Design considerations for p-i-n thyristor structures

An analysis of a high-voltage gate turn-off (GTO) thyristor structure with a double-layered n base (p-i-n structure) is presented. From integration of Poisson's equation, an expression for the forward-blocking voltage at the onset of avalanche breakdown is obtained. Simple design criteria are developed to calculate the optimal thickness and doping density of the n base of a conventional pnpn structure designed for a specific voltage-blocking capability. The same principle is applied to design for the doping densities and thicknesses of the high-resistivity region and the buffer layer of the p-i-n GTO structure. The forward-blocking voltage, as well as the on-state voltage (at a current density of 300 A cm^-2) is predicted for a wide range of base layer thicknesses and doping densities to illustrate the available tradeoff options. Lowest on-state power dissipation for high blocking voltages (>6000 V) is predicted for a doping level of 5×10¹² cm^-3 in the high-resistivity layer     

Calculating the photocurrent and transit-time-limited bandwidth ofa heterostructure p-i-n photo-detector

We have calculated the photocurrent and transit-time-limited bandwidth of a heterostructure p-i-n photodetector. The effective heights of potential barriers at the heterojunction interfaces in the valence band and conduction band have been calculated at different bias voltage and grading lengths for InP-In_0.53Ga_0.47As and Al_0.2Ga_0.8As-GaAs systems. The rates of thermionic emission from the trap can then be easily estimated for each type of material system at an applied bias and for a particular thickness of the grading layer. An expression for current through the photodetector in the presence of traps has been derived by solving rate equations for an arbitrary distribution of photogenerated carriers in the absorption region. Frequency-domain calculations are used to find the transit-time-limited 3-dB bandwidths of the photodetector. It has also been indicated how the results could be used to estimate the bandwidth of the photodetector without performing the exact calculations in the presence of interface trapping. The results from the present model show good agreement with experimental data already reported in the literature for conventional and resonant-cavity-enhanced p-i-n photodetectors     

Speed and efficiency in multiple p-i-n photodetectors

We present a theoretical evaluation of the application of multiple intrinsic layers to enhancing the quantum efficiency of high-speed p-i-n photodetectors. It is shown that the use of multiple layers may lead to substantial improvements in the efficiency of detectors operating in the 20-100-GHz range, provided that the device area is reduced to limit the intrinsic capacitance and special care is taken to avoid large parasitic effects. Potential fabrication schemes are discussed     

Design considerations for wide-band p-i-n/HBT monolithictransimpedance optical receivers

A comprehensive circuit and device model was developed for the design of wideband transimpedance optical receivers using heterojunction bipolar transistors (HBTs). This model is used to determine the device and circuit design that gives the highest combination of bandwidth, sensitivity, and stability. For optoelectronic integration, it is convenient to use the collector-base junction of the HBT device structure to fabricate the p-i-n detector. A resulting transistor transit-time effect is shown to cause shunt peaking in the closed-loop response or, at worst, instability. It is shown that the photodiode stray capacitance is not a major source of sensitivity degradation in flip-chip transimpedance receivers. Optimum device structures are determined for InP- and GaAs-based HBT receivers with fine-line as well as relaxed geometries     

Optical signal detection in the turbulent atmosphere using p-i-n photodiodes

Performance of signal detection for intensity modulated direct detection optical communications systems through the turbulent atmosphere is studied considering statistics of photoelectron count in p-i-n photodiodes. The effect of atmospheric turbulence is modeled as a lognormal process. In this system, the photoelectron count is a conditional Poisson process, where the mean count is lognormal. A normal approximation is derived for this conditional Poisson process, which results in a simple calculation for the probability of miss. The probabilities of miss for quantum-limited detection and detection in the presence of thermal electronic noise are simulated and compared with calculations using the normal approximation. Simulation results match the analytical results. For the thermal noise case, the probability of miss is compared with the probability of false alarm to determine appropriate signal length and detection threshold settings for a required performance. Applying results in this paper, a system designer can determine appropriate signal length and detection threshold settings.     

