Optimized breakdown probabilities in Al/sub 0.6/Ga/sub 0.4/As-GaAs heterojunction avalanche photodiodes

Recently, it has been shown that the noise characteristics of heterojunction Al/sub 0.6/Ga/sub 0.4/As-GaAs avalanche photodiodes (APDs) can be optimized by proper selection of the width of the Al/sub 0.6/Ga/sub 0.4/As layer. Similar trends have also been shown theoretically for the bandwidth characteristics. The resulting noise reduction and potential bandwidth enhancement have been attributed to the fact that the high bandgap Al/sub 0.6/Ga/sub 0.4/As layer serves to energize the injected electrons, thereby minimizing their first dead space in the GaAs layer. We show theoretically that the same optimized structures yield optimal breakdown-probability characteristics when the APD is operated in Geiger mode. The steep breakdown-probability characteristics, as a function of the excess bias, of thick multiplication regions (e.g., in a 1000-nm GaAs homojunction) can be mimicked in much thinner optimized Al/sub 0.6/Ga/sub 0.4/As-GaAs APDs (e.g., in a 40-nm Al/sub 0.6/Ga/sub 0.4/As and 200-nm GaAs structure) with the added advantage of having a reduced breakdown voltage (e.g., from 36.5 V to 13.7 V).     

Optimal excess noise reduction in thin heterojunction Al/sub 0.6/Ga/sub 0.4/As-GaAs avalanche photodiodes

It has been recently found that the initial-energy effect, which is associated with the finite initial energy of carriers entering the multiplication region of an avalanche photodiode (APD), can be tailored to reduce the excess noise well beyond the previously known limits for thin APDs. However, the control of the initial energy of injected carriers can be difficult in practice for an APD with a single multiplication layer. In this paper, the dead-space multiplication recurrence theory is used to show that the low noise characteristics associated with the initial-energy effect can be achieved by utilizing a two-layer multiplication region. As an example, a high bandgap Al/sub 0.6/Ga/sub 0.4/As material, termed the energy-buildup layer, is used to elevate the energy of injected carriers without incurring significant multiplication events, while a second GaAs layer with a lower bandgap energy is used as the primary carrier multiplication layer. Computations show that devices can be optimally designed through judicious choice of the charge-layer width to produce excess noise factor levels that are comparable to those corresponding to homojunction APDs benefiting from a maximal initial-energy effect. A structure is presented to achieve precisely that.     

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.     

Detection efficiencies and generalized breakdown probabilities for nanosecond-gated near infrared single-photon avalanche photodiodes

A rigorous model is developed for determining single-photon quantum efficiency (SPQE) of single-photon avalanche photodiodes (SPADs) with simple or heterojunction multiplication regions. The analysis assumes nanosecond gated-mode operation of the SPADs and that band-to-band tunneling of carriers is the dominant source of dark current in the multiplication region. The model is then utilized to optimize the SPQE as a function of the applied voltage, for a given operating temperature and multiplication-region structure and material. The model can be applied to SPADs with In/sub 0.52/Al/sub 0.48/As or InP multiplication regions as well as In/sub 0.52/Al/sub 0.48/As--InP heterojunction multiplication regions for wavelengths of 1.3 and 1.55 /spl mu/m. The predictions show that the SPQE generally decreases with decreasing the multiplication-region thickness. Moreover, an InP multiplication region requires a lower breakdown electric field (and, hence, offers a higher SPQE) than that required by an In/sub 0.52/Al/sub 0.48/As layer of the same width. The model also shows that the fractional width of the In/sub 0.52/Al/sub 0.48/As layer in an In/sub 0.52/Al/sub 0.48/As--InP heterojunction multiplication region can be optimized to attain a maximum SPQE that is greater than that offered by an InP multiplication region. This effect becomes more pronounced in thin multiplication regions as a result of the increased significance of dead space.     

Boundary effects on multiplication noise in thin heterostructure avalanche photodiodes: theory and experiment [Al/sub 0.6/Ga/sub 0.4/As/GaAs]

The history-dependent recurrence theory for multiplication noise in avalanche photodiodes (APDs), developed by Hayat et al., is generalized to include inter-layer boundary effects in heterostructure APDs with multilayer multiplication regions. These boundary effects include the initial energy of injected carriers as well as bandgap-transition effects within a multilayer multiplication region. It is shown that the excess noise factor can be significantly reduced if the avalanche process is initiated with an energetic carrier, in which case the initial energy serves to reduce the initial dead space associated with the injected carrier. An excess noise factor reduction up to 40% below the traditional thin-APD limit is predicted for GaAs, depending on the operational gain and the multiplication-region's width. The generalized model also thoroughly characterizes the behavior of dead space as a function of position across layers. This simultaneously captures the effect of the nonuniform electric field as well as the anticipatory nature of inter-layer bandgap-boundary effects.     

