Impact ionization under dynamic fields

In this paper, the influences of thermal and spectral variations on various structures of avalanche photodiodes (APDs) have been deeply investigated. The characteristics and performance of Silicon (Si), Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs) avalanche photodiodes have been evaluated under effects of different temperature degrees. The impacts of different temperature levels on the band gap energy, detector responsivity, noise equivalent power (NEP), cut-off wavelength, dark current and the photocurrent are studied and analyzed in details. The signal to noise ratio (SNR) and Bit Error Rate (BER) of these APDs are also computed and measured under the influences of different temperature levels and spectral variations. The aim of this paper is to determine the most efficient avalanche photodiode achieving the best performance according to the maximum SNR and minimum BER at high temperature environments. The analysis of the proposed model is performed by MATLAB. It was found that the experimental measured results endorse the analytical computed results. Further, it was observed that the APD (InGaAs) provides performance better than other APD structures in terms of larger SNR and less BER at higher temperature levels. Also, the temperature has a slight effect on the performance of APD (GaAs). So, it is seemed to be stable more than other APD structures and recommended to be used in warm environments and for high temperature applications.

The thinning of the multiplication (M) layers in high-speed avalanche photodiodes (APDs) is an effective way to boost the gain-bandwidth product (GBP) and reduce excess noise. However, such downscaling usually comes at the price of a huge leakage current (>1 μA) induced by direct tunneling through the final thin M-layer (<100 nm) and degradation of the RC-limited bandwidth due to the decrease in the thickness of the depletion layer. In this work, we demonstrate how a p-side up top-illuminated APD structure with a thick InP collector layer buried below the thin and cascaded In<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub>Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</sub>As based multiplication layer can effectively relax the trade-offs among the dark current, RC-limited bandwidth, multiplication gain, and avalanche delay time. This advanced device structure is then applied and the performance of APDs with different thin M-layer thicknesses (<50 nm) explored. Under 1.55 μm wavelength excitation, a device fabricated with a large active window (mesa) diameter of 10 (20) μm and an optimized M-layer thickness exhibits a dark current as low as ∼0.4 μA and a high responsivity (2.8 A/W; gain = 9.3) at 0.9 V<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub>. This device also exhibits excellent dynamic performance, including a wide optical-to-electrical bandwidth (44 GHz at 0.84 A/W), an extremely large GBP of 1.03 THz, and a high saturation current (12 mA), which corresponds to a large millimeter-wave (MMW) output power (∼0 dBm) at 45 GHz. The exceptional speed performance coupled with the wide dynamic range and simple top-illuminated structure opens up new possibilities to further enhance the sensitivity of 50 G passive optical networks (PONs).

An Al/sub x/Ga/sub 1-x/Sb avalanche photodiode (APD) with an Al composition of 0.05 has been fabricated and tested. The measured gain-bandwidth (GB) product of the highly doped diode is 90 GHz. Such a large GB product has never been previously achieved by any APD materials sensitive in the long-wavelength region. This suggests the advantage of the Al/sub x/Ga/sub 1-x/Sb material system for very high-speed operation. >

The Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic is a highly specialized spectroscopic instrument with 2 spatial channels consisting of 8 spectral channels each and a resolution of ∼0.25 nm deployed at DIII-D to measure turbulent ion temperature fluctuations. Charge exchange emissions are obtained between 528 and 530 nm with 1 μs time resolution to study plasma instabilities. A primary challenge of extracting fluctuation measurements from raw UF-CHERS signals is photon and electronic noise. In order to reduce dark current, the Avalanche Photodiode (APD) detectors are thermo-electrically cooled. State-of-the-art components are used for the signal amplifiers and conditioners to minimize electronic noise. Due to the low incident photon power (≤1 nW), APDs with a gain of up to 300 are used to optimize the signal to noise ratio. Maximizing the APDs' gain while minimizing the excess noise factor (ENF) is essential since the total noise of the diagnostic sets a floor for the minimum level of detectable broadband fluctuations. The APDs' gain should be high enough that photon noise dominates electronic noise, but not excessive so that the ENF overwhelms plasma fluctuations. A new generation of cooled APDs and optimized preamplifiers exhibits significantly enhanced signal-to-noise compared to a previous generation. Experiments at DIII-D have allowed for characterization and optimization of the ENF vs. gain. A gain of ∼100 at 1700 V is found to be near optimal for most plasma conditions. Ion temperature and toroidal velocity fluctuations due to the edge harmonic oscillation in quiescent H-mode plasmas are presented to demonstrate UF-CHERS' capabilities.

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. >

Exact analysis as well as asymptotic analysis, based on large-deviation theory (LDT), are developed to compute the bit-error rate (BER) for ultrafast avalanche-photodiode (APD) based optical receivers assuming on-off keying and direct detection. The effects of intersymbol interference (ISI), resulting from the APD's stochastic avalanche buildup time, as well as the APD's dead space are both included in the analysis. ISI becomes a limiting factor as the transmission rate approaches the detector's bandwidth, in which case the bit duration becomes comparable to APD's avalanche buildup time. Further, the effect of dead space becomes significant in high-speed APDs that employ thin avalanche multiplication regions. While the exact BER analysis at the generality considered here has not been reported heretofore, the asymptotic analysis is a major generalization of that developed by Letaief and Sadowsky [IEEE Trans. Inform. Theory, vol. 38, 1992], in which the LDT was used to estimate the BER assuming APDs with an instantaneous response (negligible avalanche buildup time) and no dead space. These results are compared with those obtained using the common Gaussian approximation approach showing the inadequacy of the Guassian approximation when ISI noise has strong presence.

