Effect Of Dead Space Research Articles

Comparison of waveguide avalanche photodiodes with InP and InAlAs multiplication layer for 25 Gb/s operation

2.5 and 10 Gb/s InP/InGaAs avalanche photodiodes (APDs) have been widely used in optical communication systems. However, the study on InP/InGaAs APDs above 10 Gb/s is insufficient. Recently, high-speed and high-sensitivity APDs for 100 Gb/s or even 400 Gb/s optical communication systems have drawn a lot of attention. On basis of the physical model for frequency response of APD including the dead space effect, a waveguide separate absorption, grading, charge, and multiplication (WG-SAGCM) InP/InGaAs APD has been designed for 25 Gb/s operation. Also, the frequency response of WG InAlAs/InGaAs APD was also simulated, which is perfectly in accordance with the experimental data. The comparison between InP/InGaAs APD and InAlAs/InGaAs APD with the same thickness of multiplication layer shows that the speeds of carriers in the nonionization layers are also important for the gain-bandwidth characteristics of SAGCM WG-APD. The higher drift velocity of carriers returned from multiplication layer and the lower drift velocity of carriers injected into multiplication layer result in a higher gain-bandwidth product and a higher dc gain. This work is helpful for the design of high-speed APDs.

Effect of Mask Dead Space and Occlusion of Mask Holes on Delivery of Nebulized Albuterol

Infants and children with respiratory conditions are often prescribed bronchodilators. Face masks are used to facilitate the administration of nebulized therapy in patients unable to use a mouthpiece. Masks incorporate holes into their design, and their occlusion during aerosol delivery has been a common practice. Masks are available in different sizes and different dead volumes. The aim of this study was to compare the effect of different degrees of occlusion of the mask holes and different mask dead space on the amount of nebulized albuterol available at the mouth opening in a model of a spontaneously breathing child. A breathing simulator mimicking infant (tidal volume [VT] = 50 mL, breathing frequency = 30 breaths/min, inspiratory-expiratory ratio [I:E] = 1:3), child (VT = 155 mL, breathing frequency = 25 breaths/min, I:E = 1:2), and adult (VT = 500 mL, breathing frequency = 15 breaths/min, I:E = 1:2) breathing patterns was connected to a collection filter hidden behind a face plate. A pediatric size mask and an adult size mask connected to a continuous output jet nebulizer were sealed to the face plate. Three nebulizers were loaded with albuterol sulfate (2.5 mg/3 mL) and operated with 6 L/min compressed air for 5 min. Experiments were repeated with different degrees of occlusion (0%, 50%, and 90%). Albuterol was extracted from the filter and measured with a spectrophotometer at 276 nm. Occlusion of the holes in the large mask did not increase the amount of albuterol in any of the breathing patterns. The amount of albuterol captured at the mouth opening did not change when the small mask was switched to the large mask, except with the breathing pattern of a child, and when the holes in the mask were 50% occluded (P = .02). Neither decreasing the dead space of the mask nor occluding the mask holes increased the amount of nebulized albuterol captured at the mouth opening.

