Morphological changes of capillaries in the rat soleus muscle following experimental tenotomy

We examined the structural changes of capillaries in the rat soleus muscle 4, 7, 14, and 35 days after experimental limb tenotomy. In the soleus muscles after tenotomy, muscle fibres degenerated and some of them were destroyed; the muscle did not seem to recover until the 35th day. On the 14th day, some small muscle fibres, probably regenerating muscle fibres, started forming within the basal-lamina tube and remained after necrosis of a pre-existing muscle fibre. Most capillaries at each stage were of the continuous type. However, about 10% of the capillaries around degenerated muscle fibres at days 4, 7 and 14 consisted of endothelial cells with a small number of fenestrae bridged by a single-layered diaphragm. On the 14th day, capillaries around small regenerating muscle fibres also often had a small number of fenestrations. Even on the 35th day, capillaries occasionally had fenestrations. Additionally, some of the fenestrated capillaries formed small pores at the fenestrated portion of the endothelial cells. The untreated muscles contained only continuous capillaries. These findings suggest that fenestrations in the endothelial cells may occur in intramuscular capillaries not only around degenerated muscle fibres but also around regenerating muscle fibres after tenotomy.     

Physical identification of gate metal interdiffusion in GaAs PHEMTs

The Ti metal interdiffusion of Ti/Pt/Au gate metal stacks in 0.15-/spl mu/m GaAs PHEMTs subjected to high-temperature accelerated lifetest has been physically identified using scanning transmission electron microscopy. Further energy dispersive analysis with X-ray (EDX) analysis confirms the Ti diffusion into the AlGaAs Schottky barrier layer and the decrease of Schottky barrier height suggests the Ti-AlGaAs intermetallic formation, which is consistent with previous Rutherford backscattering spectroscopy/X-ray photoelectron spectroscopy studies. The Ti metal interdiffusion reduces the separation of the gate metal and InGaAs channel, thus leading to a slight Gm increase, positive shift in pinchoff voltage, and S21 increase during the preliminary portion of the lifetest. Accordingly, the Ti interdiffusion effect implies that the lifetime of GaAs PHEMTs subjected to high-temperature accelerated lifetest could be dependent upon the initial thickness of the Schottky layer underneath the gate metal.     

Degradation of AlGaN/GaN HEMTs under elevated temperature lifetesting

Elevated temperature lifetesting was performed on 0.25 μm AlGaN/GaN HEMTs grown by MOCVD on 2-in. SiC substrates. A temperature step stress (starting at T_a of 150 °C with a step of 15 °C; ending at T_a of 240 °C; 48 h for each temperature cycle) was employed for the quick reliability evaluation of AlGaN/GaN HEMTs. It was found that the degradation of AlGaN/GaN HEMTs was initiated at ambient temperature of 195 °C. The degradation characteristics consist of a decrease of drain current and transconductance, and an increase of channel-on-resistance. However, there is no noticeable degradation of the gate diode (ideality factor, barrier height, and reverse gate leakage current). The FIB/STEM technique was used to examine the degraded devices. There is no detectable ohmic metal or gate metal interdiffusion into the epitaxial materials. Accordingly, the degradation mechanism of AlGaN/GaN HEMTs under elevated temperature lifetesting differs from that observed in GaAs and/or InP HEMTs. The reliability performance was also compared between two vendors of AlGaN/GaN epilayers. The results indicate that the reliability performance of AlGaN/GaN HEMTs could strongly depend on the material quality of AlGaN/GaN epitaxial layers on SiC substrates.     

Demonstration of bufferless optical packet switch with recursive stages of parametric wavelength converter

We demonstrate in an experiment an optical packet switching (OPS) using recursive parametric wavelength converters (PWCs) which uses the combination of two or more PWCs for wavelength conversions. The PWCs, which are based on four-wave mixing in highly nonlinear fibers, are possible for multiple wavelength conversion, allowing a share-per-node switching scheme, and thus significantly reducing the number of wavelength converters. Detailed demonstration of a developed OPS prototype using the joint of two PWCs indicates the generation of various wavelength conversion patterns which play an important role for OPS performance. The converted signals after two stages of PWC are obtained with error-free operations and low power penalties (BER=10^?9) of about 2.0 dB. Numerical results also show significant improvement in packet blocking probability by the proposed recursive PWC-based OPS in comparison with the previous non-recursive schemes.     

A novel monolithic HBT-p-i-n-HEMT integrated circuit with HBTactive feedback and p-i-n diode variable gain control

We report the world's first functional MMIC circuit integrating HBT's, HEMT's, and vertical p-i-n diodes on a single III-V substrate. The 1-10 GHz variable gain amplifier monolithically integrates HEMT, HBT, and vertical p-i-n diode devices has been fabricated using selective MBE and a merged processing technology. The VGA offers low-noise figure, wideband gain performance, and good gain flatness over a wide gain control range. A noise figure below 4 dB was achieved using a HEMT transistor for the amplifier stage and a wide bandwidth of 10 GHz. A nominal gain of 10 dB was achieved by incorporating HBT active feedback techniques and 12 dB of gain control range was obtained using a vertical p-i-n diode as a varistor, all integrated into a compact 1.5×0.76 mm² MMIC. The capability of monolithically integrating HBT's, HEMT's, and p-i-n's in a merged process will stimulate the development of new monolithic circuit techniques for achieving optimal performance as well as provide a foundation for high performance mixed-mode multifunctional MMIC chips     

