A novel broadband photodetector and its array having spectral response ranging from visible to shortwave infrared (VIS-SWIR) is developed for focal plane array (FPA) for military, security, industrial imaging applications, and optical communications. The photodetector is based on InGaAs and fabricated on InP substrate, exhibiting high sensitivity, high quantum efficiency, and yet cost-effective. In order to realize a small weight, power, and cost effectiveness (SWAP-C) camera and receiver, the photodetector requires having low dark current at high operating temperatures, which saves power for cooling. This paper explains the photodetector structure, design-simulation for optimizing the structural parameters, and performance of the photodetector. An analytical closed form model is derived for design and simulation of the broadband photodetector. Both theoretical and experimental results of electrical and optical characteristics of the photodetectors are also presented in this paper.
The promise of infrared (IR) imaging attaining low-cost akin to CMOS sensors success has been hampered by the inability to achieve cost advantages that are necessary for crossover from military and industrial applications into the consumer and mass-scale commercial realm despite well documented advantages. Banpil Photonics is developing affordable IR cameras by adopting new strategies to speed-up the decline of the IR camera cost curve. We present a new short-wave IR (SWIR) camera; 640x512 pixel InGaAs uncooled system that is high sensitivity low noise (<50e-), high dynamic range (100 dB), high-frame rates (> 500 frames per second (FPS)) at full resolution, and low power consumption (< 1 W) in a compact system. This camera paves the way towards mass market adoption by not only demonstrating high-performance IR imaging capability value add demanded by military and industrial application, but also illuminates a path towards justifiable price points essential for consumer facing application industries such as automotive, medical, and security imaging adoption. Among the strategic options presented include new sensor manufacturing technologies that scale favorably towards automation, multi-focal plane array compatible readout electronics, and dense or ultra-small pixel pitch devices.
The reflection loss in imaging devices is one of the major drawbacks, which degrades efficiency resulting in lower responsivity. Since the reflected light is no longer available for conversion into electrons, it is very important to reduce the reflection from the top surface of the device as much as possible. Quarter wavelength and two index antireflection (AR) coatings have been developed to reduce reflection; however, these AR coatings are wavelength dependent and have not performed effectively in a broadband range. Attempts to make AR coating for broadband wavelengths by stacking multiple index AR layers result in thicker and expensive solutions, which still do not provide proper antireflection at all desired wavelengths. Moreover, the usage of AR coatings escalates material and fabrication costs of the device. We propose a novel nanostructure, which matches the refractive index of the device to that of free space to reduce reflection from the top surface, eliminating the use of AR coatings and hence reducing the device cost. It is shown via simulation that the proposed nanostructure effectively eliminates the reflection loss over the broadband spectrum of desired wavelengths e.g. Visible, Mid-wave IR (MWIR), Short-wave IR (SWIR) spectrums, opening various application opportunities.
Banpil Photonics has developed a novel high-performance multispectral photodetector array for Short-Wave Infrared (SWIR) imaging. The InGaAs based device uses a unique micro-nano pillar structure that eliminates surface reflection to significantly increase sensitivity and the absorption spectra compared to its macro-scaled thin film pixels counterpart (non-pillar). We discuss the device structure and highlight fabrication of the novel high performance multispectral image sensor. We also present performance results of the device characterization showing low dark current suitable for high performance imaging applications for the most demanding security, defense, and machine vision applications.
Banpil Photonics has developed a high-performance Digital Read-Out Integrated Circuit (DROIC) for image sensors and camera systems targeting various military, industrial and commercial Infrared (IR) imaging applications. The on-chip digitization of the pixel output eliminates the necessity for an external analog-to-digital converter (ADC), which not only cuts costs, but also enables miniaturization of packaging to achieve SWaP-C camera systems. In addition, the DROIC offers new opportunities for greater on-chip processing intelligence that are not possible in conventional analog ROICs prevalent today. Conventional ROICs, which typically can enhance only one high performance attribute such as frame rate, power consumption or noise level, fail when simultaneously targeting the most aggressive performance requirements demanded in imaging applications today. Additionally, scaling analog readout circuits to meet such requirements leads to expensive, high-power consumption with large and complex systems that are untenable in the trend towards SWaP-C. We present the implementation of a VGA format (640x512 pixels 15μm pitch) capacitivetransimpedance amplifier (CTIA) DROIC architecture that incorporates a 12-bit ADC at the pixel level. The CTIA pixel input circuitry has two gain modes with programmable full-well capacity values of 100K e- and 500K e-. The DROIC has been developed with a system-on-chip architecture in mind, where all the timing and biasing are generated internally without requiring any critical external inputs. The chip is configurable with many parameters programmable through a serial programmable interface (SPI). It features a global shutter, low power, and high frame rates programmable from 30 up 500 frames per second in full VGA format supported through 24 LVDS outputs. This DROIC, suitable for hybridization with focal plane arrays (FPA) is ideal for high-performance uncooled camera applications ranging from near IR (NIR) and shortwave IR (SWIR) to mid-wave IR (MWIR) and long-wave IR (LWIR) spectral bands.
The use of Graphics Processing Unit (GPU) for computational work has revolutionized how complex electromagnetic problems are solved. Complex problems which required supercomputers in the past for analysis can now be tackled and solved using personal computers by channeling the computational work towards GPUs instead of the traditional computer Central Processing Unit (CPU). Finite-Difference Time-Domain (FDTD) analysis, which is a computationally expensive method of solving electromagnetic problems is highly parallel in nature and can be readily executed in a GPU. We have developed an algorithm for three dimensional FDTD analysis of optical devices with micro and nano-structures using Compute Unified Device Architecture (CUDA). The developed algorithm exploits the benefits of multiple cores of GPU chips and boosts the speed of simulation without sacrificing its accuracy. We achieved a 25-fold speed up of simulation using CUDA compared to MATLAB code in CPU.
