Effect of impact ionization in the InGaAs absorber on excess noise of avalanche photodiodes

The effects of impact ionization in the InGaAs absorption layer on the multiplication, excess noise and breakdown voltage are modeled for avalanche photodiodes (APDs), both with InP and with InAlAs multiplication regions. The calculations allow for dead space effects and for the low field electron ionization observed in InGaAs. The results confirm that impact ionization in the InGaAs absorption layer increases the excess noise in InP APDs and that the effect imposes tight constraints on the doping of the charge control layer if avalanche noise is to be minimized. However, the excess noise of InAlAs APDs is predicted to be reduced by impact ionization in the InGaAs layer. Furthermore the breakdown voltage of InAlAs APDs is less sensitive to ionization in the InGaAs layer and these results increase tolerance to doping variations in the field control layer.     

Breakdown probabilities for thin heterostructure avalanche photodiodes

The recurrence theory for the breakdown probability in avalanche photodiodes (APDs) is generalized to heterostructure APDs that may have multiple multiplication layers. The generalization addresses layer-boundary effects such as the initial energy of injected carriers as well as the layer-dependent profile of the dead space in the multiplication region. Reducing the width of the multiplication layer serves to both downshift and sharpen the breakdown probability curve as a function of the applied reverse-bias voltage. In structures where the injected carriers have an initial energy that is comparable to the ionization threshold energy, the transition from linear mode to Geiger-mode is more abrupt than in structures in which such initial energy is negligible. The theory is applied to two recently fabricated Al/sub 0.6/Ga/sub 0.4/As-GaAs heterostructure APDs and to other homostructure thin GaAs APDs and the predictions of the breakdown-voltage thresholds are verified.     

Effect of dead space on gain and noise in Si and GaAs avalanchephotodiodes

The effect of dead space on the mean gain, the excess noise factor, and the avalanche breakdown voltage for Si and GaAs avalanche photodiodes (APDs) with nonuniform carrier ionization coefficients are examined. The dead space, which is a function of the electric field and position within the multiplication region of the APD, is the minimum distance that a newly generated carrier must travel in order to acquire sufficient energy to become capable of causing impact ionization. Recurrence relations in the form of coupled linear integral equations are derived to characterize the underlying avalanche multiplication process. Numerical solutions to the integral equations are obtained and the mean gain and the excess noise factor are computed     

Design and analysis of separate-absorption-transport-charge-multiplication traveling-wave avalanche photodetectors

This paper proposes a novel type of avalanche photodiode-the separate-absorption-transport-charge-multiplication (SATCM) avalanche photodiode (APD). The novel design of photoabsorption and multiplication layers of APDs can avoid the photoabsorption layer breakdown and hole-transport problems, exhibit low operation voltage, and achieve ultra-high-gain bandwidth product performances. To achieve low excess noise and ultra-high-speed performance in the fiber communication regime (1.3/spl sim/1.55 /spl mu/m), the simulated APD is Si-based with an SiGe-Si superlattice (SL) as the photoabsorption layer and traveling-wave geometric structures. The frequency response is simulated by means of a photo-distributed current model, which includes all the bandwidth-limiting factors, such as the dispersion of microwave propagation loss, velocity mismatch, boundary reflection, and multiplication/transport of photogenerated carriers. By properly choosing the thicknesses of the transport and multiplication layers, microwave propagation effects in the traveling-wave structure can be minimized without increasing the operation voltage significantly. A near 30-Gb/s electrical bandwidth and 10/spl times/ avalanche gain can be achieved simultaneously, even with a long device absorption length (150 /spl mu/m) and low operation voltage (/spl sim/12 V). In addition, the ultrahigh output saturation power bandwidth product of this simulated TWAPD structure can also be expected due to the large photoabsorption volume and superior microwave-guiding structure.     

Temperature dependent studies of InP/InGaAs avalanche photodiodesbased on time domain modeling

Using a simplified time domain modeling approach, the temperature dependent performance characteristics, such as multiplication gain, breakdown voltage, -3 dB bandwidth, gain bandwidth product and excess noise factor, have been systematically investigated for InP/InGaAs separate absorption, grading, charge and multiplication avalanche photodiodes as a function of temperature from -50°C to 110°C. In order to model the -3 dB bandwidth versus gain dependence based on the simplified approach, empirical expressions have been proposed to consider the effects of hole diffusion, hole trapping, RC (resistance-capacitance) and gain-bandwidth product limit together with the fast Fourier transform component of the impulse response from the time domain modeling. The modeling results generally agree with or can explain the corresponding experimental results. The effects of changing material parameters on the modeling results are also discussed. In addition, we have found that E_rO, the average energy loss per collision due to optical phonon scattering at 0 K, plays a dominant role in determining the -3 dB bandwidth near breakdown and the slope of the temperature dependence of the breakdown voltage. Further, the improved performance characteristics at decreased temperatures indicate the potential application prospects of the InP/lnGaAs APDs in low temperature environments     

