Effect of dead space on the excess noise factor and time responseof avalanche photodiodes

The effect of dead space on the statistics of the gain process in continuous-multiplication avalanche photodiodes (APDs) is determined using the theory of age-dependent branching processes. The dead space is the minimum distance that a newly generated carrier must travel in order to acquire sufficient energy to cause an impact ionization. Analytical expressions are derived for the mean gain, the excess noise factor, and the mean and standard deviation of the impulse response function, for the dead-space-modified avalanche photodiode (DAPD), under conditions of single carrier multiplication. The results differ considerably from the well-known formulas derived by R.J. McIntyre and S.D. Personick in the absence of dead space. Relatively simple asymptotic expressions for the mean gain and excess noise factor are obtained for devices with long multiplication regions. In terms of the signal-to-noise ratio (SNR) of an optical receiver in the presence of circuit noise, it is established that there is a salutory effect of using a properly designed DAPD in place of a conventional APD. The relative merits of using DAPD versus a multilayer (superlattice) avalanche photodiode (SAPD) are examined in the context of receiver SNR; the best choice turns out to depend on which device parameters are used for the comparison     

Effect of doping profile on avalanche noise of silicon IMPATT diodes

The open-circuit spectral noise-voltage density e2/?f of silicon IMPATT diodes with different punchthrough factors has been measured. A noise reduction has been obtained for diodes with punchthrough factors > 1. This is in agreement with theoretical curves calculated on the basis of the noise theory of Gummel and Blue.     

Effect of finite current multiplication factor in the avalanche zone on the high-frequency noise properties of read diodes

The high-frequency noise properties of a Read diode, whose current multiplication factor in the avalanche zone can be controlled and is finite, has been analyzed. The analysis indicates that the open-circuit noise voltage and the noise figure of the diode are reduced with the lowering of the current multiplication factor. It is also found that the open-circuit noise voltage exhibits a finite peak at the avalanche frequency for finite but large values of M which, however, disappears at lower values of M(≤100).     

Temperature dependence of multiplication noise in silicon avalanche photodiodes

The temperature dependence of multiplication noise in silicon avalanche photodiodes with a low-high-low impurity density profile is calculated. The variation of multiplication noise by temperature change can be neglected in practical use at a constant multiplication factor, which is in agreement with experimental results.     

Wavelength dependence of multiplication noise in silicon avalanche photodiodes

Theoretical and experimental results on wavelength dependence of multiplication noise in silicon avalanche photodiodes are described. When the photodiode has a p-n⁺-junction and is illuminated from the n⁺-side, multiplication noise increases by decreasing optical wavelength. Effective ionization coefficient ratio keffis equal tokexp (2Kw_{a}) for a uniform junction electric field, wherekis the ratio of ionization coefficients of electrons α and holes β. The multiplication noise depends on the product of optical absorption coefficientKand the avalanche-region width wa. Calculations show that there exists an optimum wafor minimizing multiplication noise at a given wavelength. Theoretical results are shown to agree with results of experiments on diodes with a low-high-low impurity profile. Measured ionization coefficient ratiokvalues are 0.04 and 0.08 at 0.811- and 0.633-µm wavelength, respectively.     

A resonant-cavity, separate-absorption-and-multiplication,avalanche photodiode with low excess noise factor

We report on the design, fabrication, and performance of a photodiode that combines the advantages of a resonant cavity with a separate-absorption-and-multiplication avalanche photodiode. The device is grown on GaAs using molecular beam epitaxy and is designed to detect light near 900 nm. This photodetector has exhibited the following characteristics: an external quantum efficiency of 70%, a spectral linewidth of less than 7 nm, an avalanche gain in excess of 30, and low dark current. In addition, a low excess noise factor corresponding to 0.2⩽k⩽0.3 has been achieved     

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     

Effects of time dependence of multiplication process on avalanche noise

Theoretical and experimental studies of noise generated due to the randomness of the multiplication process in the avalanche region of a uniform diode are presented. The theory extends the results of McIntyre to include the time dependence of the multiplication process. It also shows the correspondence between the results of McIntyre, Gummel and Blue, Hines and Tager. The space-charge feedback and transit-time effects have been neglected in this analysis. The theoretical and the experimental results described have shown that even at frequencies well below transit-time frequency, the importance of the factor resulting from consideration of the time dependence of the multiplication process cannot be ignored.The measurements of the avalanche noise on uniform p⁺-n-n⁺ silicon diodes are found to be in good agreement with the theory presented here.     

