Low Current Operation Research Articles

One-dimensional heat conduction model for an electrical phase change random access memory device with an 8F2 memory cell (F=0.15 μm)

A one-dimensional heat conduction model is developed for a phase change random access memory device with an 8F2 memory cell structure (F=0.15 μm). The required current level for a reset operation, which corresponds to the phase switching from a crystalline (“1” state) to an amorphous phase (“0” state) of Ge2Sb2Te5, was investigated by calculating one-dimensional temperature profiles for the memory cell structure. It is revealed that a reset operation is not achieved at the current level (2 mA) reported for existing devices with a subquarter micron plug size when only TiN is used as a resistive heater. However, it is possible when an additional heating layer of 5 nm thickness is inserted between the TiN and Ge2Sb2Te5 layers, for which the electrical resistivity ρelec is higher than 105 μΩ cm, and the thermal conductivity κ and specific heat c are as low as those of Ge2Sb2Te5. In addition, it is shown that a reset operation at a low current level of 1 mA can be realized in this memory cell when amorphous carbon (κ=0.2 W/m K and ρelec=106 μΩ cm) is used as an additional heating layer. It is believed that this relatively simple one-dimensional heat conduction model is a useful tool for analyzing the device operation of phase change random access memory devices and for selecting the proper conditions for an additional heating layer allowing for low-current operation.

Optical gain and saturation characteristics of quantum-dot semiconductor optical amplifiers

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs.

Low-Current, Xenon Orificed Hollow Cathode Performance for In-Space Applications

An experimental investigation of the operating characteristics of 3.2-mm diameter orificed hollow cathodes was conducted to examine low current and low flow rate operation. Cathode power was minimized with an orifice aspect ratio of approximately one and the use of an enclosed keeper. Cathode flow rate requirements were proportional to orifice diameter and the inverse of the orifice length. The minimum power consumption in diode mode was 10-W, and the minimum mass flow rate required for spot-mode emission was approximately 0.08-mg/s. Cathode temperature profiles were obtained using an imaging radiometer and conduction was found to be the dominant heat transfer mechanism from the cathode tube. Orifice plate temperatures were found to be weakly dependent upon the flow rate and strongly dependent upon the current.

Effect of cold wire inclination on temperature fluctuation measurements in a heated jet

Hot-wire anemometers at low operating currents are used as fast response resistance thermometers for the study of heated turbulent flows. Simultaneous measurement of temperature and velocity is generally performed with multi-wire arrays. In order to give good spatial resolution a new layout has been tested which uses an inclined temperature wire positioned parallel to the nearest inclined velocity wire. This leads to an asymmetric wire arrangement relative to the mean flow direction. As expected, a reduction in thermal interference from the velocity wires results when compared with an array containing a temperature wire placed normal to the flow. However, measurement of higher order moments of fluctuating quantities in an axisymmetric jet shows considerable distortion of radial distributions which is traced to alteration of the temperature field sensed by the temperature wire. When inclined velocity sensitive wires contain a temperature component, the latter may be affected by the same phenomenon.

L-band DFB laser diodes with output power of 106 mW at 1625 nm

For the first time, distributed-feedback lasers emitting at 1625 nm with high output power and low operation current are fabricated. Temperature-induced performance degradation is minimised by optimising carrier and optical confinement. Consequently, these L-band lasers show no additional degradation in threshold current with rising temperature compared to C-band lasers.

SMALL-SCALE InGaP/GaAs HETEROJUNCTION BIPOLAR TRANSISTORS FOR HIGH-SPEED AND LOW-POWER INTEGRATED-CIRCUIT APPLICATIONS

Small-scale InGaP/GaAs heterojunction bipolar transistors (HBTs) with high-speed as well as low-current operation are demonstrated. To reduce the emitter size SE and the base-collector capacitance CBC simultaneously, the HBTs are fabricated by using WSi/Ti as the base electrode and by burying SiO 2 in the extrinsic collector region. WSi/Ti metals simplify and facilitate processing to fabricate small base electrodes, and the buried SiO 2 reduces the parasitic CBC under the base electrode. The cutoff frequency fT of 156 GHz and the maximum oscillation frequency f max of 255 GHz were obtained at a collector current Ic of 3.5 mA for the HBT with SE of 0.5 μ m ×4.5 μ m , and fT of 114 GHz and f max of 230 GHz were obtained at IC of 0.9 mA for the HBT with SE of 0.25 μ m ×1.5 μ m . A 1/8 static frequency divider operated at a maximum toggle frequency of 39.5 GHz with a power consumption per flip-flop of 190 mW. A transimpedance amplifier provides a gain of 46.5 dB·Ω with a bandwidth of 41.6 GHz at a power consumption of 150 mW. These results indicate the great potential of our HBTs for high-speed. low power integrated circuit applications.

