni oxidation methods are studied to build a thin, but durable oxide for the tunnel barrier. Antenna coupled metal-insulator-metal (ACMIM) tunnel junctions are fast electromagnetic wave detectors shown to respond to radiation of wavelength as short as 1.6µm. In the design and fabrication of these devices, it is crucial to keep the RC time constant of the tunnel junction small to achieve the requisite cut-off frequency and adequate rectification efficiency. Junction geometry and the properties of the insulation layer between the metal antenna parts play an important role in determining both the time constant and the rectification effectiveness. In this paper we have designed and fabricated Ni-NiOx-Ni devices and performed three oxidation processes to optimize the insulation layer. Ease of manufacture over large areas is also a requirement for successful implementation. We detail two simple and low cost processes for the fabrication of Ni-NiOx-Ni tunnel junctions. These methods allow for the large area array implementations of the antenna coupled MIMs for the application in low-cost energy harvesting.

Low-Resistivity 10 nm Diameter Magnetic Sensors

Resistivities of 5.4 μΩ·cm were measured in 10-nm-diameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20-30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.

Study of the electron transfer properties of nanostructured bimetallic films of polymerized porphyrins

The amperometric response of gold electrodes modified with bimetallic films of metalloporphyrins (FeCuPP) to H2O2 is dramatically higher than the corresponding to the bare electrode (up to 60 times) and far exceeding the individual CuPP or FePP. This response is attributed to the incorporation of H2O2 as ligand of both metal centers. Polymeric mono and bimetallic porphyrins doped with multiwalled carbon nanotubes were studied by EIS and voltammetry to understand the differential response. A better electron transfer rate was observed in the composites allowing the study of the effect of ligands on the electrochemical response. The cyclic voltammetry showed a reversible wave for azide reduction only with the bimetallic composite. The EIS experiment showed two different RC time constants suggesting that H2O2 and azide intercalate as linking the metal centres. The anion nitrite inhibits these differences and does not affect the monometallic structures.Polymers deposited onto gold electrodes reveal an amorphous structure by SEM micrograph, whereas the corresponding composites prepared by electro-co-deposition present a network of interconnected fibrils with diameters similar to the nanotubes serving as backbones of the porphyrin aggregates.

Frequency-selective contrast on variably doped p-type silicon with a scanning microwave microscope

We report on frequency-dependent contrast in d(S11)/dV measurements of a variably doped p-type silicon sample in the frequency range from 2 GHz to 18 GHz. The measurements were conducted with a scanning microwave microscope. The measurements were done at selected frequencies while varying the DC tip voltage. The measured d(S11)/dV signal shows a maximum for doping concentrations (NA) of 1015 cm−3−1016 cm−3 at 2.3 GHz. As the microscope operating frequency is increased, this maximum sequentially “switches” through the regions of increasing dopant concentration, displaying a maximum for NA of 1017 cm−3−1018 cm−3 at 17.9 GHz. The frequency dependent “switching” is attributed to the physics of tip-to-sample interaction, particularly as related to the frequency-dependent local surface resistance and the depletion capacitance that control the RC time constant of tip-to-sample interaction. This provides a unique platform for local, frequency-selective, spatially resolved microwave spectroscopy of semiconducting materials.

Reservations on the reservoir

Reservations on the reservoir

Active matrix touch sensor detecting time-constant change implemented by dual-gate IGZO TFTs

The dual-gate IGZO TFT is proposed to be used in an active matrix touch sensing circuit. The circuit contains only one TFT with a RC low-pass filter, since the dual-gate IGZO TFT can be controlled by both its top and bottom gates. The simplest structure maximizes the sensing pad area in the pixel. A touch event on the sensing pad forms a capacitance and thus increases the RC time-constant of the scan pulse fed to the gate of TFT. Thus, a significant transient ON current is generated to be the sensing signal. The current is so large that it can be easily read out and thus the power and cost of peripheral ICs can be reduced. In this paper, the robustness of the circuit to environment in operations is discussed.

Rapid potential step charging of paper-based polypyrrole energy storage devices

Abstract Symmetric paper-based supercapacitor devices containing polypyrrole (PPy)–cellulose composite electrodes and aqueous electrolytes can be charged using either potential step or constant current charging. Potential step charging provides better control of the charging and can result in significantly shorter charging times, enabling charging in 22 s for devices with cell capacitances of 12.2 F when charged to 0.8 V. The paper-based electrode material was compatible with charging currents as large as 5.9 A g−1 due to the rapid counter ion mass transport resulting from the porous composite structure and the thin PPy coatings. The charging times were controlled by the RC time constants of the devices and the cell resistance was found to decrease with increasing electrode area. For small cells, the cell resistance was determined to a large extent by the electrolyte resistance and contact resistances, whereas the resistance of the current collectors dominated for larger cells. The specific cell capacitance was 38.3 F g−1 or 2.1 F cm−2, normalized with respect to the total electrode weight and electrode cross section area respectively, and the devices showed 80–90% capacitance retention after 10 000 potential step charge and discharge cycles. These results, which demonstrate that potential step charging can be advantageous for conducting polymer based energy storage devices, are very encouraging for the development of new up-scalable paper-based energy storage devices.