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.     

Silicon avalanche photodiodes with low multiplication noise and high-speed response

Low-noise and high-speed silicon avalanche photodiodes with low breakdown voltage are reported. The diode structure with a low-high-low impurity density profile is proposed to have low-noise characteristics. Multiplication noise and depletion layer width of several structures are compared theoretically, and effects of impurity density profile of the avalanche region are discussed. Built-in field is also provided to realize high-speed response without increasing operating voltage. Silicon avalanche photodiodes with the above mentioned structure have been fabricated with long time substrate annealing, ion implantation, and epitaxial growth. Attained performances are as follows: noise parameter k = 0.027 - 0.040, output pulse half width τ = 260 ps for a mode-locked Nd:YAG laser pulse, gain-bandwidth product up to 300 GHz at M = 400, quantum efficiency 0.55 - 0.66 at the 0.81- to 0.83-µm wavelength, and breakdown voltage about 100 V.     

Excess noise analysis of separate absorption multiplication regionsuperlattice avalanche photodiodes

The operation of a separate absorption multiplication region avalanche photodiode (SAM-APD) introduces noise as results of randomness in the number and in the position at which dark carrier pairs are generated, randomness in the photon arrival number, randomness in the carrier multiplication, and the number and the position of the photogenerated carriers in the bulk of the diode. The dark current results in a smaller mean multiplication gain in excess noise factor versus mean multiplication plot due to the partial multiplication process of these generated carriers compared to the usual values associated with carriers injected at one edge of the diode. Previous analyses of mean multiplication and excess noise factor for an arbitrary superposition of injected carriers are extended to allow the presence of dark carriers in the multiplication region under the model, which admits variation (with position) of the band-gap, dark generated rate, and ionization coefficients with each stage for the superlattice APD, and the presence of impact ionization in the absorption region. The calculations reveal the presence of impact ionization carriers in the absorption region which results in a larger excess noise factor than the usual values associated with carriers injected at one edge of the device, and fits well with experimental results     

低过剩噪声级联倍增雪崩探测器设计

基于弛豫空间倍增理论数值模型和修正的弛豫空间倍增理论模型,分析了不同倍增级数和不同载流子初始能量时级联倍增雪崩探测器的过剩噪声.研究了不同碰撞离化倍增层厚度、不同电子预加热层厚度、不同电场控制层掺杂浓度对过剩噪声因子的影响.同时,比较了DSMT模型、Van Vilet模型和McIntyre模型得到的结果.通过调整碰撞离化倍增层厚度、电子预加热层厚度和电场控制层掺杂浓度,DSMT数值模拟获得了一个相对优化的结构,其过剩噪声与Van Vliet模型k_s=0. 057时相当.     

Scanning electron microscopy study of the avalanche multiplication factor

A method is presented for examining the distribution of the multiplication process over the active area of shallow junction avalanche diodes using a scanning electron microscope. The multiplication factor is measured as a function of both diode voltage and electron beam energy, and the results are compared with those obtained by more conventional techniques.     

温度对恒虚警补偿下APD倍增因子的影响

雪崩光电二极管倍增因子、噪声都会受到温度影响,在恒虚警下APD倍增因子、噪声之间会相互作用,为了研究温度变化对APD性能造成的影响并对其进行评估,分析了温度对暗电流、热噪声、倍增因子的影响及恒虚警补偿方式工作原理。建立了恒虚警下温度与倍增因子的关系模型,对模型进行了实验验证,利用模型对不同背景光、不同虚警概率下温度对倍增因子的影响进行了分析,提出了温度对恒虚警下APD性能影响的评估方法。结果表明:恒虚警下,倍增因子随温度的升高而降低;同一温度下,背景光越强或倍增因子越小时,虚警概率低,倍增因子较小;且背景光越弱、虚警概率越低时,温度对倍增因子的影响越明显。     

Effect of thermal noise and APD noise on the performance ofOPPM-CDMA receivers

The effect of thermal noise and avalanche-photodiode (APD) noise on the performance of optical overlapping pulse-position modulation/code-division multiple-access (OPPM-CDMA) systems with and without double optical hardlimiters is examined. Comparisons with the corresponding optical on-off keying/CDMA (OOK-CDMA) systems are presented as well. The maximum data rate that can be achieved under laser pulsewidth and bit error rate constraints is determined. It is shown that about 10 dB increase in the average power, with respect to the Poisson shot-noise-limited system, is required to compensate for the performance degradation due to thermal noise and APD noise. Moreover, it is indicated that for a bit error rate not exceeding 10^-9, a laser pulsewidth of 0.03 ns, and an average received optical power of -55 dBm, a data rate of more than 3 Gb/s can be achieved per channel when using OPPM-CDMA systems with double optical hardlimiters     