Impedance stability and bandwidth considerations forelevated-radial antenna systems

An examination is made of the variations in feed-point impedance that occur in an elevated antenna system due to changes in the electrical characteristics of the underlying soil. The bandwidth of such antennas is also investigated by looking at deviations in the input impedance as the operating frequency is varied about its nominal value. In all cases, results for the elevated antenna are compared with those of the conventional ground-mounted system. The computer program used for this work was NEC (Numerical Electromagnetics Code), a method-of-moments software package     

Optical phaselock receiver with multigigahertz signal bandwidth

Describes a balanced pinFET receiver which employs two signal amplification paths for homodyne detection of multigigabit-per-second, pilot-carrier PSK optical signals. Using narrow-line 1.51 mu m semiconductor lasers, the authors have employed this receiver to phaselock a local oscillator to a 295 pW pilot carrier with 8 degrees RMS phase error.<>     

Priority-based dynamic bandwidth allocation in Ethernet Passive Optical Networks

In this article, new Dynamic Bandwidth Allocation (DBA) algorithms which give high priority to some ONUs corresponding to important agencies are studied in order to manage emergency cases causing severe network traffic. In order to deal with such cases, the network state is divided into two states, namely the normal and the emergency states. We apply the Weighted Round Robin (WRR)-based algorithm to the normal state, and three priority-based DBA algorithms are applied to the emergency state. These algorithms are different from each other according to the mode of protecting non-priority ONUs from starving. Each algorithm is analyzed in terms of utilization of the uplink bandwidth and the average packet delay. Simulation and theoretical results show that the proposed algorithms are working properly at the emergency state and can satisfy more stringent QoS requirements than WRR algorithms.

Optical bandwidth and fabrication tolerances of multimodeinterference couplers

Analytical expressions are derived which relate the optical bandwidth and the fabrication tolerances of multimode interference (MMI) couplers to their most important design parameters. These expressions compare adequately with mode analysis simulations. For strong guided structures, the optical bandwidth is shown to he inversely proportional to the number of input and output ports N and to the length of the device. The fabrication tolerances are independent of N and proportional to the output channel separation D. Measurements on MMI couplers in InP/InGaAsP corroborate the theoretical predictions     

Carrier thermalization by phonon absorption in quantum-wellmodulators and detectors

In quantum-confined Stark effect based modulators and quantum-well detectors, carriers are produced at the band-edge by optical absorption. In most applications these earners generate photocurrent and to do so these initially “cold” electrons must thermalize since the photocurrent is dominated by over the barrier escapes of the carriers. The intrinsic speed of the device is thus limited by this thermalization time. We carry out a Monte Carlo simulation to study the carrier heating by phonon absorption in quantum-well structures as a function of well size and barrier height. Carrier thermalization times are dominated by intrasubband polar optical phonon processes and typical times are 1-4 ps depending upon the well and barrier design     

Optical ISL tracking considerations in meeting ISDN requirements

Burst errors in an optical intersatellite link (ISL) are short term (of the order of a millisecond) degradations of the link due to antenna mistracking. These errors degrade the average bit error rate (BER), decrease coding gain, complicate calculations of tracking accuracy, and give rise to concern about the quality of the link for carrying digital signals. After reviewing previous work on these topics, the digital performance of the link is examined in terms of how these considerations affect the ability of an optical ISL to meet ISDN (Integrated Services Digital Network) criteria for an ISL. A link with on-board regeneration is assumed. Since no allocation has yet been made for an ISL, an allocation of 25 per cent of the degradation permitted for the satellite link by CCITT Rec. G.821 and CCIR Rec. 614 is assumed here. It is found (1) that if the requirement derived from Rec. 614 is met, the requirements derived from Rec. G.821 are also met and (2) that an optical ISL should have little difficulty in meeting these requirements.     

光学全并行矩阵乘法

本文提出一种只用极少数成像系统就能完成的一种全并行矩阵运算器.     

Optical energy considerations for diode-clamped smart pixel opticalreceivers

We calculate the switching time as a function of optical energy for a single-stage smart pixel receiver. We find that operating the receivers dynamically using modelocked pulses is the most energy-efficient method of operation. We also show that the receivers no longer operate with constant optical input energy, like the symmetric self electro-optic effect device (S-SEED), but rather the product of the required optical energy and the switching time is constant, as a result of the introduction of electrical voltage gain     

