Showing papers on "Switching time published in 1990"

Fatigue and switching in ferroelectric memories: Theory and experiment

Abstract: A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.

Proposal of novel electron wave coupled devices

Abstract: We propose a novel structure for electron wave devices that utilizes the mutual coupling effect between electron wave guides. The structure consists of two parallel electron wave guides with a coupling region controlled by a gate voltage. Using simple theoretical calculations, we estimate the switching time and the coupling length required for electron transfer to be 2 ps and 0.28 μm, respectively.

33 ps optical switching of symmetric self‐electro‐optic effect devices

Abstract: We describe a new method of operation of symmetric self‐electro‐optic effect devices (S‐SEEDs) using excitation pulses shorter than the diode sweep out times. We shall call this dynamic SEED switching. Previous SEEDs are optically bistable logic devices which use the unique properties of multiple quantum well structures placed in the intrinsic i regions of two p‐i‐n diodes reverse biased in series. Conventional operation of S‐SEEDs is at the exciton wavelength where absorption decreases with increasing electric field. Utilizing the dynamic properties of the S‐SEED we demonstrate that switching is possible at any wavelength where there is a nonlinear response showing a threshold. The major advantages of dynamic over conventional operation, is that it can be faster while requiring less optical energy, is automatically reset each cycle, and is less susceptible to saturation.

A 2 kbit array of symmetric self-electrooptic effect devices

Abstract: A 64*32 array of symmetric self-electrooptic effect devices, each of which can be operated as a memory element or logic gate, is discussed. The required optical switching energies of the devices were approximately 800 fJ and approximately 2.5 pJ at 6 and 15 V bias, respectively, and the fastest switching time measured was approximately 1 ns. Either state of the devices could be held with continuous or pulsed incident optical signals with an average optical incident power per input beam of approximately 200 nW or less than 1 mW for the entire array. Photocurrent and reflectivity were measured for all 2048 devices. Only one device failed to have the negative resistance required for bistability, and only nine of the devices fell outside a band of +or-20% of the mean. Additionally, over 200 devices in the array were operated in parallel using low-power semiconductor laser diodes. >

A low-voltage 8*8 Ti:LiNbO/sub 3/ switch with a dilated-Benes architecture

Abstract: An 8*8 switch array with a dilated-Benes architecture that greatly relaxes the crosspoint extinction ratio requirements needed to achieve low overall switch array crosstalk is discussed. This, combined with the low uniform switching voltages (9.2+0.2 V) of the 48 directional coupler crosspoints, facilitates high-speed low-crosstalk operation. The crosspoints can be switched in about 1 ns. The switch array is fully packaged with permanently attached single-mode fiber pigtails. The high data transfer rate inherent in lithium niobate switches in general, combined with the low crosstalk and high switching speed of this switch array, is a good match to the requirements of time-multiplexing switching. >

Frequency synthesizer allowing rapid frequency switching

Abstract: A frequency synthesizer is designed to modify voltage value data read out of a memory in accordance with the output voltage of a filter, and apply a d.c. voltage corresponding to the modified data to a second variable-capacitance diode at the control input of a VCO, thereby allowing the VCO frequency switching without imposing a significant fluctuation of the application voltage to a first variable-capacitance diode and accomplishing a short channel switching time and stable operation against temperature fluctuation. The device is further designed to supply a voltage setup value read out of the memory to the second variable-capacitance diode thereby to reduce the frequency matching time at channel switching, and to determine such another voltage setup value as to reduce the time of phase matching based on the trend of change in the filter output voltage and supply the value to the second variable-capacitance diode.

Modeling GaAs high-voltage, subnanosecond photoconductive switches in one spatial dimension

Abstract: Subnanosecond high-voltage gallium arsenide photoconductive switches are studied to understand how to improve their switching speed, efficiency, and durability. Two possible mechanisms for such switching are discussed: field-compression-induced ionization of valence states and field-dependent trapping of charge carriers. Analysis and computations suggest that field compression is limited to roughly a factor of two at low initial fields. This shows that one cannot achieve arbitrary amounts of field enhancement and so obtain avalanche-like performance at will. At initial field strengths of 10 to 50 kV/cm, Gunn instabilities produce large field compression, but carrier trapping and recombination quench intrinsic photoavalanching, according to the calculations presented. Observed avalanching may therefore be due to extrinsic effects related to deep levels. As an alternative to intrinsic impact ionization, it is shown that field-dependent trap filling can yield an avalanche-like rise in current and may account for two other experimental observations, the existence of a voltage threshold and a voltage-dependent delay between the start of illumination and the occurrence of switching. >

Direct digital synthesizer/direct analog synthesizer hybrid frequency synthesizer