Excess noise in GaAs avalanche photodiodes with thin multiplicationregions

It is well known that the gain-bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP-InGaAsP-InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This paper presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite “size effect” for multiplication regions less than approximately 0.5 μm. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process     

Geiger mode operation of an In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As avalanche photodiode

Low-temperature photon counting with gated mode quenching is demonstrated with separate absorption, charge, and multiplication avalanche photodiodes that have an In/sub 0.52/Al/sub 0.48/As multiplication layer. A minimum of ten dark counts per second and single-photon detection efficiency of 16% were achieved at 130 K.     

BV/sub CEO/--BV/sub CBO/ separation and sharpness of breakdown in high-speed bipolar transistors

The effect of secondary impact ionization by the noninitiating carrier on the near avalanche behavior of high-speed n-p-n bipolar transistors is studied. We show that secondary collector ionization by generated holes traveling back toward the base layer significantly reduces BV/sub CBO/ if the hole ionization coefficient is higher than that of electrons [/spl beta//sub p/(E)>/spl alpha//sub n/(E)]: positive feedback associated with a strong secondary ionization sharpens the breakdown characteristic by speeding up carrier multiplication and decreases separation between the open-base collector-emitter (BV/sub CEO/) and the open-emitter base-collector (BV/sub CBO/) breakdown voltages. The effect of secondary ionization on the BV/sub CEO/-BV/sub CBO/ separation has not previously been described. Multiplication coefficient comparisons for representative InP, GaAs, and Si collectors indicate all structures can sustain low-current above BV/sub CEO/ operation from a transport (nonthermal) point of view, although the different degrees of secondary ionization in various semiconductors lead to fundamental differences when InP is compared to GaAs and Si since for the latter materials /spl beta//sub p/(E)相似文献   

Double junction AlInAs/GaInAs multiquantum well avalanche photodiodes

A separate absorption, grading, and multiplication avalanche photodiode with an AlInAs/GaInAs multiquantum well multiplication region is reported. This device exhibits a low excess-noise factor and a gain-bandwidth product of 50 GHz, due to the high ratio of ionisation rates of the multiplication material. In addition, a large bandwidth is obtained owing to the use of an undoped (n type) GaInAs absorption layer, fully depleted when multiplication occurs.<>     

Frequency response of InP/InGaAsP/InGaAs avalanche photodiodes

A theoretical model for the frequency response of InP/InGaAs avalanche photodiodes (APDs) is presented. Included in the analysis are resistive, capacitive, and inductive parasitics, transit-time factors, hole trapping at the heterojunction interfaces, and the avalanche buildup time. The contributions of the primary electrons, primary holes, and secondary electrons to the transit-time-limited response are considered separately. Using a measurement apparatus which consists of a frequency synthesizer and a spectrum analyzer controlled by a microcomputer, the frequency response of InP/InGaAsP/InGaAs APDs grown by chemical-beam epitaxy are measured. Good agreement with the calculated response has been obtained over a wide range of gains     

Ultra-low noise avalanche photodiodes with a "centered-well" multiplication region

We report avalanche photodiodes with a "centered-well" multiplication region that have achieved high gain, low noise, and low dark current. The multiplication region consists of an /spl sim/80 nm-thick Al/sub 0.2/Ga/sub 0.8/As layer sandwiched between two thin (10/spl sim/20 nm) layers of Al/sub 0.6/Ga/sub 0.4/As. Monte Carlo simulation shows the beneficial effect of spatial modulation of the ionization rates in this structure compared to homojunctions.     

