Charge Packets Research Articles

Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cm dc fields

This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model.

CCD-based range-finding sensor

Integration-time-based, time-domain computation provides an area-efficient way to process image information by directly handling photo-created charge during photo-sensing. We have fabricated and tested a CCD-based range-finding sensor which uses the time-of-flight method for range measurement. The sensor exploits two charge packets for light integration and detects the delay of the received light pulse relative to the transmitted light pulse. It has detected a 10 cm distance difference at the range of 150 cm in the dark background.

Packet-like space charges and conduction current in polyethylene cable insulation

Packet-like space-charge behavior in polyethylene for cable insulation was studied by utilizing the laser-induced pressure-pulse technique. Space charge observation under various conditions showed that the charge packet was formed in the specimen doped with antioxidant, especially when the specimen is oxidized. Periodic formation and transport of charge packets led to a small current oscillation. The charge packet seemed to be formed in the bulk, and the internal field of 1.2 to 1.4 MV/cm, in the region where the packet was formed was periodically enhanced from 2 to 2.5 MV/cm by space-charge accumulation. This suggests that the packet formation is caused by carrier generation under the highly enhanced field. The antioxidant deteriorated by oxidation was shown to be a possible origin of the carrier generation.

VLSI implementation of pulsating neural networks

We describe a generic approach for realizing networks of pulsating neurons based on charge pumping of interface states situated in the channel of MOS transistors. Two basic building blocks will be described: the pulse activated charge pumping (PSCP) synapse, and the charge sensitive oscillator (CSO). The PSCP synapse which operates as either a short or a long term memory device which produces a charge packet proportional to the number of pulses applied to its input, will be described in detail together with experimental results demonstrating its capability. The CSO circuit which is a charge controlled oscillator will be described together with simulations of its output frequency dependence on its input voltage, and the relation between the temporal dependence of output waveform on its input charge.

Time of flight analysis of charge transport in insulators for the nondispersive trap limited domain

We consider the unipolar carrier transport problem through an insulating media with traps. The charge packet drifts under the influence of an external electric field applied between two electrodes. We solve the Poisson equation exactly and find the electric field distribution in the device. The displacement current on the electrodes is obtained for the case of a non-dispersive thin charge packet, with deep trapping characterized by a single time constant τ. A generalization of the induced charge “splitting theorem” in such devices is obtained for the trapping case.

The double-sided floating-surface detector: an enhanced charge-detection architecture for CCD image sensors

A new high speed, low noise, nondestructive charge detector, called the Double-Sided Floating-Surface Detector (DS-FSD), which is fabricated in a standard CCD image sensor process, is reported. This detector can be integrated in CCD image sensors and is capable of detecting large charge packets at very low noise levels. Typical values are 5-8 noise electrons (within a bandwidth of 5 MHz) for a charge packet size of 100000 to 250000 electrons. The detector is used as the first MOS transistor in a three-stage source-follower configuration with a bandwidth of 150 MHz. The performance of both the traditional Floating-Surface Detector and Double-Sided Floating-Surface Detector (DSFSD) are calculated using a new, simple, model, This model is experimentally verified.

A 9-b charge-to-digital converter for integrated image sensors

Charge-to-digital conversion offers advantages over conventional charge readout techniques because it performs digitization directly in the charge domain. The approach consolidates hardware, reduces power and weight, and eliminates many sources of noise and nonlinearity. This paper introduces an architecture for a charge-to-digital converter (CDC) that is tailored toward a charge-coupled device (CCD) implementation. New methods of generating charge, sensing charge, and comparing charge packets are described that improve conversion accuracy. Factors limiting device performance are discussed. Measured results are presented for two prototype CDCs. The first, using buried channel CCDs, is optimized for resolution. It achieves 56 dB spurious free dynamic range (SFDR) at a 2 MHz sampling rate and operates from 5 V. The second, using surface channel CCDs, is optimized for power and speed. It achieves 49 dB SFDR at a 15 MHz sampling rate and consumes 13 mW power at its maximum sampling rate of 22 MHz.

A 22-kHz multibit switched-capacitor sigma-delta D/A converter with 92 dB dynamic range

A time- and capacitor-multiplexing technique for use in a highly linear switched-capacitor multibit DAC in sigma-delta data converters is presented. The technique uses subintervals in the sample clock to deliver multiple charge packets to holding capacitors. It avoids distortion effects caused by mismatched capacitors and finite opamp gain. A five-level switched-capacitor DAC using the proposed technique was designed as part of an audio-band multibit sigma-delta D/A converter that achieved a dynamic range of 92 dB and a THD of -93 dB with a low oversampling ratio of 32. No trimming, calibration, or dynamic matching scheme was required. The five-level SC DAC has been fabricated in a 2-/spl mu/m CMOS process, and testing confirmed the anticipated theoretical results.

A technique to measure trap characteristics in CCD's using X-rays

An important type of radiation damage in CCD's used for X-ray spectroscopy is the degradation of charge transfer efficiency (CTE). Traps associated with radiation-induced defects are the basic cause of the damage. Here, we describe a method to extract trap characteristics using small charge packets produced by individual X-ray photon interactions in rectangular imaging CCD's. The method applies the principles of trap occupancy to the framestore CCD configuration, and uses data from CCD's operating in their normal transfer mode. We have measured trap characteristics in radiation damaged CCD's in a range of operating temperatures from 170-200 K, and have found that these data compare well to the expected phosphorus-vacancy (P-V) trap characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A proposed SEU tolerant dynamic random access memory (DRAM) cell

A novel DRAM cell technology consisting of an n-channel access transistor and a bootstrapped storage capacitor with an integrated breakdown diode is proposed. This design offers considerable resistance to single event cell errors. The informational charge packet is shielded from the single event by placing the vulnerable node in a self-compensating state while the cell is in standby mode. The proposed cell is comparable in size to a conventional DRAM cell, and computer simulations show an improvement in critical charge of two orders of magnitude over conventional 1-T DRAM cells. >

Displacement currents by propagating charge packets in capacitor structures: Part II: Analysis of the dispersive case

An analytical derivation for the currents induced by charge packet drift ininsulator devices is presented. The theoretical formalism allows for quantization of the time of flight (TOF) current shapes in the special case of general (dispersive) charge spreading. The results are used to extract dispersion parameters of the electrical transport in insulating media from TOF experiments.