Abstract: A digital/analog hybrid frequency synthesizer having a digital frequency synthesizer for digitally generating an analog output signal of a predetermined frequency within a frequency range of f to f+Δf; an input stage analog frequency synthesizer for, receiving the digital frequency synthesizer output signal and I analog input stage input signals, each input stage input signal separated in frequency from a next one by a frequency increment of Δf wherein a first one and a last one of the input stage input signals are respectively of a frequency of f A and f A +(I-1)Δf, selecting one input stage input signal from the input stage input signals, mixing the selected input stage input signal with the digital frequency synthesizer output signal and providing a resultant input stage output signal; and an output stage analog frequency synthesizer for, receiving the input stage output signal and K analog output stage input signals, each output stage input signal separated in frequency from a next one by a frequency increment of IΔf wherein a first one and a last one of the output stage input signals are respectively of a frequency of f N and f N +(K-1)IΔf, selecting one output stage input signal from the output stage input signals, mixing the selected output stage input signal with the input stage output signal and providing a resultant output stage output signal. Additional intermediate frequency stages may be employed to facilitate greater bandwidth expansion.

Output buffer having capacitive drive shunt for reduced noise

Abstract: An output buffer maintains low noise across a range of process variations, temperatures, and voltages. This is achieved by limiting the drive signal so as to reduce the switching speed of the output buffer as the other variables tend to increase the speed. This is accomplished by limiting the current through the pre-driver stage by controlling the conductance of a shunt transistor connected to a grounded capacitor.

Wavelength tunable optical filter

Abstract: A wavelength tunable optical filter having a characteristic single transmission peak for enhanced tunability comprises a phase control section positioned between a Distributed Bragg Reflector section and a high reflectivity mirror. The Distributed Bragg Reflector section comprises a periodic region grating positioned over an end portion of a passive waveguide whereas the phase control section comprises the portion of the passive waveguide region located between the grating and the mirror. Continuous tunability in the range of several THz may be achieved by controlling the refractive index in either the phase control or distributed Bragg reflector section. The optical filter is expected to have a frequency switching time of a few nanoseconds and, thus, is applicable to both transmission and switching applications.

Fast Optical Switching in Polymer Waveguide Using Ferroelectric Liquid Crystal

Abstract: A fast electro-optical switching in polymer waveguide has been demonstrated using ferroelectric liquid crystal. In this device, the surface stabilized ferroelectric liquid crystal layer is used as the active material with the passive polymer waveguide. The switching time is of the order of several microseconds.

High resolution direct digital synthesizer

Abstract: The frequency resolution of a direct digital synthesizer is increased by varying the phase increment used to accumulate wave lookup table addresses between two integer values. The time periods during which the larger integer value is employed is proportional to a fractional part of the desired phase increment for providing an analog output at a selected frequency.

Photonic wavelength-division and time-division hybrid switching system utilizing coherent optical detection

Abstract: The application of coherent optical detection to a photonic wavelength-division (WD) and time-division (TD) hybrid switching system for a large-capacity switching is studied. Wavelength switching time is 1.8 ns, and the wavelength is stable after wavelength switching as wide as 3-4 AA. The results suggest the feasibility of switching over 20 channels (TD 4 channels, WD approximately 6 channels) for 100-Mb/s signals. This channel number can be increased to 256 channels (WD 64 channels) in view of the maximum wavelength sweep range (40 AA) of wavelength tunable laser diodes obtained so far. >

All‐optical switching of picosecond pulses in a GaAs quantum well waveguide coupler

Abstract: A time‐resolved pump‐probe configuration employing 10 ps laser pulses has been used to investigate the all‐optical switching characteristics of zero‐gap directional couplers containing a single quantum well. All‐optical switching of the weak probe pulses from one port into the adjacent one was obtained at pump input pulse energies of 150 pJ. The switching takes place within the duration of the laser pump pulses. The recovery time of the device was found to be ∼1.5 ns which indicates that carrier diffusion plays a major role in the device switching speed.

Liquid crystal optical routing switch.

Abstract: The design and experimental results of a new 2 × 2 optical routing switch based on liquid crystal is reported. The switch uses nematic liquid crystal for polarization beamsplitting and ferroelectric liquid crystal for polarization rotation. The polarization insensitive, broadcast capable, full duplex switch has a signal-to-crosstalk ratio of ~20 dB, a switching speed below 250 μs and a measured insertion loss of 1.4 dB. We show that the basic 2 × 2 switch can be extended to form a N × N rearrangeable permutation network.

Modeling and characterization of embedded electrode performance in transverse electrooptic modulators.

