In Resonant Cavity Enhanced Photodetectors (RCE-PDs), the trade-off between the bandwidth and the quantum efficiency in the conventional photodetectors is overcome. In RCE-PDs, large bandwidth can be achieved using a thin absorption layer while the use of a resonant cavity allows for multiple passes of light in the absorption which boosts the quantum efficiency. In this paper, a complete bias-dependent model for the Resonant Cavity Enhanced-Separated Absorption Graded Charge Multiplication-Avalanche Photodetector (RCE-SAGCM-APD) is presented. The proposed model takes into account the case of drift velocities other than the saturation velocity, thus modeling this effect on the photodetector different design parameters such as Gain, Bandwidth and Gain-Bandwidth product.
KEYWORDS: Endoscopes, Mirrors, Modulation transfer functions, Prototyping, Lens design, Combined lens-mirror systems, Objectives, Monte Carlo methods, Endoscopy, Imaging systems
Autofluorescence endoscopy is a promising functional imaging technique to improve screening of pre-cancerous or early cancer lesions in the gastrointestinal (GI) tract. Tissue autofluorescence signal is weak compared to white light reflectance imaging. Conventional forward-viewing endoscopes are inefficient in the collection of light from objects of interest along on the GI luminal wall. A key component of a complete autofluorescence endoscope is the light collection module. In this paper, we report the design, optimization, prototype development, and testing of an endoscope objective that is capable of acquiring simultaneous forward and radial views. The radial-view optical design was optimized for a balance between image quality and light collection. Modulation transfer function (MTF), entrance pupil radius, manufacturability, and field-of-view were parameters used in the lens optimization. In comparison with the typical forward-viewing endoscopes, our nonsequential ray trace simulations suggest the proposed radial-view design is more practical in the light collection. To validate the proposed simulation methods, a 3:1 scaled-up prototype was fabricated. Contrast measurements were taken with the prototype, and then compared with the simulated MTF.
We have investigated the influence of carbon concentration on the low frequency noise (LFN) of Si/SiGe:C
Heterojunction Bipolar Transistors (HBTs). The HBTs are supplied by ST-Microelectronics Crolles and are based on a
0.13 &mgr;m BiCMOS technology. Three types of transistors were studied; they only differ by the amount of carbon
incorporated. When carbon is incorporated, representative noise spectra of the input current spectral density, SiB, show
important generation-recombination (G-R) components, while no such components are observed in carbon free
transistors. When the 1/f noise component is unambiguously observed, the associated figure of merit KB has a very good
value close to 4.10-10 &mgr;m2. In this paper we focus on the analysis of the G-R components associated with the presence of the carbon. Most of the observed Lorentzians are associated with Random Telegraph Signal (RTS) noise. No RTS noise
is found in carbon free devices. The RTS noise appears to be due to electrically active defects formed by the addition of
carbon, typically observed for concentrations above the bulk solid solubility limit in silicon. The RTS noise, amplitude
&Dgr;IB and the mean pulse widths (tH, tL), are analyzed as a function of bias voltage and temperature. The RTS amplitude is
found to scale with the base current and to decrease exponentially with temperature, independently of the carbon
concentration. The mean pulse widths are found to decrease rapidly with bias voltage, as 1/exp(qVBE/kT) or stronger. Our results confirm that electrically active C-related defects are localized in the base-emitter junction, and the RTS amplitude is explained by a model based on voltage barrier height fluctuations across the base-emitter junction induced by trapped
carriers in the space charge region. The observed bias dependence of mean pulse widths seems to indicate that two
capture processes are involved, electron and hole capture. These C-related defects behave like recombination centers
with deep energy levels rather than electron or hole traps involving trapping-detrapping process.
Waveguide Photodetector (WGPDs) are considered leading candidates to overcome the bandwidth-efficiency trade-off presented in conventional photodetectors. In this paper, we present a physical model of the waveguide-separated absorption charge multiplication-avalanche photodetector (WG-SACM-APD). Both time and frequency modeling for this photodetector are presented. The frequency response has been simulated for different thicknesses of the absorption and multiplication layers and for different areas of the photodetector.
The gain-bandwidth characteristic of WG-SACM-APD is studied for different areas and different thicknesses of both the absorption and the multiplication layers showing the dependence of the performance of the photodetector on the dimensions, the material parameters and the multiplication gain. In addition, the characteristics of WG-SACM-APD are studied for the case of an inductor added in series to the load resistor and better performance is achieved in comparison to the case with no inductor.
The results obtained from the model that is presented in this work are compared with published experimental results and good agreement has been obtained.