Apparent Dead Space with the Anesthetic Conserving Device, AnaConDa®

The anesthetic conserving device (ACD) reduces consumption of volatile anesthetic drug by a conserving medium adsorbing exhaled drug during expiration and releasing it during inspiration. Elevated arterial CO2 tension (PaCO2) has been observed in patients using the ACD, despite tidal volume increase to compensate for larger apparatus dead space. In a test lung using room temperature dry gas, this was shown to be due to adsorption of CO2 in the ACD during expiration and release of CO2 during the following inspiration. The effect in the test lung was higher than in patients. We tested the hypothesis that a lesser dead space effect in patients is due to higher temperature and/or moisture attenuating rebreathing of CO2. The lungs of 6 postoperative cardiac surgery patients were ventilated using a conventional heat and moisture exchanger (HME) or an ACD. The ACD was studied with a test lung at varying temperatures and moistures. Infrared spectrometry was used to measure apparent dead space by the single-breath test for CO2 as well as rebreathing of CO2. In patients, the median apparent dead space was 136 mL (95% confidence interval [CI,] 120-167) larger using the ACD compared with an HME (after correction for difference in internal volume 100 and 50 mL, respectively). Median rebreathing of CO2 using the ACD was 53% (range 48-58) of exhaled CO2 compared with 29% (range 27-32) with an HME. The median difference in CO2 rebreathing was 23% (95% CI, 18-27). In the test lung apparent dead space using ACD was unaffected by body temperature but decreased from 360 to 260 mL when moisture was added. This decreased rebreathing of CO2 from 62% to 48%. The use of an ACD increases apparent dead space to a greater extent than can be explained by its internal volume. This is caused by adsorption of CO2 in the ACD during expiration and release of CO2 during inspiration. Rebreathing of CO2 was attenuated by moisture. The dead space effect of the ACD could be clinically relevant in acute respiratory distress syndrome and other diseases associated with ventilation difficulties, but investigations with larger sample sizes would be needed to determine the clinical importance.

S41 Impact of alveolar nitrogen excretion on washout accuracy and progression in a model of multiple breath nitrogen washout

BackgroundThe advantages of multiple breath washout tests, and in particular lung clearance index (LCI), over conventional spirometry in cystic fibrosis (CF) are now well established. Although the majority of studies...

Evaluation of the Impact of Alveolar Nitrogen Excretion on Indices Derived from Multiple Breath Nitrogen Washout

BackgroundA large body of evidence has now accumulated describing the advantages of multiple breath washout tests over conventional spirometry in cystic fibrosis (CF). Although the majority of studies have used exogenous sulphur hexafluoride (SF6) as the tracer gas this has also led to an increased interest in nitrogen washout tests, despite the differences between these methods. The impact of body nitrogen excreted across the alveoli has previously been ignored.MethodsA two-compartment lung model was developed that included ventilation heterogeneity and dead space (DS) effects, but also incorporated experimental data on nitrogen excretion. The model was used to assess the impact of nitrogen excretion on washout progress and accuracy of functional residual capacity (FRC) and lung clearance index (LCI) measurements.ResultsExcreted nitrogen had a small effect on accuracy of FRC (1.8%) in the healthy adult model. The error in LCI calculated with true FRC was greater (6.3%), and excreted nitrogen contributed 21% of the total nitrogen concentration at the end of the washout. Increasing DS and ventilation heterogeneity both caused further increase in measurement error. LCI was increased by 6–13% in a CF child model, and excreted nitrogen increased the end of washout nitrogen concentration by 24–49%.ConclusionsExcreted nitrogen appears to have complex but clinically significant effects on washout progress, particularly in the presence of abnormal gas mixing. This may explain much of the previously described differences in washout outcomes between SF6 and nitrogen.

Gain and bandwidth analysis and comparison for gallium arsenide and silicon avalanche photodiodes with very thin multiplication layer

We describe the calculation of gain and bandwidth of an n + -i-p + avalanche photodiode (APD) for a multiplication layer down to tens of nanometers. The computed results are used to make a comparative study of gallium arsenide (GaAs) and silicon (Si) APDs. In the analysis, the depletion region is discretized into equal energy segments to take into account the discontinuous nature of impact ionization in the multiplication layer due to dead-space effect. Also, the carrier diffusion from undepleted regions is considered to study the effect of low bias on the frequency response. Carrier distribution within the structure is obtained by a numerical solution of coupled equations and recurrence relations. The model is verified with some experimental data taken from literature. Results show that gain increases with bias more rapidly for thinner multiplication layer. The Si APD is thinner than GaAs APD for the same gain at a given bias. Diffusion causes significant reduction of bandwidth at a low gain, with the change being sharper for GaAs APD than for Si APD.

