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.     

III-V alloy heterostructure high speed avalanche photodiodes

Heterostructure avalanche photodiodes have been successfully fabricated in several III-V alloy systems: GaAlAs/GaAs, GaAlSb/GaSb, GaAlAsSb/GaAlSb, and InGaAsP/InP. These diodes cover optical wavelengths from0.4 to 1.8 mum. Early stages of development show very encouraging results. High speed response of <35 ps and high quantum efficiency >95 percent have been obtained. The dark currents and the excess avalanche noise will also be discussed. A direct comparison of GaAlSb, GaAlAsSb, and InGaAsP avalanche photodiodes is given.     

Signal and noise response of high speed germanium avalanche photodiodes

Germanium avalanche photodiodes, providing gain at microwave frequencies, have been fabricated and tested. The diodes employ a guard ring structure to achieve a uniform, microplasma-free, multiplying region with an active diameter of 40 microns. Low-frequency chopped light current gains of greater than 200, and small-signal 6 GHz current gains of greater than 10 have been obtained at room temperature for a carrier wavelength of 1.15 microns. In the normal operating range, the signal output power is found to vary as the square of the multiplication, and the noise is found to vary as the cube of the multiplication. This limits the maximum useful multiplication of the diode to that level which gives a diode noise equal to the receiver noise. A small-signal equivalent circuit with lumped elements corresponding to the physical processes occurring within the diode, is introduced to describe the small signal behavior. The model is valid over the entire multiplication range, up to frequencies of about 10 GHz.     

Ga1-xAlxSb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga1-xAlxSb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10^-4A/cm²have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found forx = 0.065(300 K) where the ratiobeta/alphaexceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy Eg, the threshold energy for hole initiated impact ionization reaches the smallest possible value (E_{i} = E_{g}) and the ionization process occurs with zero momentum. This leads to a strong increase of β atDelta/E_{g} = 1. The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.     

Detrimental effect of impact ionization in the absorption region on the frequency response and excess noise performance of InGaAs-InAlAs SACM avalanche photodiodes

It is shown that optimization of the electric field profile in the absorption region of separate absorption, charge, and multiplication InGaAs-InAlAs avalanche photodiodes is critical to achieve low excess noise and high gain bandwidth product.     

Pulse response of avalanche photodiodes

A study has been made of the time response of heterostructure avalanche photodiodes for InGaAs and InP/InGaAs material systems. A transfer/scattering matrix method, where the matrix parameters are related to the ionization coefficients, has been used. A time domain study has been carried out to find the time variation of electron and hole number densities and currents     

Calculated electron and hold spatial ionization profiles in bulkGaAs and superlattice avalanche photodiodes

The author presents ensemble Monte Carlo calculations of the electron and hole spatial and temporal impact ionization profiles in bulk GaAs and GaAs-AlGaAs superlattice structures. The results indicate that transient effects are much more pronounced for electrons than for holes, which greatly influences the dynamics of each type. It is concluded that the heterointerface plays less of a role in the hole ionization process than that for electrons due to the small valence band edge discontinuity and to the weaker nonlinear, transient effects experiences by the holes     

The probability distribution of gains in avalanche photodiodes

The probability density function for the gain of an avalanche photodiode is derived. The derivation is based on a Lagrange expansion of the implicit form of the moment generating function. The results confirm previously published conjectures.     

Absolute noise characterisation of avalanche photodiodes

Excess noise in four types of commercially obtained avalanche photodiodes (a.p.d.s) has been measured absolutely, by comparing avalanche noise from the a.p.d. with shot noise from an illuminated p-i-n diode. The method used yields directly the noise-current spectral density, simplifies the deduction of the quantum efficiency keff and hence the true value of the multiplication factor, and ultimately yields a measured value of the noise parameter x.     

Arrays of Geiger mode avalanche photodiodes

Following our previous work which has led us to fabricate single pixels of geiger mode avalanche photodiodes (GMAPs), we present in this letter the results regarding the fabrication and characterization of a bidimensional array of GMAPs. Low dark count rates and very good uniformity over the sensor are reported. High quantum efficiency in the visible range has been measured. Measurements indicate that not all the nominal active area is effectively sensitive. We have some preliminary evidence that no crosstalk effects are present in our device. Notwithstanding this, in view of a near future shrinking of all dead regions, an optical trench process has been developed and is illustrated here. Possible future trends are highlighted.     

