Exact Analytical Formula for the Excess Noise Factor for Mixed Carrier Injection Avalanche Photodiodes

The well-known analytical formula for the excess noise factor associated with avalanche photodiodes (APDs), developed by R. J. McIntyre in 1966, assumes the injection of either an electron or a hole at the edge of the APD\u27s avalanche region. This formula is based on the statistics of the probabilities of carriers gaining and losing energy subject to high electric fields. However, this analytical formula, is not applicable in cases when photons are absorbed inside the avalanche region (even though the physics of the high field transport remains the same), and its use may severely underestimate or overestimate the actual excess noise factor depending on the absorption profile and the hole-to-electron ionization coefficient ratio, k. Here, an easy-to-use exact analytical formula is derived for the excess noise factor of APDs while taking into account a mixed-carrier initiated avalanche multiplication process, which is triggered by a parent electron-hole pair at an arbitrarily specified location within the multiplication region. The derivation relies on analytically solving a special case of a previously reported recursive integral equations [Hayat et al., IEEE Trans. Electron Devices, vol. 39, no. 3, pp. 546-552, Mar. 1992.], and the result matches the formula reported by McIntyre in 1999 using a different and limited technique. In addition, an expression for the excess noise factor is presented in the case when the location of the parent electron-hole pair within the multiplication region obeys an arbitrary exponential distribution. The results show that in contrast to the case of edge parent-electron injection, when mixed injection is allowed even a small level of hole ionization (e.g., small k ~ 0.0001) causes the excess noise factor to increase dramatically, depending on the absorption profile as it ranges from narrow to flat within the multiplication region. The theoretical results are validated against experimental results for Si APDs

Mitigating the twin problems of malnutrition and wheat blast by one wheat variety, ‘BARI Gom 33’, in Bangladesh

For the first time in history outside of Latin America, deadly wheat blast caused by the fungus Magnaporthe oryzae pathotype triticum (MoT) emerged in the 2015–2016 wheat (Triticum aestivum L.) season of Bangladesh. Bangladesh, a country in South Asia, has a population of nearly 160 million, of which 24.3% are classified as poor. Consequently, malnutrition and micronutrient deficiency are highly prevalent, particularly among school going children and lactating women. Bangladesh Wheat and Maize Research Institute (BWMRI), with the technical support of the International Maize and Wheat Improvement Center (CIMMYT), Mexico, has developed and released a new wheat ‘BARI Gom 33’. The new wheat is a zinc-enriched (Zn) biofortified wheat, resistant to the deadly wheat blast disease. ‘BARI Gom 33’ provides 5–8% more yield than the check varieties in Bangladesh. Rapid dissemination of it in Bangladesh, therefore, can not only combat wheat blast but also mitigate the problem of Zn deficiency and ensure income for resource-poor wheat farmers. Importantly, a large portion of the current wheat area in India and Pakistan is vulnerable to wheat blast, due to the similarities of the agro-climatic conditions of Bangladesh. As wheat blast is mainly a seed-borne disease, a rapid scaling out of the new wheat in Bangladesh can reduce the probability of MoT intrusion in India and Pakistan, and thereby generate positive externalities to the food security of more than 1 billion people in South Asia. This study explains the development process of ‘BARI Gom 33’; the status of malnutrition in Bangladesh, and the possible economic gain from a rapid scaling out of ‘BARI Gom 33’ in Bangladesh. A few policies are recommended based on the discussions

Low-Noise Speed-Optimized Large Area CMOS Avalanche Photodetector for Visible Light Communication

Mean-gain and excess-noise measurements are presented for a 350 × 350 μm 2 P+/N-well/P-sub and a 270 × 270 μm 2 N-well/P-sub avalanche photodetectors fabricated using 0.13-μm CMOS technology. The active area of the P+/N-well/P-sub device was divided into multiple subsections to decrease transit time and increase speed. For the P+/N-well structure, remarkably low excess-noise factors of 4.1 and 4 were measured at a mean gain of 16 corresponding to a k value of approximately 0.1, using a 542 (633) nm laser. For a variant N-well/P-sub structure, excess-noise factors of 6.5 and 6.2 were measured at a mean-gain of 16 corresponding to a k value of approximately 0.3. The proposed CMOS APDs with high gain, low noise, low avalanche breakdown voltage (below approximately 12 V) and low dark-currents (approximately nA) would be attractive for low-cost optical receivers in visible-light communication systems

Linear Mode CMOS Compatible p-n Junction Avalanche Photodiode for Smart-lighting Applications

There is a need in emerging smart lighting concepts for a high-speed sensing capability to enable adaptive lighting (smart spaces) and visible light communication. One approach to address this need is to design and manufacture a novel complementary-metal—oxide—semiconductor (CMOS) compatible, cost-effective detector array and readout circuit (ROIC) that incorporates integrated waveguide detectors and avalanche photodiodes (APDs). This thesis focuses on the APD design and fabrication component of the sensing capability required by smart-lighting systems. Silicon CMOS compatible APDs are expected to provide high-speed and high-sensitivity sensors in terms of simplicity of design, low power consumption and cost-effectiveness for smart-lighting applications. To date, most of the CMOS-based APD devices have been dedicated to the Geiger mode, which aims to count individual photons under ultralow light conditions. This thesis reports on the modeling, design, fabrication,and characterization of CMOS compatible p-n junction Si APDs to be operated in the linear avalanche mode. The recursive dead-space multiplication theory (DSMT), is applied to the recently fabricated thin Si n+p APDs to predict the avalanche and breakdown properties including low excess noise factor. The low excess noise factor is due to the presence of dead space effect and the initiation of avalanche process by the photogenerated electron in the depletion region of Si APDs. The calculated mean gain, avalanche breakdown voltage, excess noise factor, electron and hole ionization coefficients, electric fields are reported. Moreover, measured dark current, photocurrent, mean gain, capacitance, spectral response, and breakdown voltages are also reported supporting low-voltage operation across the visible electromagnetic spectrum. A mean gain of ~50 has been obtained for the fabricated structure at a reverse bias breakdown voltage of ~8.67 V. The type of APD developed in this thesis can be integrated with waveguide structures to provide enhanced sensitivity and high speed detection capability as well as uniformity across colors

Low-Cost High-Sensitivity Color Sensor for Smart-Lighting Applications

There is a need in emerging smart-lighting concepts for a high-speed sensing capability to enable adaptive-lighting (smart spaces) and visible-light communication. One approach to address this need is to design and manufacture a low-cost, high-sensitivity color-sensor to be used in lighting enabled systems and applications. This novel sensor network will process light coming from the right direction, right color and intensity. The color-sensor will enable automatic adjust of right lighting for us at any given time, optimized for human health and productivity.This poster focuses on the advanced low-cost color-sensor component to enable high quality, energy efficient, color tunable lighting with minimal human intervention

CMOS Compatible Dual Avalanche Photodiode for Algorithmic Visible Spectral Sensing

A previously reported CMOS-compatible dual avalanche photodiode design is exploited to develop a maximum-likelihood spectral-sensing algorithm, which maps the dual photocurrents to the monochromatic light\u27s wavelength. Optimization over the reverse biases of the two APDs yields a spectral resolution of 10 nm within 440-650 nm