Effect Of Dead Space Research Articles

On the frequency response of a resonant-cavity-enhanced separate absorption, grading, charge, and multiplication avalanche photodiode

In this article the frequency response of a resonant-cavity-enhanced (RCE) separate absorption, grading, charge, and multiplication (SAGCM) avalanche photodiode (APD) has been investigated. This study on APDs includes the development of a model for the avalanche buildup of carriers including the dead-space effect. In this model, the carriers are characterized by their energy and position in the region of multiplication. The excess energy of the carriers above threshold are assumed to be equally distributed among the carriers after impact ionization. The analysis shown in this article can be used to study the frequency response of the APD for a wide range of reverse bias voltages. For low bias voltages, an analytical expression has been derived to study the emission of carriers from the interface trap. The delay due to diffusion of photogenerated carriers from the undepleted region of the absorption layer and the low velocity of carriers at low field have also been considered. The model is used to calculate gains at different biases and bandwidth at different dc gains. The results show good agreement with published experimental data on a RCE SA(G)CM APD.

Intravenous Drug Infusion: Modeling of the Effects of Flow Rate and Tubing Dead Space on Drug Delivery

Intravenous Drug Infusion: Modeling of the Effects of Flow Rate and Tubing Dead Space on Drug Delivery

Changes in regional ventilation after autologous blood clot pulmonary embolism.

Previous studies have suggested that pulmonary embolism (PE) and pulmonary artery occlusion result in a shift in alveolar ventilation away from unperfused regions. This study aimed to directly assess changes in regional specific ventilation (sV(A)) due to autologous blood clot PE using positron emission tomography. Pulmonary embolism was created in six anesthetized, paralyzed, and mechanically ventilated sheep by injecting cylindrical clots of autologous blood (7 mm in diameter and height). Clots were progressively infused into a central vein until a stable mean pulmonary artery pressure between 30 and 40 mmHg was achieved. A multislice positron emission tomography camera was used to image 15 contiguous, 6.5-mm-thick transverse cross-sections of the chest beginning just above the diaphragm. sV(A) from perfused regions (sV(A),(p)) was assessed as the ventilatory turnover rate of the tracer NN after central venous injection of NN-labeled saline. Pulmonary embolism obstructed flow to 64% of imaged areas. Before PE, (sV(A),(p))was equivalent in areas that would remain perfused and those that would become embolized after PE (0.021 +/- 0.007 0.021 +/- 0.006 s(-1); P = nonsignificant). After PE, sV(A),(p) of areas remaining perfused increased to 0.033 +/- 0.011 s (-1) (P < 0.005). This effect on regional sV(A),(p) could have been caused by active redistribution of sV(A),(p) or by a reduction in tracer concentration of perfused areas due to the dead space common to perfused and embolized regions. Model simulations indicated that the common dead-space effect could only explain a small part of the sV(A),(p) increase. An increase in sV(A),(p) of perfused regions occurs following PE with 7-mm autologous blood clots. This increase is most likely caused by a shift in ventilation away from embolized areas mediated by hypocapnic pneumoconstriction.

Calculation of gain and noise with dead space for GaAs and Al/sub x/Ga/sub 1-x/As avalanche photodiode

It is well known that, as a result of the nonlocal nature of impact ionization, the noise of avalanche photodiodes decreases as the thickness of the multiplication region is reduced. In this paper, we present an alternative technique to calculate the gain distribution, including the dead-space effect, by numerical solution of the recursive equations. This method yields the average gain, the multiplication noise, and gain distribution curves. The results are compared with simple Monte Carlo simulation and the Z-transform technique.

Gain-bandwidth characteristics of thin avalanche photodiodes

The frequency-response characteristics of avalanche photodiodes (APDs) with thin multiplication layers are investigated by means of a recurrence technique that incorporates the history dependence of ionization coefficients. In addition, to characterize the autocorrelation function of the impulse response, new recurrence equations are derived and solved using a parallel computer. The mean frequency response and the gain-bandwidth product are computed and a simple model for the dependence of the gain-bandwidth product on the multiplication-layer width is set forth for GaAs, InP, Al/sub 0.2/Ga/sub 0.8/As, and In/sub 0.52/Al/sub 0.48/As APDs. It is shown that the dead-space effect leads to a reduction (up to 30%) in the bandwidth from that predicted by the conventional multiplication theory. Notably, calculation of the power-spectral density of the photocurrent reveals that the presence of dead space also results in a reduction in the fluctuations in the frequency response. This result is the spectral generalization of the reduction in the excess noise factor in thin APDs and reveals an added advantage of using thin APDs in ultrafast receivers.