The advent of ultrathin crystalline silicon (c-Si) solar cells has significantly reduced the cost of silicon solar cells by consuming less material. However, the very small thickness of ultrathin solar cells poses a challenge to the absorption of sufficient light to provide efficiency that is competitive to commercial solar cells. Light trapping mechanisms utilizing nanostructure technologies have been utilized to alleviate this problem. Unfortunately, a significant portion of light is still being lost even before entering the solar cells because of reflection. Different kinds of nanostructures have been employed to reduce reflection from solar cells, but reflection losses still prevail. In an effort to reduce reflection loss, we have used an array of modified nanostructures based cones or pyramids with curved sides, which matches the refractive index of air to that of silicon. Moreover, use of these modified nano-pyramids provides a quintic (fifth power) gradient index layer between air and silicon, which significantly reduces reflection. The solar cells made of such nanostructures not only significantly increase conversion efficiency at reduced usage of crystalline silicon material (e.g. thinner), but it also helps to make the c-Si based solar cell flexible. Design and optimization of flexible c-Si solar cell is presented in the paper.
Crystalline Silicon (c-Si) solar cell has presented itself as the ultimate solution for solving the cost and efficiency dilemma for the solar industry. We at Banpil have been working on the development of novel solar cells based on nanostructures to increase the conversion efficiency significantly over standard solar cells. These nanostructure based c-Si solar cells could potentially break the cost barrier that has thwarted the photovoltaic industry. In this work, we have designed ultrathin c-Si solar cells based on nanostructures, enabling a light trapping phenomenon to achieve a power density ranging from 0.91 W/g to 3.5 W/g, which is from 3.5 to more than 10 times over available standard c-Si solar cells.
Banpil Photonics has developed a novel InGaAs based photodetector array for Short-Wave Infrared (SWIR) imaging, for the most demanding security, defense, and machine vision applications. These applications require low noise from both the detector and the readout integrated circuit arrays. In order to achieve high sensitivity, it is crucial to minimize the dark current generated by the photodiode array. This enables the sensor to function in extremely low light situations, which enables it to successfully exploit the benefits of the SWIR band. In addition to minimal dark current generation, it is essential to develop photodiode arrays with higher operating temperatures. This is critical for reducing the power consumption of the device, as less energy is spent in cooling down the focal plane array (in order to reduce the dark current). We at Banpil Photonics are designing, simulating, fabricating and testing SWIR InGaAs arrays, and have achieved low dark current density at room temperature. This paper describes Banpil’s development of the photodetector array. We also highlight the fabrication technique used to reduce the amount of dark current generated by the photodiode array, in particular the surface leakage current. This technique involves the deposition of strongly negatively doped semiconductor material in the area between the pixels. This process reduces the number of dangling bonds present on the edges of each pixel, which prevents electrons from being swept across the surface of the pixels. This in turn drastically reduces the amount of surface leakage current at each pixel, which is a major contributor towards the total dark current. We present the optical and electrical characterization data, as well as the analysis that illustrates the dark current mechanisms. Also highlighted are the challenges and potential opportunities for further reduction of dark current, while maintaining other parameters of the photodiode array, such as size, weight, temperature of peak performance (lowest dark current), and power consumption.
Device-level reduction of dark current is critical for improving the Signal to Noise ratio and dynamic range of
system using III-V based optical components. We report on low-noise, back-illuminated p-i-n photodetector and its
arrays with high spectral response for optical communication and also optical interconnects applications at 1.3 μm
and 1.55 μm wavelengths. The photodetector is based on planar structure fabricated from InP having a latticematched
InGaAs-absorber layer, and it is designed specifically for high sensitivity and low dark current (nA) at
room temperature by minimizing leakage current. Electrical and optical performance of both individual diodes and
small-pixel array test structures will be presented in this paper.
Banpil Photonics (Banpil) has developed a low-cost high-performance multispectral camera system for Visible to Short-
Wave Infrared (VIS-SWIR) imaging for the most demanding high-sensitivity and high-speed military, commercial and
industrial applications. The 640x512 pixel InGaAs uncooled camera system is designed to provide a compact, smallform
factor to within a cubic inch, high sensitivity needing less than 100 electrons, high dynamic range exceeding 190
dB, high-frame rates greater than 1000 frames per second (FPS) at full resolution, and low power consumption below
1W. This is practically all the feature benefits highly desirable in military imaging applications to expand deployment to
every warfighter, while also maintaining a low-cost structure demanded for scaling into commercial markets. This paper
describes Banpil’s development of the camera system including the features of the image sensor with an innovation
integrating advanced digital electronics functionality, which has made the confluence of high-performance capabilities
on the same imaging platform practical at low cost. It discusses the strategies employed including innovations of the key
components (e.g. focal plane array (FPA) and Read-Out Integrated Circuitry (ROIC)) within our control while
maintaining a fabless model, and strategic collaboration with partners to attain additional cost reductions on optics,
electronics, and packaging. We highlight the challenges and potential opportunities for further cost reductions to achieve
a goal of a sub-$1000 uncooled high-performance camera system. Finally, a brief overview of emerging military,
commercial and industrial applications that will benefit from this high performance imaging system and their forecast
cost structure is presented.