Model of a small steam engine for renewable domestic CHP (combined heat and power) system

A small steam expander reciprocating engine is proposed for the conversion and utilization of low-grade heat resources of different nature, such as geothermal, solar, and recovery of waste heat. The engine, still to be developed for production, should be able to work with small flow rates and low upper temperature (100–150 °C), rejecting heat at a level still interesting for heating or cooling (with an absorption machine), that is, 50–80 °C. The device should be compact, simple and capable of easy control in order to match electric production and loads.The thermodynamic model uses real-fluid and real-cycle assumptions. It includes a heat transfer model for non-adiabatic compression and expansion, losses through admission/discharge valves, and the effects of dead space. Quasi-stationary modeling of the system is applied: the results are also compared with those of a dynamic model, working under perfect-gas assumptions. The model allows to calculate and analyze the performance of the system, including its dependence on the main design parameters; it includes a preliminary design, with special reference to valve sizing. The results indicate that the proposed technical solution can be applied and that the performance of the CHP (combined heat and power) system is competitive with respect to other technologies for renewable energies.

Thin 3D Multiplication Regions in Plasmonically Enhanced Nanopillar Avalanche Detectors

We demonstrate a nanopillar (NP) device structure for implementing plasmonically enhanced avalanche photodetector arrays with thin avalanche volumes (∼ 310 nm × 150 nm × 150 nm). A localized 3D electric field due to a core-shell PN junction in a NP acts as a multiplication region, while efficient light absorption takes place via surface plasmon polariton Bloch wave (SPP-BW) modes due to a self-aligned metal nanohole lattice. Avalanche gains of ∼216 at 730 nm at -12 V are obtained. We show through capacitance-voltage characterization, temperature-dependent breakdown measurements, and detailed device modeling that the avalanche region is on the order of the ionization path length, such that dead-space effects become significant. This work presents a clear path toward engineering dead space effects in thin 3D-confined multiplication regions for high performance avalanche detectors for applications in telecommunications, sensing and single photon detection.

The Effect of Alveolar Dead Space on the Measurement of End-Expiratory Lung Volume by Modified Nitrogen Wash-Out/Wash-In in Lavage-Induced Lung Injury

The accuracy of end-expiratory lung volume measurement by the modified nitrogen wash-out/wash-in method (EELV-N(2)) depends on the precise determination of carbon dioxide elimination (V(CO(2))), which is affected by alveolar dead space (V(D-alv)). The purpose of this study was to investigate the influence of V(D-alv) on EELV-N(2). Six piglets with lavage-induced acute lung injury were mechanically ventilated in a decremental PEEP trial that was reduced from 20 to 4 cm H(2)O in steps of 4 cm H(2)O every 10 min. EELV was measured by the modified EELV-N(2) method and computed tomography scan (EELV-CT), volumetric capnography, blood gas measurements, and hemodynamic data were recorded at each PEEP level. The data were divided into higher and lower PEEP groups. During the decremental PEEP trial, EELV-N(2) exhibited a high correlation (r(2) = 0.86, P < .001) with EELV-CT, with a bias of -48.6 ± 150.7 mL (1 ± 18%). In the higher PEEP group, EELV-N(2) was not correlated with EELV-CT, with a bias of -168.1 ± 171.5 mL (-14 ± 14%). However, in the lower PEEP group, EELV-N(2) exhibited a high correlation (r(2) = 0.86, P < .001) with EELV-CT, with a bias of 11.2 ± 97.2 mL (6 ± 17%). The measurement bias was negatively correlated with V(D-alv) (r(2) = 0.44, P = .04) and V(CO(2)) (r(2) = 0.47, P = .03) in the higher PEEP group. In this surfactant-depleted model, EELV measurement by the modified EELV-N(2) method reveals a systematic underestimation at high PEEP levels that is partly due to an increase in V(D-alv).