A High-Speed Multi-Channel Readout for SSPM Arrays

Solid-state photomultiplier (SSPM) arrays are a new technology that shows great promise to be used in PET detector modules. To reduce the number of channels in a PET scanner, it is attractive to use resistor dividers, which multiplex the number of channels in each module down to four analog output channels. It is also attractive to have SSPMs with large pixels (3×3 or 4×4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). However, large area SSPMs have correspondingly large capacitances (up to 1 nF) and directly coupling them to a resistive network will create a low-pass filter with a high RC time constant. In order to overcome this, we have developed an application specific integrated circuit (ASIC) that “hides” the intrinsic capacitance of the SSPM array from a resistive network with current buffers, significantly improving the rise time of the SSPM signals when connected to the resistive network. The ASIC is designed for a wide range of SSPM sizes, up to 1 nF (equivalent to 4×4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and for input currents of 1 to 20 mA per channel. To accommodate various sizes of SSPM pixels, the ASIC uses adjustable current sources (to keep the feedback loop stable). A test ASIC has been fabricated that has 16 input channels, an internal resistor divider array that produces four analog outputs, 16 buffers that isolate the SSPM capacitance from the resistor array, and four output buffers that can drive 100 ohm loads. Thus, detector modules based on SSPMs and this ASIC should be compatible with the block detector readout electronics found in many PET cameras. Tests of this ASIC show that its rise time is <; 2 ns (and it will thus not significantly degrade the ~7 ns rise time of the SSPM pixels) and that the analog decoding circuitry functions properly.

A High-Speed Adaptively-Biased Current-to-Current Front-End for SSPM Arrays

Abstract Solid-state photomultiplier (SSPM) arrays are an interesting technology for use in PET detector modules due to their low cost, high compactness, insensitivity to magnetic fields, and sub-nanosecond timing resolution. However, the large intrinsic capacitance of SSPM arrays results in RC time constants that can severely degrade the response time, which leads to a trade-off between array size and speed. Instead, we propose a front-end that utilizes an adaptively biased current-to-current converter that minimizes the resistance seen by the SSPM array, thus preserving the timing resolution for both large and small arrays. This enables the use of large SSPM arrays with resistive networks, which creates position information and minimizes the number of outputs for compatibility with general PET multiplexing schemes. By tuning the bias of the feedback amplifier, the chip allows for precise control of the close-loop gain, ensuring stability and fast operation from loads as small as 50pF to loads as large as 1nF. The chip has 16 input channels, and 4 outputs capable of driving 100 n loads. The power consumption is 12mW per channel and 360mW for the entire chip. The chip has been designed and fabricated in an AMS 0.35um high-voltage technology, and demonstrates a fast rise-time response and low noise performances.

Low power, high resolution CMOS variable-delay element

This letter proposes a novel CMOS variable-delay element (VDE). It employs a RC-based differentiator to control the pMOS transistor of a CMOS inverter, whose delay time becomes dependent on the time constant RC. By implementing the RC differentiator with transistor components, the delay can be controlled by variations of both R and C, for fine and coarse delay tuning, respectively. Simulated results show good delay linearity, high resolution and low power consumption for the proposed circuit.

Carbon black supercapacitors employing thin electrodes

Carbon black supercapacitors that employ thin (∼1 μm) electrodes were produced by coating and inkjet printing on a conventional current collector or directly on a separator membrane. The simplicity and diversity of ultrathin electrode fabrication were facilitated by the physical form of carbon black, which can be described as a fine particle of around 100 nm in size. It was established that the performance of carbon black in thin supercapacitor electrodes may be compared with carbon nanotube and graphene materials in those instances where fast frequency response is desired from the supercapacitor. At the same time, the fast response supercapacitors that employ nanotubes and graphenes have presently involved more elaborate fabrication routes. RC time constant of 354 μs and phase angle of −75° at 120 Hz is demonstrated for carbon black supercapacitors that employ SC3 carbon black of 1800 m 2 g −1 surface area.

Fabrication of a novel lumped electro-absorption modulator with a lower RC-time constant

A novel lumped electro-absorption modulator with a charge layer and an undercut ridge waveguide (DU-EAM) was fabricated and measured. Also, two other kinds of EAM with straight ridge waveguides, one with a charge layer (D-EAM) and another with no charge layer (N-EAM), were fabricated and tested to ensure that the design of the DU-EAM would reduce the RC-time constant. The measured capacitance of the D-EAM and the DU-EAM is lower than that of the N-EAM under reverse bias voltage from −1 to −8 V due to the inserted charge layer. The capacitances of the N-EAM, the D-EAM and the DU-EAM are 0.375, 0.225 and 0.325 pF, respectively, at −3 V. In addition, the DU-EAM had a larger extinction ratio (25 dB at −3 V) and higher modulation efficiency (13 dB/V between −1 and −2 V) than two other straight-ridge-waveguide ones (the D-EAM performed 22 dB and 10 dB/V, the N-EAM performed 20 dB and 10 dB/V) due to the 5.2 μm wider active region.