Effect of dead space on avalanche speed [APDs

The effects of dead space (the minimum distance travelled by a carrier before acquiring enough energy to impact ionize) on the current impulse response and bandwidth of an avalanche multiplication process are obtained from a numerical model that maintains a constant carrier velocity but allows for a random distribution of impact ionization path lengths. The results show that the main mechanism responsible for the increase in response time with dead space is the increase in the number of carrier groups, which qualitatively describes the length of multiplication chains. When the dead space is negligible, the bandwidth follows the behavior predicted by Emmons but decreases as dead space increases.

Excess noise characteristics of Al/sub 0.8/Ga/sub 0.2/As avalanche photodiodes

The avalanche noise characteristics of Al/sub 0.8/Ga/sub 0.2/As have been measured in a range of p-i-n and n-i-p diodes with i-region widths /spl omega/ varying from 1.02 to 0.02 μm. While thick bulk diodes exhibit low excess noise from electron initiated multiplication, owing to the large /spl alpha///spl beta/ ratio (1/k), the excess noise of diodes with /spl omega/ < 0.31 μm were found to be greatly reduced by the effects of dead space. The thinnest diodes exhibit very low excess noise, corresponding to k = 0.08, up to a multiplication value of 90. In contrast to most III-V materials, it was found that both thick and thin Al/sub 0.8/Ga/sub 0.2/As multiplication layers can give very low excess noise and that electrons must initiate multiplication to minimize excess noise, even in thin structures.

Dead space effect in space-charge region of collector of AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.

Treatment of soft threshold in impact ionization

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been measured and analyzed using a model for impact ionization which includes the effect of dead space, modified to allow for a gradual onset of ionization, with realistic threshold energies. Good agreement is achieved between the predictions of this “soft dead space” model and measurements of multiplication and excess noise, both on a range of submicron diodes with uniform electric fields and also on a p+n diode with a highly nonuniform electric field.

Off‐line exhaled nitric oxide measurements in children

The concentration of exhaled nitric oxide (eNO) is a useful marker of asthmatic bronchial inflammation. eNO can now be measured away from the laboratory (off-line), even in children. Short exhalation maneuvers (8 sec) and small samples (1 L) of exhaled gas are probably sufficient in children, but more information is needed about the effect of different measurement conditions. As a preliminary step before conducting epidemiological studies in schoolchildren, we investigated the effects of expiratory flow, dead space, and expiratory time on eNO concentrations collected in 1-L mylar collection bags. We studied 101 cooperative subjects (62 males) aged 5–18 years (30 healthy volunteers, 51 asthmatics, and 20 children with various other respiratory diseases) in our pulmonary function laboratory. On-line and off-line eNO were compared in a single session, and analyzed with a Sievers NOA™ 280 nitric oxide analyzer. For both methods of collecting expired gas, subjects did a single exhalation without breath-holding against an expiratory pressure 10 cm H2O. We investigated the effects of expiratory flow, dead space, and exhalation time on eNO; we also compared on-line and off-line eNO measurements, and the repeatability of both techniques at a given flow rate. Expiratory flows of 58 mL/sec provided more reproducible data than lower flows (coefficient of repeatability 1.1 ppb for 58 mL/sec vs. 2.8 for 27 mL/sec vs. 5.7 for 18 mL/sec). eNO concentrations were about 25% higher in off-line than in on-line recordings if the initial 250 mL of exhaled gas were not eliminated, and 37% higher if exhalation lasted longer (16 sec vs. 8 sec). Eliminating 250 mL of dead space and shortening the filling time to 8 sec yielded off-line eNO values close to those on-line (geometric mean off-line eNO 14.4 ppb, 95% confidence interval: 12.2–17.0) vs. on-line eNO 13.8 ppb (95% confidence interval: 11.6–16.5). On-line and off-line results were highly correlated (r = 0.996, P = 0.000) and had similar coefficients of variation (on-line eNO 2.6%, off-line 2.8%). Neither agreement nor repeatability of eNO measurements were affected by disease status or baseline FEV1 (% predicted values). Once standardized, the off-line eNO technique using 1-L gas collection bags will provide results similar to those recorded on-line. Pediatr Pulmonol. 2001; 32:159–167. © 2001 Wiley-Liss, Inc.