We propose a simple expression to relate the total excess noise factor of a single-carrier multiplication staircase avalanche photodiode (APD) to the excess noise factor and gain given by the individual conduction band discontinuities. The formula is valid when electron impact ionization dominates hole impact ionization; hence, it is especially suited for staircase APDs with In-rich multiplication regions, as opposed, for example, to GaAs/AlGaAs systems where hole ionization plays an important role. The formula has been verified by accurate means of numerical simulations based on a newly developed nonlocal history dependent impact ionization model.

Digital alloy (DA) InAlAs on the InP substrate exhibits a lower excess noise compared to a traditional In0.52Al0.48As random alloy as the multiplication layer in avalanche photodiodes (APDs). This work implements DA InAlAs as the multiplication layer in a 1550 nm separate absorption, grading, charge, and multiplication APD and characterizes the performances through various analyses. The device reaches a maximum gain of 221 before avalanche breakdown, with a maximum gain-bandwidth product of more than 140 GHz. At 90% breakdown voltage, the dark current density is 4.1 mA/cm2, and the responsivity is 0.48 A/W at unit gain. Excess noise factors were identified, yielding an effective k value of around 0.15, which is lower than that of random alloy In0.52Al0.48As APDs (k ∼0.2). These findings show that DA InAlAs has the potential to be a promising material for high-performance APDs.

A novel planar separate absorption, charge sheet, grading and multiplication avalanche photodiode (APD) structure incorporating a partial charge sheet in the device periphery is described, which allows for straightforward fabrication of APD devices without the use of guard rings. Metalorganic chemical vapor deposition grown, Zn-diffused InP/InGaAs APD devices have been fabricated. High dc gains well in excess of 100 and a low primary dark current of 0.1 nA at 0.99 of the breakdown voltage VB have been measured for a 40-μm-diam device. The receiver sensitivity for a bit error rate of 10−9 at a bit rate of 400 Mbit/s was −41 dBm. The −3 dB electrical bandwidth was 2.5 GHz, and the gain-bandwidth product was 30 GHz.

Data receiving frontends using avalanche photodiodes are used in optical free-space communications for their effective sensitivity, large detection area, and uncomplex operation. Precise control of the high voltage necessary to trigger the avalanche effect inside the photodiode depends on the semiconductor’s excess noise factor, temperature, received signal power, background light, and also the subsequent thermal noise behavior of the transimpedance amplifier. Several prerequisites must be regarded and are explained in this document. We focus on the application of using avalanche photodiodes as data receivers for the on/off-keying of modulated bit streams with a 50% duty cycle. Also, experimental verification of the performance of the receiver with background light is demonstrated.

Development of high performance Avalanche Photodiodes and dedicated analog systems for HXI/SGD detectors onboard the Astro-H mission

We present an in-depth numerical study of the design of a long wavelength infrared (LWIR) avalanche photodiode (APD) that incorporates a compositional gradient to improve performance in terms of gain, noise, and dark current. The modeling uses a combination of drift–diffusion to assess dark current mechanisms and Monte Carlo (MC) simulations to assess gain and noise, which are impact ionization-related properties. The HgCdTe material parameters used in this study were carefully tuned to published experimental data for devices with a uniform composition, and then applied to the design of a proposed device with a compositional gradient. The proposed device is shown to significantly reduce the impact of band-to-band tunneling, which extends the operational bias range, resulting in an APD with higher gain and lower dark current. Furthermore, it is shown that in certain cases, absorption of illumination within the multiplication region can degrade the superb gain and noise performance of HgCdTe APDs, an effect that is eliminated with the proposed molar grading. This manuscript also discusses the implementation of a robust technique to simulate smoothly graded composition within the MC framework.

We demonstrate low noise short wavelength infrared (SWIR) Sb-based type II superlattice (T2SL) avalanche photodiodes (APDs). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on a GaSb substrate. At room temperature, the device exhibits a 50% cut-off wavelength of 1.74 µm. The device was revealed to have an electron-dominated avalanching mechanism with a gain value of 48 at room temperature. The electron and hole impact ionization coefficients were calculated and compared to give a better prospect of the performance of the device. Low excess noise, as characterized by a carrier ionization ratio of ~0.07, has been achieved.

Large gain-bandwidth product and low-noise avalanche photodiodes (APDs) in the long-wavelength region are required in long-haul gigabit-rate lightwave systems. We have reported a practical device structure for planar GaInAs APDs with a successful guard ring effect in buried structure APDs.1 To obtain a large gain-bandwidth product, a higher carrier concentration n-InP multiplication layer must be employed to reduce avalanche buildup time. However, this results in deterioration of the excess noise factor for GaInAs APDs. We describe a design condition for achieving a large gain-bandwidth product and low-noise performance at the same time for GaInAs buried structure APDs grown by liquid phase epitaxy (LPE).

In this letter, an avalanche photodiode (APD) for ultraviolet detection was fabricated on a 4H-SiC epi-layer with a radius of 150 µm. By adopting passive quenching method, the impact of quenching resistor on single photon detection performance of the fabricated APD was investigated for the first time. It is found that both dark count rate (DCR) and single photon detect efficiency (SPDE) were reduced with the increasing quenching resistance. When the DCR fixed at 5 Hz/μm2 , the SPDE is 7.1% /6.7%/5.4%/5.2% corresponding to the quenching resistance of 10/20/50/100 kΩ. Variation of the SPDEs can be ascribed to the changing death time by comparing the photon counting spectra with various resistors. The obtained results have built up a good basis for the design of SiC APD single photon detection.