Rapid measurement of threshold non-uniformity in ccd multiplexers for hybrid focal plane arrays

A new, simple and rapid method of measurement for threshold non-uniformity in CCD multiplexers for hybrid Focal Plane Arrays (FPAs) is presented. The method is based on simple oscilloscope measurement of non-uniformity in the readout of CCD multiplexers wherein identical charge packets are loaded on the whole array through background suppression drain.

Displacement currents by propagating charge packets in capacitor structures I: Basic formalism applied to the case of a thin charge packet

In this paper the displacement current dependence on the detailed electric field spatiotemporal dependence is revisited in connection with charge transport current forms obtained in capacitive structures by the time-of-flight (TOF) technique. The formalism developed may be used to analyze experimental TOF currents in insulating devices where small packets of charges in transit constitute their main operating process. In particular, it is shown that the “Shockley-Ramo” current theorem is a special case of the displacement current concept in such devices.

Performance of modulation-doped charge-coupled devices (MD-CCD's) in the microwave and millimeter-wave bands

A two-phase, modulation-doped charge coupled device (MD-CCD) has been characterized by both phase shift and charge transfer efficiency (CTE) measurements from 1.25 MHz to 16.4 GHz. Both two-dimensional transient simulations and experimental evidence support the conclusion that the cutoff frequency for transport of discrete charge packets emulates the cutoff frequency of small signal HEMT devices in the short gate length regime. These simulations predict a CTE of almost 0.999 at 40 GHz for an In/sub 0.53/Ga/sub 0.47/As channel device. The device is fabricated using conventional MMIC processing techniques. In addition, measurement methods used for characterization of a prototype 5-stage delay line chip agree well with simulations using a new CCD SPICE model. >

Direct observation of time-dependent space charge profiles in XLPE cable under high electric fields

Space charge distribution was measured, applying an electric field of >1 MV/cm to 3 mm thick XLPE cable insulation. Improving the conventional pulse electro-acoustic method, a new method was developed to measure space charge when applying HV to cables. Under high field, hetero charges were formed soon after voltage application, followed by an injection from the cathode. Then intermittent injections of charge packets from the anode took place. The space charge distribution kept changing without becoming stabilized. A simulation was done assuming hysteresis of the injection characteristics. The observed intermittent injection near the anode was qualitatively reproduced using computer simulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An orthogonal-transfer CCD imager

We describe a new two-dimensional CCD imager structure capable of transferring charge packets in all four directions, as well as experimental results on a 64/spl times/64-pixel prototype device. Applications include astronomical imaging where the charge shifting can be made to track the tip-tilt correction for atmospheric turbulence and thereby improve image resolution. >

Modeling imager deterministic and statistical modulation transfer functions

A unified two-step approach for evaluation of deterministic and statistical modulation transfer functions (MTF's) is applied to a time-delay-and-integration charge-coupled imager. The deterministic MTF's include the well-known spatial and temporal aperture MTF's, as well as the charge-coupled imager interpixel and intrapixel synchronism MTF's, which are derived here. These latter MTF's originate from nonsynchronous motion (velocity mismatch) between the image on the focal plane and the charge packets. The statistical evaluation results in phase and jitter MTF's.

Study of photogenerated charge carrier dispersion in chlorinated a-Se:0.3%As by the interrupted field time-of-flight technique

Dispersion of photogenerated charge carriers due to their mutual Coulombic repulsion hitherto has been neglected in considering the spread of the photocurrent in a typical drift mobility experiment such as the time-of-flight transient photoconductivity measurement. Interrupted field time-of-flight experiments carried out on a high-resistivity semiconductor material (a-Se) with a well-defined mobility clearly show that the overwhelming mechanism for the dispersion of the charge packet in the absence of an applied field is due to Coulombic repulsion. Dispersion is directly proportional to time and the amount of photogenerated charge and puts a lower limit to the resolution of photodetector devices operating in the constant mobility regime.

Numerical study of charge and spin separation in low-dimensional systems.

We study numerically the charge and spin separation in low-dimensional systems with local interactions. We consider finite Hubbard systems in one and two dimensions and calculate the ground state. An electron is added to the system in a spatially localized state. The time evolution of this state is obtained and the charge and spin densities are evaluated for different times. In one dimension the charge and spin-wave packets propagate with different velocities. The results are interpreted in terms of the Bethe-ansatz solution. In two dimensions the effect of charge and spin separation is not clearly observed, probably because of the small size of the system.

VLSI implementation of pulse activated synapses

A generic circuit, the pulse stimulated charge pumping (PSCP) synapse, is presented. The PSCP synapse is a device based on charge pumping of interface states: it produces at its output a charge packet proportional to the number of pulses applied to its input, and to the electrically programmed density of interface states. It is particularly suitable for realising artificial neural networks in which the neuron activity is implemented as a pulse train, closely resembing the behaviour of biological neural networks. The PSCP synapse can be realised easily in less than 50 μm 2 , and has a large dynamic range. Preliminary experimental results are presented showing the current against frequency behaviour of the device for various densities of interface states