Short-Term Effects of Humidification Devices on Respiratory Pattern and Arterial Blood Gases During Noninvasive Ventilation

The impact of humidification devices on ventilatory and arterial blood gases parameters during noninvasive ventilation (NIV) remains controversial. The aim of the study was to compare the short-term impact of heat and moisture exchangers (HMEs) and heated humidifiers (HHs) during NIV for either hypercapnic or hypoxemic acute respiratory failure. Consecutive subjects receiving NIV were successively treated with HME and HH in randomized order for 30 min each. At the end of each period, arterial blood gases were measured and ventilatory parameters were recorded. Eighty-one subjects were enrolled, of whom 52 were hypercapnic (with or without acidosis) and 29 hypoxemic. Minute ventilation was greater with the HME, in comparison with the HH (15 [12-18] vs 12 [10-16] median [interquartile range], P < .001), while P(aCO(2)) was increased when using HME, indicating a dead space effect. This effect was observed in all subjects, but was more pronounced in hypercapnic subjects (P(aCO(2)) 62 ± 17 mm Hg with HME vs 57 ± 14 with HH, P < .001). In a subgroup of 19 subjects with respiratory acidosis, alveolar hypoventilation improved only with the HH. The amplitude of the dead space impact was a function of the degree of hypercapnia. Use of an HME decreased CO(2) elimination during NIV, despite increased minute ventilation, especially in hypercapnic subjects.

Relationships between the lung clearance index and conductive and acinar ventilation heterogeneity

The lung clearance index (LCI) derived from a multiple breath washout test has regained considerable popularity in recent years, alternatively being promoted as an early detection tool or a marker of small airways function. In this study, we systematically investigated the link between LCI and indexes of acinar and conductive airways ventilation heterogeneity (Sacin, Scond) to assess potential contributions from both lung zones. Relationships were examined in 55 normal subjects after provocation, where only Scond is known to be markedly increased, and in 55 asthma patients after bronchodilation, in whom both Scond and Sacin ranged between normal and abnormal. LCI was correlated to Scond in both groups (R = 0.37-0.43; P < 0.01 for both); in the asthma group, LCI was also tightly correlated to Sacin (R = 0.70; P < 0.001). Potential mechanisms operational at various levels of the bronchial tree were identified by considering washout curvilinearity in addition to LCI to distinguish specific ventilation and dead space effects (also illustrated by simple 2-compartment model simulations). Although the asthma data clearly demonstrate that LCI can reflect very peripheral ventilation heterogeneities, the normal provocation data also convincingly show that LCI increases may be the exclusive result of far more proximal ventilation heterogeneities. Because LCI potentially includes heterogeneities at all length scales, it is suggested that ventilation imaging in combination with LCI measurement at the mouth could identify the scale of relevant ventilation heterogeneities. In the meantime, interpretations of LCI results in the clinic based on washout curves collected at the mouth should be handled with caution.

EMPIRICAL FORMULAE FOR EXCESS NOISE FACTOR WITH DEAD SPACE FOR SINGLE CARRIER MULTIPLICATION

In this letter, two empirical equations are presented for the calculation of the excess noise factor of an avalanche photodiode for single carrier multiplication including the dead space effect. The first is an equation for calculating the excess noise factor when the multiplication approaches infinity as a function of parameters that describe the degree of the dead space effect. The second equation can be used to find the minimum value of the excess noise factor for any multiplication when the dead space effect is completely dominant, the so called "deterministic" limit. This agrees with the theoretically known equation for multiplications less than or equal to two.