Long-wavelength In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As large-area avalanche photodiodes and arrays

Large-area (500-/spl mu/m diameter) mesa-structure In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As avalanche photodiodes (APDs) are reported. The dark current density was /spl sim/2.5/spl times/10/sup -2/ nA//spl mu/m/sup 2/ at 90% of breakdown; low surface leakage current density (/spl sim/4.2 pA//spl mu/m) was achieved with wet chemical etching and SiO/sub 2/ passivation. An 18 /spl times/ 18 APD array with uniform distributions of breakdown voltage, dark current, and multiplication gain has also been demonstrated. The APDs in the array achieved 3-dB bandwidth of /spl sim/8 GHz at low gain and a gain-bandwidth product of /spl sim/120 GHz.     

SAGCM-APD增益影响因子分析与优化

应用二维漂移扩散模型研究具有分立吸收层、渐变层、电荷层和倍增层结构(SAGCM)的InGaAsP-InP雪崩光电探测器(APD),仿真分析了不同电荷层、倍增层厚度和掺杂浓度对电场分布、电流响应及击穿电压的影响,特别是参数变量对增益计算模型的影响,载流子传输过程的时间依赖关系和倍增层中所处位置的影响,仿真结果表明:较高掺杂浓度和较薄电荷层结构可以改变器件内部的电场分布,进而提高增益值.当入射光波长为1.55μm,光功率为500 W/m2时,光电流响应量级在10-2A;阈值电压降低到10V以下,击穿电压为42.6V时,器件倍增增益值大于100.     

Optimization of charge and multiplication layers of 20-Gbps InGaAs/InAlAs avalanche photodiode

We calculated the correlation between the doping concentration of the charge layer and the multiplication layer for separate absorption, grading, charge, and multiplication InGaAs/InAlAs avalanche photodiodes (APDs). For this purpose, a predictable program was developed according to the concentration and thickness of the charge layer and the multiplication layer. We also optimized the design, fabrication, and characteristics of an APD for 20 Gbps application. The punch-through voltage and breakdown voltage of the fabricated device were 10 V and 33 V, respectively, and it was confirmed that these almost matched the designed values. The 3-dB bandwidth of the APD was 10.4 GHz, and the bit rate was approximately 20.8 Gbps.     

MBE生长的PIN结构碲镉汞红外雪崩光电二极管

对中波红外碲镉汞雪崩光电二极管(APD)特性进行理论计算,获得材料的能量散射因子及电离阈值能级与材料特性的相互关系,从而计算器件的理论雪崩增益与击穿电压.通过对材料特性(组分,外延厚度,掺杂浓度等)的优化,设计并生长了适合制备PIN结构红外雪崩光电二极管的碲镉汞材料,并进行了器件验证.结果显示,在10V反偏电压下,该器件电流增益可达335.     

A simplified approach to time-domain modeling of avalanchephotodiodes

A simplified algorithm for calculating time response of avalanche photodiodes (APDs) is presented. The algorithm considers the time course of avalanche processes for the general case of position-dependent double-carrier multiplications including the dead space effect. The algorithm is based on a discrete time setting ideally suited for computer modeling and can be applied to any APD structure. It gives a fast and accurate estimation of the time and frequency response of APDs. As an example, the present method is applied to InP-InGaAs separate absorption, grading, charge, and multiplication (SAGCM) APDs. The variation of multiplication pain with bias voltage and 3-dB electrical bandwidth at different multiplication gain obtained using the new algorithm show good agreement with experimental results. The algorithm can be used to study temperature dependence of APD characteristics and can be easily extended to calculate the excess noise factor     

Dead-space-based theory correctly predicts excess noise factor forthin GaAs and AlGaAs avalanche photodiodes