High-performance 1.55-μm wavelength GaInAsP-InP distributed-feedback lasers with wirelike active regions

High-performance 1.55-/spl mu/m wavelength GaInAsP-InP strongly index-coupled and gain-matched distributed-feedback (DFB) lasers with periodic wirelike active regions mere fabricated by electron beam lithography, CH/sub 4//H/sub 2/-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, whose index-coupling coefficient was more than 300 cm/sup -1/. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width mere investigated both theoretically and experimentally. A record low threshold current density of 94 A/cm/sup 2/ among 1.55-/spl mu/m DFB lasers was successfully obtained for a stripe width of 19.5 /spl mu/m and a cavity length of 600 /spl mu/m. Moreover, a record low threshold current of 0.7 mA was also realized at room temperature under CW condition for a 2.3-/spl mu/m-wide buried heterostructure with a 200-/spl mu/m-long cavity. Finally, we confirmed stable single-mode operation due to a gain-matching effect between the standing-wave profile and the wirelike active region.

The future of bipolar power transistors

Silicon based bipolar power transistor (BPT) as a switching power transistor has been replaced by other superior power devices in the past two decades. This transformation is primarily due to the poor performance of the BPT. Among many problems of the BPT, low current gain and small safe operation area (SOA) caused by the second breakdown have been most detrimental to silicon BPT's fate. However, BPT performance based on newer materials, such as wide bandgap semiconductors, has not been previously studied. This paper systematically compares the BPTs based on wide bandgap semiconductor materials. Device figures-of-merit for conduction and switching losses are proposed. Comparison of the BPT based on total power loss is then provided. Based on this work, it is concluded that BPTs based on wide bandgap materials overcome the critical disadvantages of silicon BPTs, and are capable of switching power operation at several hundred kilohertz frequencies at very high current densities and voltages. Therefore, BPTs based on wide bandgap materials are still very attractive switching power devices for the future.

Theoretical Analysis of GaInAsP/InP Multiple Micro-Cavity Laser

Fundamental lasing properties of a new type of laser, which consists of a multiple micro-cavity structure with vertical grooves, was theoretically investigated based on an improved perturbation feedback theory in combination with the transfer matrix method. In order to suppress a diffraction loss between semiconductor micro-cavities, an MMC laser having grooves buried with low loss polymer (Benzocyclobutene: BCB) was proposed. Attainable maximum reflectivity was calculated to be higher than 98% for quarter wavelength wide grooves buried with BCB polymer (refractive index of 1.546, number of the micro-cavity elements of 8), which was higher than that of the semiconductor micro-cavity/air groove periodic structure. As a result, a moderately low threshold current operation (1 mA for a stripe width of 1 µm) can be obtained for the total cavity length of 45 µm with the micro-cavity length of 4.90 µm (9 elements).

EBSD in FESEM and LVSEM

Abstract Background Electron backscattered diffraction (EBSD) is a very powerful analysis technique to understand the crystallography of a sample in the SEM. Typically, a high beam current is needed to obtain good results because of inefficient collection of the patterns. In W-filament SEM's, the high beam current necessitates a large spot size and requisite low image resolution. The large beam current can also be a source of problems for insulating samples or sample mounts by causing charging and the resulting image drift. To reduce the spot size with adequate beam currents for imaging, field-emission (FE) guns are used. For most FESEM's, the beam current is markedly reduced from that produced by W-filament instruments. Even though the image resolution is better, the EBSD performance tends to suffer at low beam currents for standard hardware configurations. To regain the performance of a comparable W-filament system, the EBSD hardware should be changed for the low beam current operation. This paper will address the hardware considerations for low beam current applications.

Role of Dislocations in InGaN-Based LEDs and Laser Diodes

UV InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG at both low- and high-current operation due to the lack of localized energy states formed by In composition fluctuations. In order to improve the lifetime of laser diodes (LDs), ELOG had to be used because the operating current density of the LDs is much higher than that of LEDs. A violet InGaN multi-quantum-well GaN/AlGaN separate-confinement-heterostructure LD was grown on ELOG on sapphire. The LDs with cleaved mirror facets shows an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 10 mW was more than 2,000 hours under CW operation at an ambient temperature of 60°C.

ROLE OF DISLOCATIONS IN InGaN-BASED LEDs AND LASER DIODES

UV InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG at both low- and high-current operation due to the lack of localized energy states formed by In composition fluctuations. In order to improve the lifetime of laser diodes (LDs), ELOG had to be used because the operating current density of the LDs is much higher than that of LEDs. A violet InGaN multi-quantum-well GaN/AlGaN separate-confinement-heterostructure LD was grown on ELOG on sapphire. The LDs with cleaved mirror facets shows an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 10 mW was more than 2,000 hours under CW operation at an ambient temperature of 60°C.

Room-Temperature Dephasing in InAs Quantum Dots

Summary form only given. Semiconductor quantum dots (QDs) are receiving increasing attention for fundamental studies on zero-dimensional confinement and for device applications. Quantum-dot lasers are expected to show superior performances, like high material gain, low and temperature-independent threshold current and chirp-free operation, due to the delta-like density of states (DOS). We have measured the dephasing time at room temperature of InAs QDs embedded in a waveguide to estimate the lower limit for the energy-broadening of the DOS given by the homogeneous linewidth. The sample consists of 3 stacked layers of InAs-InGaAs-GaAs quantum dots.