History-Dependent Impact Ionization Theory Applied to HgCdTe e-APDs

The variation of the gain and the excess noise factor in HgCdTe avalanche photodiodes (APDs) with different junction geometries are compared with published theoretical and numerical work. It is shown that, although some features of the gain curves are reproduced, such as the constant exponential increase in the gain, the theoretical work fails to predict the observed variation of the gain as a function of multiplication layer width. In contrast, a new analytical gain model based on local impact ionization coefficients and a first direct comparison of the prediction of history-dependent impact ionization theory are shown to give a good general fit to the experimental gain data. A generic model of the gain in HgCdTe APDs has been obtained by fitting the analytical local model to gain curves of APDs with various geometries and cut-off wavelengths. The study of different hypotheses on the electric field dependence of the dead-space length and the saturation value of the impact ionization coefficient has shown that a variable dead-space effect has a direct impact on the excess noise of APDs, which is why exact excess noise measurements are necessary to achieve a pertinent estimation of the nonlocal impact ionization function.

Rebuttal from Sheel and McKenzie

POINT:COUNTERPOINTPulmonary edema does/does not occur in human athletes performing heavy sea-level exerciseRebuttal from Sheel and McKenziePublished Online:01 Oct 2010https://doi.org/10.1152/japplphysiol.01353.2009cMoreSectionsPDF (41 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Within the Point presented by Dr. Hopkins (2) several case studies, which have little to do with the debate, are presented as “evidence” of the development of pulmonary edema during heavy sea-level exercise. Anecdotal reports of pulmonary edema induced by emotional stress, sexual intercourse, and exercise at high altitude are all distractions from the central question. Moreover, left ventricular diastolic dysfunction is likely causative in these instances (8) and they represent an entirely different physiological scenario than that of the healthy athlete performing near-maximal exercise at sea level.We disagree with the assertion that the rapid infusion of 20 ml/kg of normal saline into resting humans can be used as supporting evidence for exercise-induced interstitial edema (4). Rather, it produces a nonphysiological situation (i.e., volume overload by ∼1.5 liters), which results in airflow obstruction and changes to airway responsiveness that are not commensurate with the typical exercise response seen in the population being discussed in this debate.Our colleague (2) and others (5) argue that the persistence of V̇A/Q̇ inequality during recovery from heavy exercise supports the idea of an exercise-induced increase in extravascular lung water. Specifically, the logic is that interstitial edema compresses the small airways and blood vessels resulting in a mismatch of V̇A/Q̇. However, this is not direct evidence of edema and should not be interpreted as such. As clearly stated within the concluding statements of the original manuscript by Schaffartzik et al. (5): “It [persistent V̇A/Q̇ mismatch owing to pulmonary interstitial edema] is of course not proof of the hypothesis.” We agree. Alternative explanations for V̇A/Q̇ heterogeneity include changes in airway resistance and increases in dead-space ventilation (6, 7). Moreover, increased V̇A/Q̇ heterogeneity with sustained heavy exercise does not result in a change in MRI-derived lung density measures 20 min following exercise (1). The multiple inert-gas elimination technique for determining V̇A/Q̇ has never been directly validated (3) which makes definitive statements about the development of exercise-induced edema difficult. Furthermore, regional estimates of where the edema (i.e., peribronchial and perivascular “cuffing”) occurs based on V̇A/Q̇ heterogeneity measures in humans are speculative. It appears that the indirect methods offer only cause for speculation on the presence of pulmonary edema while direct measurements via CT or MRI consistently fail to demonstrate changes in lung density.Accumulation of scattered anecdotal reports and circumstantial evidence is not proof of interstitial edema. In our view, the hypothesis continues to await experimental support.REFERENCES1. Burnham KJ , Arai TJ , Dubowitz DJ , Henderson AC , Holverda S , Buxton RB , Prisk GK , Hopkins SR. Pulmonary perfusion heterogeneity is increased by sustained, heavy exercise in humans. J Appl Physiol 107: 1559–1568, 2009.Link | ISI | Google Scholar2. Hopkins SR. Counterpoint: Pulmonary edema does occur in human athletes performing heavy sea-level exercise. J Appl Physiol; doi:10.1152/japplphysiol.01353-2009.Google Scholar3. Hopkins SR , Levin DL , Emami K , Kadlecek S , Yu J , Ishii M , Rizi RR. Advances in magnetic resonance imaging of lung physiology. J Appl Physiol 102: 1244–1254, 2007.Link | ISI | Google Scholar4. Prisk GK , Olfert IM , Arai TJ , Wagner PD , Hopkins SR. Rapid intravenous infusion of 20 ml/kg saline does not impair resting pulmonary gas exchange in the healthy human lung. J Appl Physiol 108: 53–59, 2010.Link | ISI | Google Scholar5. Schaffartzik W , Poole DC , Derion T , Tsukimoto K , Hogan MC , Arcos JP , Bebout DE , Wagner PD. VA/Q distribution during heavy exercise and recovery in humans: implications for pulmonary edema. J Appl Physiol 72: 1657–1667, 1992.Link | ISI | Google Scholar6. Tsukimoto K , Arcos JP , Schaffartzik W , Wagner PD , West JB. Effect of common dead space on VA/Q distribution in the dog. J Appl Physiol 68: 2488–2493, 1990.Link | ISI | Google Scholar7. Wagner PD , Gale GE , Moon RE , Torre-Bueno JR , Stolp BW , Saltzman HA. Pulmonary gas exchange in humans exercising at sea level and simulated altitude. J Appl Physiol 61: 260–270, 1986.Link | ISI | Google Scholar8. Wilmshurst PT. Pulmonary oedema induced by emotional stress, by sexual intercourse, and by exertion in a cold environment in people without evidence of heart disease. Heart (Br Cardiac Soc) 90: 806–807, 2004.Crossref | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 109Issue 4October 2010Pages 1274-1275 Copyright & PermissionsCopyright © 2010 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.01353.2009cPubMed20940440History Published online 1 October 2010 Published in print 1 October 2010 Metrics