Numerical simulation of avalanche breakdown within InP-InGaAs SAGCMstandoff avalanche photodiodes

The breakdown location within a planar InP/In_0.53Ga_0.47As (InGaAs) separate absorption, grading, charge sheet, and multiplication (SAGCM) avalanche photodiode (APD), using the standoff breakdown suppression design to replace guard rings, depends on the two-dimensional (2-D) geometry of the Zn diffused well. Since the geometry of this p⁺ diffusion is dependent upon the surface etch, the effects of varying the etch depth (t_standoff) and length of the sloped etch edge (w_slope ) are studied using a two-dimensional drift-diffusion simulator. It is determined that the etch depth brackets a region where center breakdown dominance is possible. To ensure center breakdown within this region it is concluded that there is a maximum value that the slope of the etch walls must not exceed     

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_0.2Ga_0.8As, and In_0.52Al_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     

短波碲镉汞电子雪崩二极管的模拟仿真

碲镉汞e-APD器件可用于低弱信号检测,碲镉汞材料工艺、器件工艺及器件结构设计对器件性能非常重要。本文采用silvaco软件对平面PIN结构的短波碲镉汞e-APD器件的暗电流、雪崩增益和量子效率进行了仿真分析。结果表明:1)高工作电压下,器件暗电流的主要成分是带间直接隧穿电流;2)工艺因素引入的接近禁带中心的陷阱能级决定器件在中等偏压下的暗电流特性;3)带间直接隧穿和电子碰撞电离主要发生在低掺杂的雪崩放大区;4)在固定偏压下,器件的暗电流和雪崩增益随着雪崩放大区宽度的上升而减少;5)在固定偏压下,器件的雪崩增益随吸收层厚度的增加而轻微增加,同时量子效率逐渐下降。为实现高性能的短波碲镉汞e-APD器件,需要合理设计器件结构和优化材料生长工艺及器件制造工艺。     

Trapping phenomena in avalanche photodiodes on nanosecond scale

A technique for measuring the release of minority carriers emitted from deep levels in avalanche photodiodes (APDs) at operating conditions is discussed. The method, time-correlated carrier counting (TCCC), is very sensitive and accurate. Densities of filled traps were measured down to 10⁹ cm^-3 and lifetimes in the nanosecond range. This technique can be useful in tailoring gettering processes for APDs and in studies of traps at high electric fields     

Investigation of microplasmas in InP avalanche photodiodes

The electron beam induced current technique has been used to investigate microplasmas in InP avalanche photodiodes. It is shown that microplasmas develop along growth induced doping striations. Slope discontinuities in the I-V characteristic have been correlated to the turn on voltage of microplasmas.     

中波碲镉汞雪崩光电二极管(APD)增益特性

采用不同工艺制备了中波碲镉汞雪崩二极管(HgCdTe APD)器件,利用不同方法对其结特性和增益随偏压变化关系进行了表征,并基于Beck模型和肖克莱解析式进行了拟合分析。结果表明,不同工艺制备的APD器件饱和耗尽区宽度分别为1.2μm 和2.5μm,较宽的耗尽层有效抑制了高反偏下器件的隧道电流,器件有效增益则从近100提高至1000以上。采用拟合HgCdTe APD器件增益-偏压曲线获得了较好的效果,且拟合得到的参数与Sofradir的Rothman的结果相似。     

Frequency responses of graded-bandgap low-noise avalanche photodiodes

Frequency responses of graded-bandgap low-noise avalanche photodiodes using AlcGa1-cAs, GaAS1-cSbc, and IncGa1-cAs have been computed using a novel hybrid computational technique. Response characteristics of nongraded structures have also been computed and a comparison is made.     

Characterisation of avalanche photodiodes having nearly-unilateral-carrier multiplication

Theoretical and experimental results for the output noise spectral density of avalanche photodiodes with nearly-unilateral-carrier multiplication are combined to obtain both the quantum efficiency and the noise factor from noise measurements over the operating range of multiplication factors. A noise factor of 2+0.012 M is reported for a silicon diode.