Avalanche multiplication characteristics of Al/sub 0.8/Ga/sub 0.2/As diodes

The avalanche multiplication characteristics of Al/sub 0.8/Ga/sub 0.2/As have been investigated in a series of p-i-n and n-i-p diodes with i-region widths, w, varying from 1 /spl mu/m to 0.025 /spl mu/m. The electron ionization coefficient, /spl alpha/, is found to be consistently higher than the hole ionization coefficient, /spl beta/, over the entire range of electric fields investigated. By contrast with Al/sub x/Ga/sub 1-x/As (x/spl les/0.6) a significant difference between the electron and hole initiated multiplication characteristics of very thin Al/sub 0.8/Ga/sub 0.2/As diodes (w=0.025 /spl mu/m) was observed. Dead space effects in the diodes with w/spl les/0.1 /spl mu/m were found to reduce the multiplication at low bias below the values predicted from bulk ionization coefficients. Effective /spl alpha/ and /spl beta/ that are independent of w have been deduced from measurements and are able to reproduce accurately the multiplication characteristics of diodes with w/spl ges/0.1 /spl mu/m and breakdown voltages of all diodes with good accuracy. By performing a simple correction for the dead space, the multiplication characteristics of even thinner diodes were also predicted with reasonable accuracy.

Dynamic holography for optical interconnections II Routing holograms with predictable location and intensity of each diffraction order

An analysis of dynamic phase-only holograms, described by fractional notation and recorded onto a pixelated spatial light modulator (SLM) in a reconfigurable optical beam-steering switch, is presented. The phase quantization and arrangement of the phase states and the SLM pixelation and dead-space effects are decoupled, expressed analytically, and simulated numerically. The phase analysis with a skip-rotate rule reveals the location and intensity of each diffraction order at the digital replay stage. The optical reconstruction of the holograms recorded onto SLM's with rectangular pixel apertures entails sinc-squared scaling, which further reduces the intensity of each diffraction order. With these two factors taken into account, the highest values of the nonuniform first-order diffraction efficiencies are expected to be 33%, 66%, and 77% for two-, four-, and and eight-level one-dimensional holograms with a 90% linear pixel fill factor. The variation of the first-order diffraction efficiency and the relative replay intensities were verified to within 1 dB by performing the optical reconstruction of binary phase-only holograms recorded onto a ferroelectric liquid crystal on a silicon SLM.

The Effects of Artificial Dead Space on the Pulmonary Ventilation of Intubated Children with Mechanical Ventilation

This study was done to evaluate the effect reducing artificial dead space on intubated children. Data were collected from July 1st, 1998 to August 31st, 1999. The subjects were selected from a pediatric intensive care unit of 'S' hospital and intubated with 3.5 mm or 4.5 mm endotracheal tube after o...

Impact-ionization and noise characteristics of thin III-V avalanche photodiodes

It is, by now, well known that McIntyre's localized carrier-multiplication theory cannot explain the suppression of excess noise factor observed in avalanche photodiodes (APDs) that make use of thin multiplication regions. We demonstrate that a carrier multiplication model that incorporates the effects of dead space, as developed earlier by Hayat et al. provides excellent agreement with the impact-ionization and noise characteristics of thin InP, In/sub 0.52/Al/sub 0.48/As, GaAs, and Al/sub 0.2/Ga/sub 0.8/As APDs, with multiplication regions of different widths. We outline a general technique that facilitates the calculation of ionization coefficients for carriers that have traveled a distance exceeding the dead space (enabled carriers), directly from experimental excess-noise-factor data. These coefficients depend on the electric field in exponential fashion and are independent of multiplication width, as expected on physical grounds. The procedure for obtaining the ionization coefficients is used in conjunction with the dead-space-multiplication theory (DSMT) to predict excess noise factor versus mean-gain curves that are in excellent accord with experimental data for thin III-V APDs, for all multiplication-region widths.

Avalanche multiplication and ionization coefficient in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

The hole-initiated impact ionization multiplication factor Mp−1 and the ionization coefficient αp in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors (HBTs) are presented. A large discrepancy is observed at low electric field when the measured data from the p-n-p HBTs are compared with those given from avalanche photodiode. The results show that the conventional impact ionization models, based on local electric field, substantially overestimate the hole impact ionization multiplication factor Mp−1. We believe that the hole ionization coefficient in p-n-p HBTs where significant dead space effects occur in the collector space charge region.

A visual aid for teaching ventilation-perfusion relationships.