The conventional McIntyre carrier multiplication theory for avalanche photodiodes (APDs) does not adequately describe the experimental results obtained from APDs with thin multiplication-regions. Using published data for thin GaAs and Al_0.2Ga_0.8As APDs, collected from multiplication-regions of different widths, we show that incorporating dead-space in the model resolves the discrepancy. The ionization coefficients of enabled carriers that have traveled the dead space are determined as functions of the electric field, within the confines of a single exponential model for each device, independent of multiplication-region width. The model parameters are determined directly from experimental data. The use of these physically based ionization coefficients in the dead-space multiplication theory, developed earlier by Hayat et al. provide excess noise factor versus mean gain curves that accord very closely with those measured for each device, regardless of multiplication-region width. It is verified that the ratio of the dead-space to the multiplication-region width increases, for a fixed mean gain, as the width is reduced. This behavior, too, is in accord with the reduction of the excess noise factor predicted by the dead-space multiplication theory     

A physically-based MOS transistor avalanche breakdown model

A physically based breakdown model for MOSFET's is presented to rectify the unexplained experimental breakdown behaviors. The drain avalanche breakdown in the MOS transistor can be caused by either infinite multiplication (MI) or finite multiplication with positive feedback of the substrate current (MF) due to the impact ionization in the pinch-off region. The breakdown voltages of these two modes of breakdown have different dependencies on the biasing conditions and device parameters. For MI mode of breakdown, the breakdown voltage increases slowly with the gate voltage and can be approximated by the drain saturation voltage plus a constant offset. For MF mode of breakdown, the breakdown voltage decreases as the drain saturation current becomes larger. The calculated breakdown characteristics agree well with the measured ones for devices with effective channel length in the range of 0.44~10 μm     

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.     

Short-Wave Infrared HgCdTe Avalanche Photodiodes

Short-wave infrared (SWIR) HgCdTe avalanche photodiodes (APDs) have been developed to address low-flux applications at low operating temperature and for laser detection at higher temperatures. Stable multiplication gains in excess of 200 have been observed in homojunction APDs with cutoff wavelengths down to 2.8???m and operating temperatures up to 300?K, associated with low excess noise F?<?1.3 and low 1/f noise. The measured dark current density at 200?K of 6.2???A/cm² is low enough to enable high-sensitivity single-element light detection and ranging (lidar) applications and time-of-flight imaging. Corresponding APD arrays have been hybridized on a readout integrated circuit (ROIC) designed for low-flux low-SNR imaging with low noise and frame rates higher than 1500?frames/s. Preliminary focal-plane array characterization has confirmed the nominal ROIC performance and showed pixel operability above 99.5% (pixels within ±50% of average gain). The bias dependence of the multiplication gain has been characterized as a function of temperature, cadmium composition, and junction geometry. A qualitative change in the bias dependence of the gain compared with mid-wave infrared (MWIR) HgCdTe has motivated the development of a modified local electric field model for the electron impaction ionization coefficient and multiplication gain. This model gives a close fit to the gain curves in both SWIR and MWIR APDs at temperatures between 80?K and 300?K, using two parameters that scale as a function of the energy gap and temperature. This property opens the path to quantitative predictive device simulations and to estimations of the junction geometry of APDs from the bias dependence of the gain.     

Study of reverse dark current in 4H-SiC avalanche photodiodes

Temperature-dependent current-voltage (I-V) measurements have been used to determine the reverse dark current mechanisms in 4H-SiC avalanche photodiodes (APDs). A pn junction vertical mesa structure, passivated with SiO/sub 2/ grown by plasma enhanced chemical vapor deposition, exhibits predominate leakage current along the mesa sidewall. Similar APDs, passivated by thermal oxide, exhibit lower dark current before breakdown; however, when the temperature is higher than 146/spl deg/C, an anomalous dark current, which increases rapidly with temperature, is observed. This current component appears to be eliminated by the removal of the thermal oxide. Near breakdown, tunneling is the dominant dark current mechanism for these pn devices. APDs fabricated from a pp/sup -/n structure show reduced tunneling current. At room temperature, the dark current at 95% of breakdown voltage is 140 fA (1.8 nA/cm/sup 2/) for a 100-/spl mu/m diameter APD. At a gain of 1000, the dark current is 35 pA (0.44 /spl mu/A/cm/sup 2/).     

To the Problem of Optimization of Parameters of a Double Heterostructure Based on Direct-Gap Semiconductors for Avalanche Photodiodes

A double heterostructure based on direct-gap semiconductors with a photoabsorption middle layer at the avalanche breakdown voltage is considered. Such structures are used in the development of avalanche photodiodes with separate absorption and multiplication regions (APD with SAMR). It is shown that impact generation of electron–hole pairs should be considered in calculating the maximum possible characteristics of APDs with SAMR even in the absorption layer; therewith, this can be performed analytically.