Abstract: A mathematical model with experimental verification is presented to characterize the performance of surface and embedded electrodes in 2-D electrooptic modulators. From the solution of a discretized integral equation for the electrode surface charge, the electrode capacitance and the electric field penetration and uniformity are related to the switching voltage, speed, and uniformity of the electrooptic modulation. Fabricated surface and embedded electrodes in 9/65/35 PLZT are then evaluated with respect to the predictions of the model and the saturated quadratic response of the electrooptic material. These results provide important insight into the design trade-offs of switching speed, halfwave voltage, switching energy, and modulation uniformity of surface and embedded modulator geometries.

Switching-time limitation in tunable multisection lasers

Abstract: The impact of the thermal transient on the frequency switching of multisection tunable lasers is studied. A simple thermal model is used to calculate the amplitude and time evolution of the frequency transient due to the thermal properties of both the laser chip and its mount. Transient time constants as large as 200 mu s for the laser chip and several hundred milliseconds for the diode mount are measured, limiting the applicability of these devices to systems where the frequency is switched at low rates. A method to compensate electrically for this transient by means of a passive net work is demonstrated. >

Synthesized selective call receiver having variable characteristics

Abstract: A paging receiver has a synthesizer for governing the receive frequency. The paging receiving further has characteristics which are varied in response to the receive frequency. These characteristics include varying the bandwidth of a loop filter within a phase lock loop within the synthesizer as well as varying the time in which a detector circuit used to extract a DC level from a recovered audio signal is disabled. Furthermore, the bandwidth of the loop filter is varied in response to switching from a first receive frequency to a second receive frequency in order to provide for either a uniform frequency lock time or for a rapid frequency lock time. Furthermore, the time in which the detector circuit is disabled is correspondingly changed.

Analysis of the device performance of quantum interference transistors utilizing ultrasmall semiconductor T structures

Abstract: We present a theoretical study of a recently proposed class of quantum interference transistors that utilize quantum interference effects in ultrasmall semiconductor T structures. Our analysis reveals that the attractive features of these transistors are the very low power‐delay product and multifunctionality; whereas the major drawbacks are extreme sensitivity of the device characteristics to slight structural variations, low gain, and low extrinsic switching speed in digital circuits caused by a large resistance‐capacitance (RC) time constant arising from an inherently low current‐carrying capability. The low switching speed of the transistors can however be improved dramatically by switching the device optically rather than electronically, using virtual charge polarization caused by optical excitation. This mode of switching (which is possible because of the small value of the threshold voltage) eliminates the RC time constant limitation on the switching time and results in an ultrafast optoelectronic ...

Electro-optic imaging of the internal fields in a GaAs photoconductive switch

Abstract: A technique has been developed that allows single‐shot measurement of the electric field over an extended area. A LiTaO3 crystal, covering the entire active area of the GaAs photoconductive switch, coupled the surface field with the polarization of an optical probe pulse. When imaged onto a two‐dimensional detector array, this system produced snapshots of the surface field profile with 200‐ps time resolution and 3‐μm spatial resolution. The technique was used to monitor the collapse of the electric field in the GaAs as it was switched with a λ=1.06 μm optical pulse. The switching speed and generation of field enhancements were found to be dependent on the illumination configuration and the electric field. In particular, the switching efficiency was found to decrease with increasing field.

Dielectric anisotropy and dielectric torque in ferroelectric liquid crystals and their importance for electro‐optic device performance

Abstract: Measurements of the relevant anisotropic part of the dielectric constant for ferroelectric liquid crystals allows a calculation of the dielectric torque acting upon the director during addressing conditions in a matrix‐multiplexed device. The frequency dependence of the dielectric anisotropy is presented for different commercially available ferroelectric liquid‐crystal materials in the range 103–106 Hz. The field dependence of the switching time has been measured for one of the materials in order to determine the field at the minimum value. The ratio between this field and the field at which switching ceases has been determined and found to be in good agreement with the theoretically calculated value. The results enable us to consider the torque balance as a function of field and frequency, and thereby identify appropriate driving conditions.

Fast thin film vanadium dioxide microwave switches

Abstract: A new concept in microwave and millimeter-wave switching structures is demonstrated. The switch is built on a GaAs substrate and is compatible with IC technology. The approach used to implement the concept is to place a variable-resistance element spanning a very short gap (compared to the width of the line) on a metal transmission line. Switching the gap material from the high resistance state to the low resistance state turns the line from an open circuit to the connected state. The topics discussed include the material system used, a specific demonstration of a microwave switch using this system, possible applications of the switch, and its present limitations and how they could be overcome. A switch is built which at 40 GHz has an insertion loss of less than 1 dB in the on-state and more than 12 dB in the off-state. Switching time is under 30 ns. >

Method for controlling the switching speed of bipolar power devices

Abstract: The switching speed of bipolar power rectifiers is increased by formation of misfit dislocations in the depletion region, spaced from the substrate/epitaxial layer interface, in order to reduce minority carrier lifetime. The misfit dislocations are formed by the introduction of germanium during epitaxy, and are distributed along the silicon/silicon-germanium interface. Preferably, the germanium containing layer is located proximate the center of the depletion region.