A DC-20-GHz InP HBT balanced analog multiplier for high-data-ratedirect-digital modulation and fiber-optic receiver applications

This paper reports on a dc-20-GHz InP heterojunction bipolar transistor (HBT) active mixer, which obtains the highest gain-bandwidth product (GBP) thus far reported for a direct-coupled analog mixer integrated circuit (IC). The InP HBT active mixer is based on the Gilbert transconductance multiplier cell and integrates RF, local oscillator, and IF amplifiers, High-speed 70-GHz f_T and 160-GHz f_max InP HBT devices along with microwave matching accounts for its record performance. Operated as a down-converter mixer, the monolithic microwave integrated circuit achieves an RF bandwidth (BW) from dc-20 GHz with 15.3-dB gain and benchmarks a factor of two improvement in GBP over state-of-the-art analog mixer ICs. Operated as an up-converter, direct-digital modulation of a 2.4-Gb/s 2³¹ -1 pseudorandom bit sequence (PRBS) onto a 20-GHz carrier frequency resulted in a carrier rejection of a 28 dB, clock suppression of 35 dBc, and less than a 50-ps demodulated eye phase jitter. The analog multiplier was also operated as a variable gain amplifier, which obtained 20-dB gain with a BW from dc-18 GHz, an third-order intercept of 12 dBm, and over 25 dB of dynamic range. A single-ended peak-to-peak output voltage of 600 mV was obtained with a ±35-mV 15 Gb/s 2⁵-1 PRES input demonstrating feasibility for OC-192 fiber-telecommunication data rates. The InP-based analog multiplier IC is an attractive building block for several wideband communications such as those employed in satellites, local multipoint distribution systems, high-speed local area networks, and fiber-optic links     

Numerical simulation of a pulsed laser pumped distributed-feedback waveguided dye laser by coupled-wave theory

A dynamic model of pulsed laser pumped distributed-feedback (DFB) waveguided dye laser based on a coupled-wave theory is described. Due to the periodical distribution of the intensities of pump source and stimulated emission along the waveguide, the rate equations of the population densities are turned into the equations of the Fourier coefficients. Coupled-wave equations of optical fields are used to simulate the laser oscillation. Besides the temporal evolution of the output intensity, the spectra can also be obtained by the Fourier transform of the optical fields. Two different configurations of the waveguided dye laser, prefabricated DFB (mainly index coupling), first- and second-order holographic DFB (dynamic gain-coupling), are considered in the model. The simulation shows that: 1) the temporal waveforms of the holographic DFB consist of sharp spikes; 2) the broadened spectral widths resulted from the possible nonuniformities in propagation constant or grating period are less than 50 pm except for the second-order holographic DFB; and 3) strong parasitic oscillations can be observed in the second-order holographic DFB with terminal reflection. These results and the comparisons of some of them to the experiments are reported.     

A 100-Gb/s throughput ATM switch MCM with a 320-channel paralleloptical I/O interface

For an ATM switch system, we have developed a 100-Gb/s input/output (I/O) throughput optical I/O interface ATM switch multichip module (MCM) that has 320-ch optical I/O ports. This MCM is fabricated using ceramic (MCM-C) technology and very-small highly-parallel O/E and E/O optical converters. It uses 0.25-μm complementary metal oxide semiconductors (CMOS) ATM switch large scale integrations (LSIs) and has a total I/O throughput of up to 160 Gb/s. A prototype module with total I/O throughput of 100 Gb/s has been partially assembled using eight optical I/O interface blocks, each composed of a 40-ch O/E converter and a 40-ch E/O converter; the data rate per channel is from dc to 700 Mb/s. Using this module we developed an optical I/O interface ATM switch system and confirmed the operation of the optical interface     

A 108-GHz InP-HBT monolithic push-push VCO with low phase noise andwide tuning bandwidth

This paper reports on what is believed to be the highest frequency bipolar voltage-controlled oscillator (VCO) monolithic microwave integrated circuit (MMIC) so far reported. The W-band VCO is based on a push-push oscillator topology, which employs InP HBT technology with peak f_T's and f_max's of 75 and 200 GHz, respectively. The W-band VCO produces a maximum oscillating frequency of 108 GHz and delivers an output power of +0.92 dBm into 50 Ω. The VCO also obtains a tuning bandwidth of 2.73 GHz or 2.6% using a monolithic varactor. A phase noise of -88 dBc/Hz and -109 dBc/Hz is achieved at 1- and 10-MHz offsets, respectively, and is believed to be the lowest phase noise reported for a monolithic W-band VCO. The push-push VCO design approach demonstrated in this work enables higher VCO frequency operation, lower noise performance, and smaller size, which is attractive for millimeter-wave frequency source applications     

A Current-Mode based Field-Programmable Analog Array for Signal Processing Applications

This paper presents a new approach to develop Field Programmable Analog Arrays (FPAAs),¹ which avoids excessive number of programming elements in the signal path, thus enhancing the performance. The paper also introduces a novel FPAA architecture, devoid of the conventional switching and connection modules. The proposed FPAA is based on simple current mode sub-circuits. An uncompounded methodology has been employed for the programming of the Configurable Analog Cell (CAC). Current mode approach has enabled the operation of the FPAA presented here, over almost three decades of frequency range. We have demonstrated the feasibility of the FPAA by implementing some signal processing functions.