Nonlinearities in p-i-n microwave photodetectors

The nonlinearities in p-i-n photodetectors have been measured and numerically modeled. Harmonic distortion measurements were made with two single-frequency offset-phased-locked Nd:YAG lasers which provide a source dynamic range greater than 130 dB and a 1 MHz-50 GHz frequency range. Carrier transport is analytically described by three coupled nonlinear differential equations, Poisson's equation and the hole and electron continuity equations. These equations are numerically solved to investigate and isolate the various nonlinear mechanisms. The numerical solution incorporates diffusion since our treatment includes carrier generation in the highly doped p-region of the device. This p-region absorption and carrier-dependent carrier velocities associated with a perturbed electric field (due to space-charge and loading effects) are shown to dominate photodetector nonlinear behavior. The numerical model was extended to predict that maximum photodetector currents of 100 mA should be possible in 20 GHz bandwidth devices before a sharp increase in nonlinear output occurs. In addition, second harmonic distortion improvements of 40-60 dB may be possible if photodetectors can be fabricated with strictly-depleted absorbing regions     

Optical nonlinearities in GaAs-GaAlAs multiple quantum-wellhetero-nipi waveguides

The authors report on measurements of large optical nonlinearities in GaAs-GaAlAs multiple quantum-well hetero-nipi (MQW-H nipi) waveguides using a Mach-Zender interferometer. At normal operational powers and absorption levels the lateral spreading of the photogenerated carriers is so large that the nonlinearity must be defined from the ratio between the total optical power and the width of the MQW-H nipi region rather than from the optical intensity alone. The study concludes that the nonlinearity measured in a GaAs-GaAlAs MQW-H nipi waveguide at a wavelength far below the absorption edge where the absorption coefficient is in the range 10-20 cm^-1, is on the order of 10 ^-5 cm²/W. This value is two to three orders of magnitude larger than the optical nonlinearity measured in a typical MQW p-i-n diode waveguide at similar absorption, but the increased nonlinearity is obtained at the expense of speed     

A survey of dynamic bandwidth allocation algorithms for Ethernet Passive Optical Networks

Ethernet Passive Optical Network (EPON) has been widely considered as a promising technology for implementing the FTTx solutions to the “last mile” bandwidth bottleneck problem. Bandwidth allocation is one of the critical issues in the design of EPON systems. In an EPON system, multiple optical network units (ONUs) share a common upstream channel for data transmission. To efficiently utilize the limited bandwidth of the upstream channel, an EPON system must dynamically allocate the upstream bandwidth among multiple ONUs based on the instantaneous bandwidth demands and quality of service requirements of end users. This paper introduces the fundamental concepts on EPONs, discusses the major issues related to bandwidth allocation in EPON systems, and presents a survey of the state-of-the-art dynamic bandwidth allocation (DBA) algorithms for EPONs.     

Study on bandwidth control for multiple transmission rates in IEEE 802.11e EDCAF

IEEE 802.11e supports the guaranteed quality of service (QoS) by providing different transmission priorities. IEEE 802.11e improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). Signal quality may affect the available bandwidth and transmission rate, because the characteristic of communication channel in wireless environment is in random time‐variation manner. Generally a station using a lower transmission rate will occupy communication channel for a longer time and degrade system performance, which causes unfairness and cannot provide the guaranteed QoS for the stations with higher transmission rates. We propose a bandwidth control scheme (BCS) by combining the IEEE 802.11e enhanced distributed channel access function (EDCAF) protocol to overcome the guaranteed bandwidth issue in multirate environments. A multirate discrete Markov chain model is analyzed for the multirate transmission system in this paper. According to the obtained results, BCS improves performance especially in throughput and makes the different QoS requirements be processed efficiently and flexibly. Copyright © 2009 John Wiley & Sons, Ltd.     

Nitride-based p-i-n bandpass photodetectors

Nitride-based p-i-n bandpass photodetectors with semitransparent Ni-Au electrodes were successfully fabricated and characterized. The photodetectors exhibit a 20-V breakdown voltage and a small dark current of 40 pA at 4-V reverse bias. It was found that spectral responsivity shows a narrow bandpass characteristics from 337 to 365 nm. Moreover, the peak responsivity was estimated to be 0.13 A/W at 354 nm, corresponding to a quantum efficiency of 44%. The relatively high response at shorter wavelength is due to the unoptimized thickness of p-Al/sub 0.1/Ga/sub 0.9/N absorption layer. At low frequency, the noise of the photodetector is dominant by the 1/f-type noise. For our 330/spl times/330 /spl mu/m/sup 2/ device, given a bias of -3.18 V, the corresponding noise equivalent power and normalized detectivity D/sup */ are calculated to be 5.6/spl times/10/sup -12/ W and 3.34/spl times/10/sup 11/ cmHz/sup 0.5/ W/sup -1/, respectively.