Numerical simulation of impact ionization in Ge/AlxGa1−xAs avalanche photodiode

Impact ionization in Ge/AlxGa1−xAs p-i-n heterostructures has been studied using the Monte Carlo technique. The thin (&amp;lt;300 nm) Ge/AlxGa1−xAs single heterojunction structure was found to exhibit large hole (β) to electron (α) ionization coefficient ratio, owing to a higher β in the Ge layer and a lower α in the AlxGa1−xAs layer, together with the dead space effects. The Ge/AlxGa1−xAs avalanche photodiodes are attractive for applications where a wide wavelength detection range is required for compatibility with multiple sources such as in the emerging active optical cable and optical interconnect applications, as well as in the established optical fiber telecommunication systems.

An Analytic Approach to Study the Effects of Optical Phonon Scattering Loss on the Characteristics of Avalanche Photodiodes

In this paper, we have introduced an efficient analytic approach to study the effects of optical phonon scattering loss on the characteristics of avalanche photodiode (APDs) with multiplication regions as narrow as 25 nm. To do so, we have considered the energy loss caused by collision between carriers and optical phonons. We have also considered the effects of the carrier's dead space and its previous ionization history to develop a new compact model for predicting multiplication gains as well as breakdown conditions in APDs valid for a vast range of temperature. These predicted values can be used to extract carriers' impact ionization coefficients and ionization threshold energies. As an example, we have used our model to predict and extract the aforementioned characteristics and parameters for Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> As-APDs operating at temperatures over the range of 13 K ¿ T ¿ 290 K. Comparisonof these results with experimental data available in literature has demonstrated the capability of the new developed model as a powerful tool for simulating APDs' behavior and interpreting their experimentally measured characteristics.