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     

Low avalanche noise characteristics in thin InP p+-i-n+ diodes with electron initiated multiplication

We have performed electron initiated avalanche noise measurements on a range of homojunction InP p⁺-i-n⁺ diodes with “i” region widths, w ranging from 2.40 to 0.24 μm. In contrast to McIntyre's noise model a significant reduction in the excess noise factor is observed with decreasing w at a constant multiplication in spite of α, the electron ionization coefficient being less than β, the hole ionization coefficient. In the w=0.24 μm structure an effective β/α ratio of approximately 0.4 is deduced from the excess noise factor even when electrons initiate multiplication, suggesting that hole initiated multiplication is not always necessary for the lowest avalanche noise in InP-based avalanche photodiodes     

Extremely Low Excess Noise in InAs Electron Avalanche Photodiodes

Measurements of the avalanche multiplication noise in InAs p-i-n and n-i-p diodes at room temperature demonstrate unambiguously that the avalanche multiplication process is dominated by impact ionization of electrons. This results in the excess noise factor for electron initiated multiplication asymptotically approaching a maximum value just less than two and becoming virtually gain-independent for higher gains. Measurements for predominantly hole initiated multiplication show corresponding high excess noise factors suggesting the electron to hole ionization coefficient ratios are comparable to those reported for $hbox{Hg}_{1-{x}}hbox{Cd}_{x}hbox{Te}$ electron avalanche photodiodes.      

Theoretical investigation of quantum-dot avalanche photodiodes for mid-infrared applications

A novel midinfrared sensor, called the quantum-dot avalanche photodiode (QDAP), is proposed which is expected to have improved signal-to-noise ratio (SNR) in the presence of Johnson noise over its quantum-dot (QD) counterpart. In the QDAP, an intersubband QD detector is coupled with a thin, low-noise GaAs avalanche layer through a tunnel barrier. The avalanche layer provides the necessary photocurrent gain required to overcome Johnson noise and nearly achieve the dark-current-limited SNR of the QD detector. In the proposed three-terminal device, the applied biases of the QD-detector and the avalanche-photodiode sections of the QDAP are controlled separately. This feature permits the control of the QDs responsivity and dark current independently of the operating avalanche gain, thereby allowing the optimization of the avalanche multiplication factor to maximize the photocurrent's SNR. Notably, a heterojunction potential-barrier layer can also be utilized to further improve the SNR. For example, when the standard deviation of the Johnson noise is four times greater than the dark current, calculations show that the SNR enhancement offered by an avalanche multiplication factor of 5 results in relaxing the cooling requirement from 20 to 80 K.     

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     

Temperature dependence of impact ionization in GaAs

The temperature dependence of electron and hole impact ionization in gallium arsenide (GaAs) has been determined from photomultiplication measurements at temperatures between 20 K and 500 K. It is found that impact ionization is suppressed by increasing temperature because of the increase in phonon scattering. Temperature variations in avalanche multiplication are shown to decrease with decreasing avalanching region width, and the effect is interpreted in terms of the reduced phonon scattering in the correspondingly reduced ionization path length. Effective electron and hole ionization coefficients are derived and are shown to predict accurately multiplication characteristics and breakdown voltage as a function of temperature in p/sup +/in/sup +/ diodes with i-regions as thin as 0.5 /spl mu/m.     

基于APD参数建模的单光子探测研究

单光子探测技术对于量子密钥分配乃至量子通信有重要的意义。针对单光子雪崩光电二极管（SPADs）的单光子量子效率（SPQE）,提出了一种严格的数学模型。模型适用于工作波长为1.3和1.5um的In052Al048As、InP倍增层和In052Al048As-InP异质结倍增层的SPADs。模型作为器件结构、工作电压、倍增层材料的函数,可用来优化SPQE,进而评估和优化盖革模式下APDs的性能。     

Single-carrier-type dominated impact ionisation in multilayer structures

A new structure for III-V avalanche photodetectors in which multiplication is dominated by a single-carrier type is proposed. Calculations for a GaAs-AlGaAs detector are reported predicting multiplication dominated by electrons. The reason for this is that electrons are injected into GaAs multiplication layers from high-electric-field AlGaAs layers, while holes are injected into the GaAs layers from low-electric-field AlGaAs layers.     

Ionization coefficient measurement in GaAs by using multiplication noise characteristics

Multiplication noise measurements for p⁺n type (100) GaAs avalanche photodiodes with various n-layer dopings ranging from 6 × 10¹⁵ to 9 × 10¹⁶ cm^?3 confirmed that the ionization coefficient of electrons α is about two times larger than that of holes β in the electric field range from 2.4 × 10⁵ to 5.6 × 10⁵ V/cm. When pure electrons were injected into the avalanche region, the multiplication noise power was proportional to the 2.7th power of the multiplication factor and the ionization coefficient ratio $\text{k =}\text{β}\text{α}$ was constant, where k = 0.5 in the above electric field range. The result was consistent with the multiplication factor dependence on light wavelength. Using the constant ionization coefficient ratio k and the multiplication factor dependence on applied bias voltage, ionization coefficients α and β for electrons and holes were estimated.