Low threshold current density operation of GaInNAs quantum well lasers grown by metalorganic chemical vapour deposition

A metal organic chemical vapour deposition grown GaInNAs quantum well laser emitting at 1.25 µm is reported. The lowest threshold current density obtained by 50 µm wide stripe lasers was 340 A/cm2 for a cavity length of 1420 µm, which is almost comparable to the lowest value reported for GaInNAs lasers grown by molecular beam epitaxy. The threshold current density per well was 170 A/cm2, which is the lowest threshold value reported to date.

Strain optimization for high differential gain and low current operation in 1.55 μm InGaAs/InGaAsP quantum well lasers

The effect of strain on threshold current and differential gain in 1.55 μm InGaAs/InGaAsP quantum well lasers is reviewed. A constant decrease in the threshold current with increasing stress is predicted if a conventional model of Auger recombination is used. We propose a more realistic model based on an accurate method for the derivation of the Auger recombination rate in quantum wells. Unlike the conventional theory, a realistic valence band structure along with Fermi–Dirac statistics and analytic expressions of the transition matrix element for both bound–bound and bound-unbound Auger processes are employed. Contrary to conventional modeling, computation results show an optimal compressive strain of approximately 1% that minimizes the threshold current and maximizes the differential gain. These results are in good agreement with the experimental results reported in the literature.

Ultraviolet InGaN and GaN Single-Quantum-Well-Structure Light-Emitting Diodes Grown on Epitaxially Laterally Overgrown GaN Substrates

Ultraviolet (UV) InGaN and GaN single-quantum-well-structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only under high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG even under low-and high-current operations, due to the lack of localized energy states formed by alloy composition fluctuations. When the emission wavelengths were in the blue and green regions, EQE was almost the same between LEDs on both ELOG and sapphire due to a large number of deep localized energy states formed by large alloy composition fluctuations. The localized energy states are responsible for the high efficiency of InGaN-based LEDs in spite of a large number of dislocations.

Operation of the LCC-type parallel resonant converter as a low harmonic rectifier

A single-phase high-frequency transformer isolated single-stage AC-to-DC controlled rectifier with low line current harmonic distortion using a variable-frequency controlled LCC-type (or series-parallel) resonant power converter (SPRC) is presented. A simple analysis and design procedure is used for designing the converter for low line current harmonic distortion and high power factor operation. The converter performance characteristics have been verified with SPICE3 simulations (without active control) and experimental prototype SPRC (rated at 150 W, with and without active control) for variation in load as well as line voltage. When operated with active current shaping, this converter operates in zero-voltage-switching mode for the complete range, maintaining power factor close to unity with low line current distortion and low peak current compared to the parallel resonant converter.

All-selective MOVPE-grown 1.3-μm strained multi-quantum-well buried-heterostructure laser diodes

Strained InGaAsP multi-quantum-well (MQW) double-channel planar-buried-hetero (DC-PBH) laser diodes (LDs) were fabricated by selective metalorganic-vapor-phase epitaxy (MOVPE). In the laser fabrication process, both the strained MQW active layer and current blocking structure were directly formed by selective MOVPE without any semiconductor etching process. The LDs are called all-selective MOVPE-grown BH LDs. The laser fabrication process can achieve both a precisely controlled gain waveguide structure and an excellent current blocking configuration, realizing the optimized DC-PBH structure. These aspects are essential to the high-performance and low-cost LD, which is strongly demanded for optical access network systems or fiber-to-the-home networks. This paper will show the excellent high-temperature characteristics for 1.3-/spl mu/m Fabry-Perot LDs which have a record threshold current of 18 mA with a low-operation current of 56 mA for 10 mW, and 74 mA for 15 mW at 100/spl deg/C with extremely high uniformity. Furthermore, reliable long-term operation at high temperature (85/spl deg/C) and high-output power of 15 mW has been demonstrated for the first time.

Low operating current and high-temperature operation of 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned structure

Low operating current and high-temperature operation has been demonstrated in 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned (RISA) structure. The RISA structure features an Al/sub 0.5/In/sub 0.5/P current blocking layer, which leads to small internal loss in the waveguide and substantially reduced operating carrier density. The resultant operating current for 50-mW continuous-wave at 70/spl deg/C is as low as 98 mA, which is almost a half of the lowest value ever reported.

Fabrication of metal field emitter arrays for low voltage and high current operation

This article describes a new fabrication process for a metal field emitter to achieve low voltage and high current operation. The key element of the fabrication process is that the isotropic silicon etching and oxidation process used in silicon tip fabrication are utilized for gate hole size reduction and gate oxide layer formation. A reliable structure with a small gate hole can be easily obtained. Metal field emitter arrays were fabricated in order to demonstrate the validity of the new process and submicron gate apertures were successfully obtained from 2.25-μm-diam disk patterns defined by a conventional contact mask aligner. The required gate voltage to obtain an anode current of a 0.1 μA/tip was 54 V, and emission currents above 20 μA/tip were stable at a gate bias of 96 V without any disruption.