Scaling Properties in the Electrical and Reliability Characteristics of Lead-Zirconate-Titanate (PZT) Ferroelectric Thin Film Capacitors

Abstract: This paper investigates the issues in the scaling of thin film PZT (Lead-Zirconate- Titanate) capacitors for DRAM (Dynamic Random Access Memories) applications. The test structures used were MIM (metal-insulator-metal) capacitors with platinum electrodes and PZT deposited using a sol-gel process. Charge storage density (Q’c), leakage current density (JL), unipolar switching time to 10% decay (ts), time dependent dielectric breakdown (TDDB) and electrical fatigue have been analyzed. Unipolar switching time has been modeled as an RC time constant, where C is electric-field dependent. Q’c at a given electric field appears to remain constant over the range of film thicknesses and electrode areas studied. Leakage current density and time-to-breakdown (tBD) for a given electric field degrade with decreasing film thickness. Unipolar stressing causes considerably less fatigue than bipolar stressing, and after 2 X 1011 cycles, a 400nm film still exhibits sufficient Q’c for DRAM operation.

Simultaneous determination of fundamental parameters for ferroelectric liquid crystals

Abstract: An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.

Fast bistable wavelength switching characteristics in two-electrode distributed feedback laser

Abstract: Fast bistable wavelength switching between two longitudinal modes separated by 1.7 nm was achieved in a two-electrode distributed feedback laser with inhomogeneous excitation. As a result, a fast wavelength switching time less than 200 ps was observed in this device. In addition, high-speed repetitive wavelength switching operation up to 1 GHz was achieved. This indicates that the device is capable of repetitive wavelength switching within 1 ns, including delay time and rise time. >

Latching type single-mode fibre switch

Abstract: A compact latching type 1 × 2 single-mode fibre switch employing a micro magnetic alloy pipe is described. The switch exhibits an insertion loss of 0.9 dB, driving power of 9mW with a switching time of <6ms, and a reproducibility of < ±0.1dB through 2 × 107 switching operations.

Electrooptic imaging of surface electric fields in high-power photoconductive switches

Abstract: A two-dimensional electrooptic probe was used to image the electric field in high-power photoconductive switches during switch operation. A LiTaO/sub 3/ crystal, covering the entire active area of the photoconductive switch, coupled the surface electric field with the polarization of an optical probe pulse. The optical probe was imaged onto a two-dimensional detector array, producing snapshots of the surface field with 200-ps time resolution and 3- mu m spatial resolution. The probe system was used to monitor the electric field in the electrode gap of high-voltage switches, the focus of investigation being the collapse of the electric field in a GaAs bulk switch as it was switched with a lambda =1.06 mu m optical pulse. The switching speed and generation of field enhancements were found to be dependent on the illumination configuration and the applied electric field, which varied from 0 to 15 kV/cm. Switching efficiency was found to decrease with increasing field. >

Dynamics of optical and electrooptical switching in photo-thermal bistable CdS crystals

Abstract: We investigate the dynamical properties of photo-thermal Self Electrooptic Effect Devices (SEEDs) fabricated of thin single crystal platelets in a wide region of excitation intensities at room temperature. Detailed characteristics of the switching processes and dynamics are precented for optical and for the first time for electro-optical types of bistable operation using an improved contact geometry. The role of the substrate of the SEED is also taken into account. It is shown that in the case of rectangular excitation pulses of light or of voltage the switching process can be divided into two regions, namely the crystal reaction time and the switching time itself. Both depend sensitively on the applied optical or electrical pulse height and also on the initial preheating. Critical slowing down is observed in the optical and the electrical case.

Phase locked loop frequency modulator using data modulated digital synthesizer as reference

Abstract: In a frequency-modulated frequency generator comprising an oscillator connected in a phase control loop, the reference frequency is generated in a digital frequency synthesizer which is adapted to be frequency-modulated by digital data signals; under control of the data signal, numerical values are alternately read out from a register to result in corresponding different frequency deviations; the register stores further predetermined additional numerical values which result in frequencies adjacent to said frequency deviations; the register is controlled by the data signal so that for every edge of the data signal predetermined additional numerical values are initially read out in such an order that the frequency values produced thereby will compensate for any overshooting which occurs upon switching between the drift frequencies.