Resonant cavity enhanced photodetectors (RCE-PDs) are promising candidates for applications in high-speed optical communications and interconnections. The parasitics effects on these high-speed photodetectors must be carefully considered since they can significantly degrade the performance of the photodetector. In this paper, we will present a complete accurate model for the time response of the RCE-PDs. We will also study the effects of the parasitics of RCE-PDs on their time response and how we can compensate for the performance degradation from these parasitics. This study has been done for both RCE-PIN-PDs and RCE-avalanche photodetectors (RCE-APDs). RCE-separated absorption graded charge multiplication-APD was taken as an example of RCE-APDs. The time response of these RCE-PDs has better performance when compared to those of non RCE-PDs.
The parasitics effects include the effects of both of the load resistance and the capacitance of the photodetector. The effects of the inductor that may be added in series with the load are also studied. It is shown that adding an external inductor results in higher performance of the photodetectors and this inductor can compensate some of the degradations resulting from other parasitics. The effects of the parasitics have been investigated for different dimensions of the photodetectors, different values of both the load resistance and the added inductor and also for different multiplication gains for the case of RCE-APDs.
The SiGeC ternary alloy seems to be an attractive material system for Si-based device applications, because the incorporation of a small amount of C in the high-mobility SiGe layer offers an additional degree of freedom for tuning the bandgap, band offsets and the lattice strain in group IV heterostructures. In this work, detailed low-frequency noise (LFN) results in SiGeC pMOSFETs are presented. Our experimental results in saturation regime of the SiGe MOSFET show that the noise in SiGeC MOSFETs at gate bias |VGS-VT|<0.4V can be referred to the gate terminal as a noise voltage SVG=VG2, which implies (ΔN) fluctuation with correlated noise in the cap and SiGeC channel currents. Overall, the trend shows that the gate referred noise voltage scales inversely with the gate area, and that the variation of the noise level has log-normal distribution. Therefore, the noise in SiGeC MOSFETs can be expressed as S=Savg*exp(t*σNp), where t=±1,...,±3 is a coefficient selected for desired confidence probability of 0.6,...,0.99 respectively, and σ is the standard deviation of the log-normal distribution of the noise level around its average Savg, later given by (ΔN-Δμ) fluctuation in the cap layer and SiGeC channel of pMOSFET.
KEYWORDS: Field effect transistors, Instrument modeling, Transistors, Resistance, Modulation, Metals, Lab on a chip, Scattering, Signal to noise ratio, Diffusion
This paper presents a thorough description of the high-frequency noise characterization and modeling of CMOS transistors for radio frequency (RF) integrated circuit (IC) design. It covers two main topics: high-frequency noise characterization and physics-based noise models. In the first section, two de-embedding procedures are presented for noise and scattering parameter de-embedding to get rid of the parasitic effects from the probe pads and interconnections in the device-under-test (DUT). With the intrinsic noise parameters, two extraction methods to obtain the channel noise, induced gate noise and their correlation in MOSFETs are discussed and experimental results are presented. Based on the noise information obtained in the first section, the second part of the paper presents physics-based noise models for the noise sources of interest in deep submicron MOSFETs. It discusses the model derivation, channel noise enhancement in deep submicron MOSFETs and impact of channel length modulation (CLM) effect. Finally a simple and accurate analytical model for channel noise calculation will be presented.
Resonant cavity enhanced photodetectors (RCE-PDs) are promising candidates for applications in high-speed optical communications and interconnections. In these high-speed photodetectors, both high bandwidth and high external quantum efficiency can be achieved simultaneously because of the multipaths of the incident light due to the presence of the Fabry-Perot cavity into which the photodetector is inserted. In this paper, state-of-the-art RCE-PDs are discussed. Different structures of the RCE-PDs such as RCE-PIN, RCE-APD, and RCE-MSM PDs are presented and discussed. The material requirements for the RCE PDs with different material system compositions for the different structures and different wavelengths of the incident light that the photodetectors are sensitive to, are discussed. An overview of the analysis and a SPICE model of the RCE-PDs will be presented. These analyses include the calculations of quantum efficiency, (QE), impulse response and frequency response of RCE-PDs. These analyses are sensitive to the standing wave effect (SWE) and to carrier diffusion, and both effects are studied. Optimization procedures for the design of ultrafast RCE-PDs will be presented, showing how the QE, bandwidth and speed of these photodetectors can be improved by adjusting the parameters of both the cavity and the photodetector itself. Finally, comparisons to experimental results and a survey of the performance of state-of-the-art of RCE-PDs will be presented.
The low frequency noise (LFN) properties of the field-effect transistors (FETs) using polymers as the semiconducting material in thin-film transistor (TFT) structures are investigated and discussed in terms of the charge carrier transport. Results obtained from several research groups are summarized. Injection-drift limited model (IDLM) for charge transport in amorphous PFETs is discussed. IDLM has some advantages in comparison to the commonly used metal-oxide-semiconductor (MOS) transistor models. A general trend of proportionality between noise power density and the DC power applied to the polymer FET’s (PFET’s) channel is observed in the data from several research groups. This trend implies mobility fluctuation in PFET as the dominant noise source.