High-K (Ba0.8Bi0.2)(Zn0.1Ti0.9)O3ceramics for high-temperature capacitor applications

Solid solutions of BaTiO(3)-Bi(Zn(1/2)Ti(1/2))O(3) were investigated for high-temperature capacitor applications. Compositions close to 0.8BaTiO(3)-0.2Bi(Zn(1/2)Ti(1/2))O(3) revealed pseudo-cubic symmetry and showed a linear dielectric response. The existence of a nearly flat temperature dependence of the relative permittivity over the temperature range of 100 to 350°C was also obtained. In this study, the effects of cation non-stoichiometry and doping were investigated in an attempt to optimize the insulation resistance for high-temperature applications. The dielectric response of (Ba(0.8)-xBi(0.2))(Zn(0.1)Ti(0.9)) O(3) ceramics where 0 ≤ X ≤ 0.08, as well as ZrO2- and Mn(2)O(3)-doped ceramics were studied. The optimum compositions exhibited a relative permittivity in excess of 1150 with a low loss tangent (tan δ < 0.05) that persisted up to a temperature of 460δC. The temperature dependence of resistivity also revealed the improved insulation resistance of Ba-deficient compositions. Additionally, we suggest that an ionic conduction mechanism is responsible for the degradation of resistivity at high temperatures. The temperature coefficient of permittivity ((τ)K) and the RC time constant were also investigated.

Time dependent model of gain saturation in microchannel plates and channel electron multipliers

Abstract We present and discuss a time dependent solution of the transmission line model of a channel electron multiplier, introduced in a previous paper and already solved in steady-state conditions. The model is applicable to all the situations in which the multiplier input current is sufficiently large so that the statistical variations of the gain for each electron can be ignored and it does not apply to photon counting detectors. By introducing the appropriate boundary conditions the time dependent non-linear equations of the model are reduced to an integral equation in implicit form, whose solution can be calculated numerically by a perturbative approach. In this way the multiplying current signal i(z,t) and the voltage V(z,t) are found as functions of the position z along the channel, and of the time t during the pulse itself, for any arbitrary shape of the input current waveform. The important case of the amplification of input current pulses with a short duration compared to the multiplier recovery time is investigated in detail, showing that the non-linear behavior can be entirely described by a general function of a conveniently defined saturation parameter and that this function is characteristic of any uniform channel multiplier. The model is then used to investigate the recovery of the multiplier after a saturating pulse, and it is found that the gain recovery from weak or moderate saturation levels is exponential to a very good approximation, but with a time constant different from the characteristic time constant RC of the multiplier. Finally the case of pulses of arbitrary shape and duration is considered and examples are given of the amplification of step pulses and of a regular sequence of identical pulses. A remarkable feature of the model is that the solution can be calculated from the time shape of the output pulse, rather than from the input. This makes possible to implement methods for pulse restoration, i.e. for recovering the original input pulse shape from a measured saturated output.

Applicability of Metal/Insulator/Metal (MIM) Diodes to Solar Rectennas

The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

Graphene electric double layer capacitor with ultra-high-power performance

We have demonstrated, for the first time, efficient 120 Hz filtering by an electric double layer capacitor (EDLC). The key to this ultra-high-power performance is electrodes made from vertically oriented graphene nanosheets grown directly on metal current collectors. This design minimized both electronic and ionic resistance and produced capacitors having RC time-constants of less than 200 μs. Significantly, graphene nanosheets have a preponderance of exposed edge planes that greatly increase stored charge over designs relying on basal plane surfaces. Collectively these factors make vertically oriented graphene nanosheet electrodes ideally suited for producing high-frequency EDLCs. Capacitors constructed with these electrodes are predicted to be significantly smaller than aluminum electrolyte capacitors that they could functionally replace plus be manufactured using standard semiconductor process equipment, creating interesting commercial opportunities.

A novel non-iterative shape method for estimating the decay time constant of the finger photoplethysmographic pulse

The photoplethysmogram (PPG) of a pulse wave, similar in appearance to the arterial blood pressure (ABP) waveform, contains rich information about the cardiovascular system. The decay time constant RC, equal to the product of peripheral resistance R and total arterial compliance C, is a meaningful cardiovascular model parameter in vascular assessment. Using or ameliorating the existing ABP methods does not achieve a satisfactory estimation of RC from the PPG volume pulse (VRC). Thus, a novel non-iterative shape method (NSM) of evaluating VRC is introduced in this paper. The mathematic expression between a novel, readily available morphological parameter called the area difference ratio (ADR) and VRC was established. As it was difficult to calculate VRC from the complicated expression analytically, we recommend estimating it using a piecewise linear interpolation criterion. Also, since the effect of the PPG magnitude is eliminated in the calculation of ADR, precaliberation or normalization is dispensable in the NSM. Results of human experiments indicated that the NSM was computationally efficient, and the simulation experiments confirmed that the NSM was theoretically available for ABP.