To help students understand the concept of the ventilation-perfusion ratio (VA/Q) and the effects that VA/Q mismatching has on pulmonary gas exchange, a "sliding rectangles" visual aid was developed to teach VA/Q relationships. Adjacent rectangles representing "ventilation" and "perfusion" are slid past one another so that portions of the ventilation and perfusion rectangles are not touching, illustrating the concepts of dead-space ventilation (VD) and shunt flow (QS). The portion of the ventilation bar representing VD is further subdivided into anatomical and alveolar VD and used to show the effects of alveolar dead space on the PO2 (PAO2) and PCO2 of alveolar air (PACO2); movement away from the "ideal" point). Similarly, the portion of the perfusion bar representing QS is used to define anatomical and physiological shunts and the effect of shunts on the PO2 (PaO2) and PCO2 of arterial blood (PaCO2). The genesis of the PAO2-PaO2 (A-a) PO2 difference as well as the effects of VA/Q mismatching and diffusion abnormalities can all be discussed with this visual aid. This approach has greatly assisted some students in mastering this traditionally difficult area of respiratory physiology.

Maintaining acid-base balance in organ donors.

An abnormal blood pH may cause the loss of donor organs through harmful physiological consequences. The organ procurement coordinator must correctly analyze the acid-base abnormality and treat its cause while normalizing the blood pH. We recommend that treatment of acidemia or alkalemia be first directed toward changing parameters on the mechanical ventilator, using the Paco2 to modify blood pH. Thereafter, hydrochloric acid or sodium bicarbonate may be administered to correct the calculated metabolic acid-base deficit. The types of acidosis or alkalosis, dead space effect during mechanical ventilation, base excess, base deficit, and the appropriate evaluation of blood lactate are also discussed as related to the correction of the acid-base status throughout donor care.

Maintaining acid-base balance in organ donors

An abnormal blood pH may cause the loss of donor organs through harmful physiological consequences. The organ procurement coordinator must correctly analyze the acid-base abnormality and treat its cause while normalizing the blood pH. We recommend that treatment of acidemia or alkalemia be first directed toward changing parameters on the mechanical ventilator, using the Paco2 to modify blood pH. Thereafter, hydrochloric acid or sodium bicarbonate may be administered to correct the calculated metabolic acid-base deficit. The types of acidosis or alkalosis, dead space effect during mechanical ventilation, base excess, base deficit, and the appropriate evaluation of blood lactate are also discussed as related to the correction of the acid-base status throughout donor care.

The merits and limitations of local impact ionization theory [APDs

Multiplication measurements on GaAs p/sup +/-i-n/sup +/s with i-region thicknesses, w, between 1 /spl mu/m and 0.025 /spl mu/m and Monte Carlo (MC) calculations of the avalanche process are used to investigate the applicability of the local ionization theory. The local expressions for multiplication are able to predict the measured values surprisingly well in p/sup +/-i-n/sup +/s with i-region thicknesses, w, as thin as 0.2 /spl mu/m before the effect of dead-space, where carriers have insufficient energy to ionize, causes significant errors. Moreover, only a very simple correction to the local expressions is needed to predict the multiplication accurately where the field varies rapidly in abrupt one-sided p/sup +/-n junctions doped up to 10/sup 18/ cm/sup -3/. However, MC modeling also shows that complex dead-space effects cause the local ionization coefficients to be increasingly unrepresentative of the position dependent values in the device as w is reduced below 1 /spl mu/m. The success of the local model in predicting multiplication is therefore attributed to the dead-space information already being contained within the experimentally determined values of local coefficients. It is suggested that these should therefore be thought of as effective coefficients which, despite the presence of dead-space effects, can be still be used with the existing local theory for efficiently quantifying multiplication and breakdown voltages.

Dead space effect on the wavelength dependence of gain and noise in avalanche photodiodes

We extend the dead space model proposed by Hayat et al. in order to determine the wavelength-dependent multiplication mean gain and excess noise factor F(/spl lambda/) in the case of mixed electron and hole injection, as it is the case when photons are absorbed within the multiplication region. We compare the predictions of the model with measurements performed on a silicon ultraviolet-selective avalanche photodiode with submicron thick multiplication region. We show that the multiplication gain is constant in the visible and near-infrared part of the spectrum, and increases in the UV range by a factor of 1.8. Furthermore, the excess noise factor is minimal for UV radiation and increases rapidly for longer wavelengths. It appears that the extended dead space model is very adequate at predicting the gain and noise measurement results. In order to unambiguously determine the effect of the dead space, we compare the predictions of our model with those of McIntyre's local noise model. The latter qualitatively describes the wavelength dependence of the gain, but greatly overestimates the excess noise factor.