The low frequency noise characteristics of double self-aligned InP/InGaAs and two types Si/SiGe heterojunction bipolar transistors (HBTs) were investigated. Spectral analysis shows no striking differences; the spectra are composed of a 1/f component and the white noise is always reached at low bias. A general trend for all the transistors was the presence of Lorentzian(s) component(s) for the smallest devices. The voltage coherence function was always one for SiGe transistors; and for the first time, it was found to be close to zero for InP devices. Concerning the 1/f noise level, both types of transistors have approximately a quadratic dependence on base current bias and an inverse dependence on the emitter area. Thus a comparison of the 1/f noise level has been made using the Kb parameter, and values around 109 μm2 for SiGe HBTs and around 108 μm2 for InP HBTs were found. These results are of same order of magnitude as the best published ones. The low frequency noise results suggest that excess noise sources are mainly located at the intrinsic emitter-base junction for the two type of SiGe devices, and for the for InP HBTs, a correlated noise source is located at the emitter periphery. To compare different devices and technologies, fc/fT where fT is the unity current gain frequency was studied as a function of collector current density and for some HBT technologies, fc/fT α Jc. The effects of different processing conditions, designs and temperature were also investigated and will be discussed.
A physically-based transient microplasma based model for low frequency noise in pn diodes is discussed and implemented in SPICE simulator. The simulation indicates that the model correctly describes the non-monotonic behavior of both the DC and the noise characteristics of diode at the onset of avalanche breakdown. Since the model is based on a new microplasma switching theory, the results of simulation confirm the findings of this theory. These are, as follows for the microplasma. Its switching threshold is the condition of equality of free- to space charge concentration in depletion layer. Its on-current is approximately twice the threshold current. It is initialized by the charge generation due to few recombination centers in microplasma region at high avalanche multiplication due to impact ionization, while the microplasma turn-off is due to carrier diffusion from microplasma region into the depletion layer at low, but larger than 1, avalanche multiplication.
For many analog integrated circuit applications, the polysilicon emitter bipolar junction transistor (PE-BJT) is still the preferred choice because of its higher operational frequency and lower noise performance characteristics compared to MOS transistors of similar active areas and at similar biasing currents. In this paper, we begin by motivating the reader with reasons why bipolar transistors are still of great interest for analog integrated circuits. This motivation includes a comparison between BJT and the MOSFET using a simple small-signal equivalent circuit to derive important parameters that can be used to compare these two technologies. An extensive review of the popular theories used to explain low frequency noise results is presented. However, in almost all instances, these theories have not been fully tested. The effects of different processing technologies and conditions on the noise performance of PE-BJTs is reviewed and a summary of some of the key technological steps and device parameters and their effects on noise is discussed. The effects of temperature and emitter geometries scaling is reviewed. It is shown that dispersion of the low frequency noise in ultra-small geometries is a serious issue since the rate of increase of the noise dispersion is faster than the noise itself as the emitter geometry is scaled to smaller values. Finally, some ideas for future research on PE-BJTs, some of which are also applicable to SiGe heteorjunction bipolar transistors and MOSFETs, are presented after the conclusions.
We investigate the impact of body biasing on the low frequency noise (LFN) performances of NMOS transistors from a transistors 130 nm CMOS technology. The body-to-source voltage VBS was varied from - 0.5 to + 0.5 V for reverse and forward mode substrate biasing. A detailed electrical characterization was performed and the benefits of the body bias analysed in terms of current and maximum transconductance variations. Noise measurements were first performed at low drain bias VDS = 25 mV and VBS = 0 V in order to discuss the noise model. Results are in agreement with the carrier number fluctuation theory. Bulk bias dependence of the LFN was investigated at VDS = VDD = 1.2 V. Significant noise reduction is observed in the subthreshold regime when applying a forward body bias. In strong inversion, the noise level is found to be approximately independent of the substrate bias VBS.
A study of the performance of an Avalanche Photodiode (APD) at low bias using time-domain modeling has been conducted. Three important considerations in this range of bias voltages are: the emission of holes from the interface-trap at the valence-band discontinuity; the diffusion of photogenerated holes from the undepleted region of the absorption layer; and the velocity of carriers at low electric fields. A new recurrence relation has been obtained for the emission of holes from the valance-band trap. The time-delays due to the diffusion of holes photogenerated in the undepleted region of the absorption layer has been taken into account in computing the impulse response of the APD in time-domain. The effect of velocity of the carriers at low- bias has been considered by using a field-dependent formulation for velocity. The frequency response of the device has been obtained by using the fast Fourier Transform of the impulse response in time-domain. The result on the gain-bias and bandwidth-gain characteristics show good agreement with the published experimental data on InP/InGaAs APDs.