     

A numerical model of avalanche breakdown in MOSFET's

An accurate numerical model of avalanche breakdown in MOSFET's is presented. Features of this model are a) use of an accurate electric-field distribution calculated by a two-dimensional numerical analysis, b) introduction of multiplication factors for a high-field path and the channel current path, and c) incorporation of the feedback effect of the excess substrate current induced by impact ionization into the two-dimensional calculation. This model is applied to normal breakdown observed in p-MOSFET's and to negative-resistance breakdown (snap-back or switchback breakdown) observed in short-channel n-MOSFET's. Excess substrate current generated from channel current by impact ionization causes a significant voltage drop across the substrate resistance in short-channel n-MOSFET's. This voltage forward-biases the source-substrate junction and increases channel current causing a positive feedback effect. This results in a decrease of the breakdown voltage and leads to negative-resistance characteristics. Current-voltage characteristics calculated by the present model agree very well with experimental results. Another model, highly simplified and convenient for device design, is also presented. It predicts some advantages of p-MOSFET's over n-MOSFET's from the standpoint of avalanche breakdown voltage, particularly in the submicrometer channel-length range.     

Statistical correlation of gain and buildup time in APDs and its effects on receiver performance

This paper reports a novel recurrence theory that enables us to calculate the exact joint probability density function (pdf) of the random gain and the random avalanche buildup time in avalanche photodiodes (APDs) including the effect of dead space. Such calculations reveal a strong statistical correlation between the gain and the buildup time for all widths of the multiplication region. To facilitate the calculation of the photocurrent statistics in the presence of this correlation, the impulse-response function of the APD is approximately modeled by a function of time whose prespecified shape is appropriately parameterized by two random variables: the gain and the buildup time. The evaluation of the variance of the photocurrent under this model leads to the definition of the shot-noise-equivalent bandwidth of the APD, which captures the statistical correlation between the gain and the buildup time. It is shown that the shot-noise-equivalent bandwidth in GaAs APDs is greater, by approximately 30%, than the traditional buildup-time-limited 3-dB bandwidth, which is calculated from the mean of the impulse-response function. A thorough analysis of the performance of APD-based integrate-and-dump digital receivers reveals that the strong correlation between the gain and the buildup time accentuates intersymbol interference (ISI) noise, and thus, adversely affects receiver sensitivity at high transmission rates beyond previously known limits.     

Evaluation of a Junction Termination Extension Avalanche Photodiode for X-Ray Detection

An avalanche photodiode (APD) with a ring structure around the active area was built. The junction termination extension (JTE) APD has three diffused rings around the main junction to reduce the electric field at the surface. This design has the advantage that it does not need a sharp bevel edge or grooves to avoid early breakdown at the surface. The JTE rings can be obtained by a well-controlled ion-implantation through a single mask. The process uses standard planar technology for silicon devices. Several APDs with 2-mm diameter active area have been built by implantation of boron with a dose of 2, 3, 4, and 5 x 10¹² cm^-2, followed by deep diffusion to 14 mum. The dark current is strongly dependent on the implantation charge, decreasing with decreasing charge. For the APDs with an implanted dose of 5 x 10¹² cm^-2, a gain of 8 is obtained at 1120 V, indicating that the devices have premature breakdown. The energy resolution from a 109Cd X-ray source (22.16 keV) was measured to be 4.7-keV full-width at half-maximum, which corresponds to 560 rms electrons noise. We have also performed simulations of the gain and breakdown voltage that correlate well with the results up to a gain of 5.     

Frequency response and modeling of resonant-cavity separateabsorption, charge, and multiplication avalanche photodiodes

A theoretical model incorporating the mechanism of resonant absorption of the multiple reflected lightwaves is presented for the frequency response of resonant-cavity (RC) separate absorption, charge, and multiplication (SACM) avalanche photodiodes (APDs). The derived theoretical expressions are general and can be readily applied to many other RC and non-RC APDs. These analytical expressions also allow for fast computation of the frequency response and bandwidth characteristics. Combining this frequency response theory with expressions of multiplication gain and ionization coefficients, an efficient approach is proposed for modeling the general performance characteristics of RC APDs. The modeling approach is applied to an InGaAs-AlGaAs RC SACM APD. The computed results are demonstrated, and the results of -3 dB bandwidth are comparable to experimental work. The validity of the modeling parameters is also discussed. It is further found that the normalized frequency response is unaffected when the value of the absorption coefficient is changed, suggesting that the standing-wave effect within the RC structure may not influence the bandwidth characteristics