Effect of mesa overgrowth on low-frequency noise in planar separateabsorption, grading, charge, and multiplication avalanche photodiodes

The purpose of this paper is to present a detailed comparison of the low-frequency noise in single-growth and mesa overgrown planar separate absorption, grading, charge, and multiplication (SAGCM) avalanche photodiodes (APD's) passivated with SiN_x. It was found that existing models did not explain the bias dependence of the low-frequency noise in the mesa overgrown devices. We have found that a low-temperature anneal decreased the low-frequency noise level in single-growth and mesa overgrown APD's. In addition, generation-recombination (G-R) noise was detected in mesa overgrown APD's     

The static and dynamic multiplication factor in avalanche photodiode optical receivers

By applying two different methods we analyze the nonlinear network which determines the multiplication factor M (t) of an avalanche photodiode (APD) in an optical receiver. For the special time dependence[cos^{2} (pit/T) ]^{K}of incident light power we give numerical results and approximate analytic expressions for the dc and ac component of the multiplication factor M. We then discuss amplification fluctuations in analogue optical receivers for both constant current and constant voltage biasing of the APD. A constant current biased APD amplifies temperature and signal independently, if the received mean optical power is constant (PPM, FM). In case of modulation schemes with signal dependent mean optical power (PFM, IM), a constant voltage biasing is more suited, although an AGC circuit is required now in order to compensate for the temperature-dependent amplification.     

Determination of doping factor,mobility ratio and excess concentration using photovoltages at extreme band bendings

Photovoltages (surface photovoltage plus Dember voltage) were calculated and plotted, taking into consideration the influence of charge exchange between a continuum of surface states and the semiconductor. For moderate band bendings the photovoltage depends on several parameters which renders their reliable determination very difficult. For extreme band bendings, however, only doping factor, mobility ratio and relative excess concentration enter into given formulas. It is shown how by means of field-modulated photovoltage measurements using short pulses of light these three parameters could be determined reliably. Ge-samples with real surfaces were used. Good agreement between parameters determined in this way and otherwise indicates validity of the model and the approximations used in this work. The mobility ratio was found constant up to excess concentrations of about 10¹⁴ cm^?3. Possible extension of such measurements to semiconductors whose doping factors are very different from unity and applications are shortly discussed.     

雪崩管最佳倍增因子线性化温控技术研究

为了进一步增强机载激光测距机在全温范围内的环境适应性,分析了温度与探测器模块输出功率信噪比的关系,推导了最佳倍增因子与温度的方程式,阐述了温度变化引起倍增因子对最佳雪崩倍增因子偏离的原因。根据雪崩管探测器雪崩击穿电压的线性温度特性,设计了机载温度范围为-55℃~70℃的基于自然对数法的最佳倍增因子雪崩偏压线性化温控电路,用于补偿因温度变化所引起的倍增因子对最佳雪崩倍增因子的偏离。结果表明,实测雪崩偏压温控系数为2.29V/℃,与理论分析值误差仅为4%。该技术用于新型机载激光测距系统中,获得了良好的试验数据,满足机载环境的特殊需求。     

Effect of stochastic dead space on noise in avalanche photodiodes

A stochastic dead-space model for impact ionization is developed and used to study the effect of the soft nature of the ionization capability of carriers on the excess noise factor of avalanche photodiodes. The proposed model is based on the rationale that the gradual, or soft, transition in the probability density function (PDF) for the distance from birth to impact ionization can be viewed as that resulting from uncertainty in the dead space itself. The resulting soft PDF, which is parameterized by a tunable softness parameter, is used to establish the limitations of the existing hard-threshold ionization models in ultrathin multiplication layers. Calculations show that for a fixed operational gain and fixed average dead space, the excess noise factor tends to increase as a result of the softness in the PDF in very thin multiplication layers (viz, <70 nm), or equivalently, under high applied electric fields (viz., >800 kV/cm). A method is proposed for extracting the softness parameter from noise versus multiplication measurements.