For fiber-optic communications at 1.3 micrometers , and 1.55 micrometers , materials lattice-matched to InP substrate are exploited to fabricate high performance photodetectors (PDS). Various types of novel structures are used for fabricating high- performance PDs. In this review, the roles of heterojunctions, resonant cavity structures and the edge- coupled structures on the bandwidth, quantum efficiency and noise performance of PDs are discussed. Their potential for the fabrication of integrated photo receivers for applications in optoelectronic integrated circuits is mentioned. Different possible structures and materials for PDs, and other important considerations for high performance PDs are also briefly discussed.
An efficient theoretical approach incorporating the mechanism of resonant absorption of the multiple reflected lightwaves is presented to model the frequency response of resonant-cavity (RC) avalanche photodiodes (APDs). Although the theoretical expressions are derived with respect to the RC separate absorption, charge and multiplication (SACM) structure, they are actually very general and can be applied to other RC APD structures and many non-RC APDs. As an example, the theoretical approach is applied to the InGaAs/InAlAs RC SACM APD. The computation results of -3 dB bandwidth based on the present theoretical approach are consistent with the experiment.
By using a simplified time domain modeling approach, the temperature dependent performance characteristics such as multiplication gain and bandwidth are studied for InP/InGaAs separate absorption, grading, charge and multiplication (SAGCM) APDs within the temperature range from -243 to 358 K. The modeling approach is improved to consider the effects of hole diffusion, hole trapping, load circuit RC and gain-bandwidth product limit together with the fast Fourier transformation component of the impulse response from the time domain computation. The modeling results agree with experiments. The effects of changing material parameters on modeling are also discussed. The improved performance characteristics also indicate the potential application prospects of InP/InGaAs SAGCM APDs in low temperature environments.
This paper reviews and also discusses some of the important issues in MOSFET Modeling for radio frequency integrated circuits (RFICs). A brief review of some popular or common MOSFET models that can predict the RF properties of MOSFETs is presented. At present, these include BSIM3v3, EKV, MOS Model 9 and adaptations of HSPICE models, and most of them are discussed here. Attention is paid to RF noise parameter extraction and modeling of MOSFETs, since this has been relatively neglected compared to the AC modeling and parameter extraction. Finally, some new and exciting result son the effects of DC electrical stresses on the microwave properties of NMOSFETs, especially the unity current-gain frequencies and maximum oscillation frequencies are presented for different stress times and at different biasing conditions. Modeling of the effects of stress on the RF properties of MOSFETs is still to be investigated.
This paper presents a brief overview of some of the common high-speed and high-sensitivity photodetectors. These devices are the key components in long-haul, high bit-rate fiber optic communication systems. In this paper, while we describe several types of photodetectors, we concentrate on avalanche photodiodes since they are the current preferred candidates for high bit-rate long-haul fiber optic communication systems. We describe and compare some analytical and stochastic modeling results with experimental data and conclude with a discussion of some state-of-the-art results on photodetectors.
Multiplied shot nose and gain-voltage characteristics of separate absorption, grading, charge and multiplication avalanche photodiodes were measured at 25 degrees C in a gain range of 3 to 30. Low optical input powers and a small spot size were used in order to minimize gain saturation effects and gain non-uniformity within the spot. The InP multiplication layer thickness and charge sheet density were extracted from capacitance - voltage characteristics and confirmed by SIMS. Electron and hole impact ionization coefficients in InP were then extracted using gain-voltage characteristics and McIntyre's expressions. Possible deep level traps within the InP multiplication layer were characterized using temperature and frequency characteristics of capacitance - voltage measurements. Peripheral and active area capacitances were separated by studying devices with different active area diameters.
Charge coupled device (CCD) imaging arrays are becoming more frequently used in space vehicles and equipment, especially space-based astronomical telescopes. It is important to understand the effects of radiation on a CCD so that its performance degradation during mission lifetime can be predicted, and so that methods to prevent unacceptable performance degradation can be found. Much recent work by various groups has focused on the problems surrounding the loss of charge transfer efficiency and the increase in dark current and dark current spikes in CCDs. The use of a CCD as the fine error sensor in the Lyman Far Ultraviolet Spectroscopic Explorer (FUSE) is limited by its noise performance. In this work we attempt to understand some of the factors surrounding the noise degradation due to radiation in a space environment. Later, we demonstrate how low frequency noise can be used as a characterization tool for studying proton radiation damage in CCDs.
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