Effects of instrumental dead space reduction during weaning from synchronized ventilation in preterm infants

A majority of the modalities of synchronized ventilation in preterm infants require the use of flow sensors that can increase dead space and may adversely affect ventilator weaning. The objective of this study was to assess the effects of flow sensor dead space during synchronized intermittent mandatory ventilation (SIMV) weaning in preterm infants. Twelve preterm infants (gestational age 25+/-2 weeks, birth weight 705+/-158 g, age: 31+/-186 days, SIMV rate: 25+/-8 breaths min(-1), peak inspiratory pressure 18+/-2 cm H(2)O, positive end-expiratory pressure: 5+/-0.5 cm H(2)O, pressure support: 9+/-3 cm H(2)O, fraction of inspired oxygen: 34+/-6%) underwent two 2.5-h weaning periods during which SIMV rate was reduced twice by 5 breaths min(-1) at 30-min intervals as tolerated, with and without reduction of flow sensor dead space, in random sequence. A 30-min baseline was obtained before each weaning period. Dead space was reduced by flushing the flow sensor with a continuous gas leak flow in the endotracheal tube connector. Transcutaneous CO(2) tension during SIMV weaning periods without and with reduced dead space did not differ from baseline, whereas total minute ventilation and tidal volume were lower during the SIMV weaning period with reduced dead space. Three infants did not tolerate SIMV weaning without while one infant did not tolerate weaning with reduced dead space. SIMV weaning elicited a compensatory rise in spontaneous ventilation. When flow sensor dead space was reduced during SIMV weaning, gas exchange was maintained with lower minute ventilation. Instrumental dead space imposes a ventilatory burden during SIMV weaning in small preterm infants.

Avalanche characteristics of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes

The mean multiplication gain and excess noise factor of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes (HAPDs) are simulated. The ionization coefficients of electron and hole in bulk GaAs and Al 0.6Ga 0.4As are used in this model to study the role of heterojunction in reducing excess noise. The band-edge discontinuities at the conduction and valence bands are included in our model which may influence the number of carrier crossing the heterojunction and hence modifies the dead space in the HAPDs. The mean multiplication gain and excess noise factor with electron- and hole-initiated multiplication for 0.1 and 0.2 μm multiplication lengths in GaAs/Al 0.6Ga 0.4As HAPDs are shown. By considering the dead space effect, our model demonstrated a small noise mainly due to the localization of carrier ionization and the limited carrier feedback ionization at heterointerface. In our model, most of the ionizations occur in the first-initiated multiplication layer which reduces the randomness of carrier ionization and noise.

Dead space effect on excess noise factor in double heterojunction avalanche photodiodes

A double heterojunction avalanche photodiode (DHAPD) model using Monte Carlo (MC) method is applied to study the effect of dead space on excess noise factor. The mean multiplication gain and excess noise factor involving electron and hole impact ionizations are simulated in our work. The higher order impact ionization occurs in three multiplication layers is considered in the model. The avalanche characteristics of DHAPD are obtained by incorporating the dead-space, dij, hole to electron coefficients ratio, kj and heterointerface probability, pim. Among these factors, the dead space effect plays an important role in reducing the excess noise factor in thin DHAPD. The dead space effect is also known in minimizing noise in homojunction APD when the device length is getting smaller. The distributions of carriers in multiplication region are demonstrated to explain the effect of dead space in DHAPD. Meanwhile, it is also shown that the hole to electron ionization coefficients ratio, k2=0.1 in the second layer gives the small excess noise factor in designing the DHAPD.

Monte Carlo investigation of avalanche multiplication process in thin InP avalanche photodiodes

An ensemble Monte Carlo simulation is presented to investigate the avalanche multiplication process in thin InP avalanche photodiodes (APDs). Analytical band structures are applied to the description of the conduction and valence band, and impact ionization is treated as an additional scattering mechanism with the Keldysh formula. Multiplication gain and excess noise factor of InP p+-i-n+ APDs are simulated and obvious excess noise reduction is found in the thinner devices. The effect of dead space on excess noise in thin APD structures is investigated by the distribution of impact ionization events within the multiplication region. It is found that the dead space can suppress the feedback ionization events resulting in a more deterministic avalanche multiplication process and reduce the excess noise in thinner APDs.