Active Pixel Sensors Papers: Prof. Glenn H. Chapman

This page contains the paper abstract and full PDF versions of publications on the Active Pixel Sensor concepts. Click on the link to download pdf version.  Papers her are listed in order of publication
Note: These papers are for academic use only and are not available for distribution or publication for other purposes.

Refereed Conference Proceedings

J. Leung, J. Dudasa, G.H. Chapman, Z. Koren, and I. Koren, “Characterization of pixel defect development during digital imager lifetime”, Electronic Imaging Sensors, Cameras, and Systems for Industrial/Scientific Applications IX, v 6816, 68160A-1- 0A12 San Jose, Jan 2008. ei08.pdf  735KB
The reliability of solid-state image sensors is limited by the development of defects, particularly hot-pixels, which we have previously shown develop continuously over the sensor lifetime.  Our statistical analysis based on the distribution and development date of defects concluded that defects are not caused by single traumatic incident or material failure, but rather by an external process such as radiation. This paper describes an automated process for extracting defect temporal growth data, thereby enabling a very wide sample of cameras to be examined and studied. The algorithm utilizes Bayesian statistics to determine the presence and absence of defects by searching through sets of color photographs.  Monte Carlo simulations on a set of images taken at 0.06 to 0.5sec exposures demonstrated that our tracing algorithm is able to pinpoint the defect development date for all the identified hot pixels within + 2 images.  Although a previous study has shown that in-field defects are isolated from each other, image processing functions applied by cameras such as the demosaicing algorithm were found to cause a single defective pixel to appear as a cluster in a color image, increasing the challenge of pinpointing the exact location of hot defects.

J. Dudas, M. L. La Haye, J. Leung, G.H. Chapman, “A Fault-Tolerant Active Pixel Sensor to Correct In-Field Hot-pixel Defects”, Proc. IEEE Int. Symposium on Defect and Fault Tolerance, pp 526-534, Rome, Italy, Oct. 2007. dft07_FTAPS.pdf 379KB
Solid-state image sensors have been shown to develop in-field defects even in benign terrestrial environments.  Previous experiments have demonstrated the growth of significant quantities of hot pixel defects that degrade the dynamic range of an image sensor and potentially limit low-light imaging.  Existing software-only techniques for suppressing hot pixels are inadequate because of the early saturation of these defective pixels.  The redundant Fault-Tolerant Active Pixel Sensor design is suggested as an effective means of isolating the point-like defects leading to hot pixels.  Circuit simulations indicate the pixel architecture can produce a usable output signal throughout the sensor’s entire dynamic range, even once the hot defect would have saturated standard pixel designs.  Experiments on hardware implementations of the FTAPS are underway to verify this behavior for the final paper. An algorithm for detecting and correcting hot pixels is also described with simulation results demonstrating the effectiveness of this fault-tolerance technique.

J. Leung, J. Dudas, G.H. Chapman, I. Koren, and Z. Koren, “Quantitative Analysis of In-Field Defects in Image Sensor Arrays”, Proc. IEEE Int. Symposium on Defect and Fault Tolerance, pp 517-525, Rome, Italy, Oct. 2007. dft07_analysis.pdf 343KB
Growth of pixel density and sensor array size increases the likelihood of developing in-field pixel defects.  An ongoing study on defect development in imagers has now provided us sufficient data to be able to quantify characteristics of defect growth.  Preliminary investigations have shown that defects are distributed randomly and no neighboring pixels were found to be defective. The fact that no defect clusters were found in the study of various digital cameras allows us to conclude that defects are not likely to be related to material degradation or imperfect fabrication but are due to environmental stress such as radiation.   Furthermore, the absence of defect clustering provides information on the size of defects and insight into the nature of the defect development. A quantitative study on cameras are included verifying the above points.

J. Dudas, C. Jung, M.L. LaHaye, and G.H. Chapman, “A Fault-Tolerant Active Pixel Sensor for Mitigating Hot Pixel Defects”, Proc. IEEE Canadian Conf. Elec. Comp. Eng. 2007, pg. 1445-1448, Vancouver, BC Apr. 2007. ccece07pixel.pdf 245KB
Hot pixel defects are unavoidable in many solid-state image sensors.  Affected pixels accumulate dark signal over the course of an exposure, grossly diminishing dynamic range and often rendering measurements unusable.  Experiments suggest the mechanisms causing hot pixels are highly localized and the defect will be confined to a single pixel.  A redundant, fault-tolerant active pixel sensor architecture that has previously been applied to other defect types is investigated for the suppression of hot pixels.  A recovery scheme using minimal computational power is also described.

J. Dudas, L.M. Wu, C. Jung, G.H. Chapman, Z. Korenb, and I. Koren, “Identification of in-field defect development in digital image sensors”, Proc. Electronic Imaging, Digital Photography III, v6502, 65020Y1-0Y12, San Jose, Jan 2007. ei07.pdf 317KB
Given the trend in digital image sensors toward shrinking pixel dimensions, more pixels per sensor, and larger sensor areas, the likelihood of developing defective pixels is greatly increased.  However, while various techniques are available to locate manufacture-time defects, little information is available about the nature or quantity of defects that develop in the field.  Anecdotal evidence from digital camera users indicates that the development over time of “dead,” “stuck”, and “hot” pixels is commonplace.  Traditional fault detection schemes require the camera be returned to the factory for evaluation and may explain the lack of information.  In this paper, we overcome that limitation and extend our defect identification methodology that uses only normally-captured images from the sensor itself.  With the aim of tracking defects in real-world sensors, models are developed for the behaviour of defects in cameras utilizing several imaging technologies.  The prevalence and response characteristics of hot and partially-stuck pixels are extracted via laboratory calibration while abnormal sensitivity defects are shown to be rare in the test devices.  Combined results also suggest that defect development is near-instantaneous and that defect behaviour remains static over time.  Clustering of defects in neighbouring pixel sites is shown to not occur in these test cases, further simplifying requirements for the described detection algorithm

M.L. La Haye, C. Jung, D. Chen, G.H. Chapman and J. Dudas, “Fault Tolerant Active Pixel Sensors in 0.18 and 0.35 Micron Technologies”, . IEEE Int. Symposium on Defect and Fault Tolerance., pp 448 – 456, Washington, DC Sept 2006 dft06_ftaps.pdf 220KB
A Fault Tolerant Active Pixel Sensor (FTAPS) has been designed and fabricated to correct for point defects that occur in CMOS image sensors both at manufacturing and over the lifetime of the sensor.  For some time it has been known that fabrication of CMOS image sensors in processes less than 0.35µm would generate significant performance changes, yet imagers are being fabricated in 0.18µm technology or smaller.  Therefore the characteristics of the FTAPS are presented for pixels fabricated in both a standard 0.18µm and 0.35µm CMOS process and compared for consistency

J. Dudas, C. Jung, L. Wu, G.H. Chapman, I. Koren, and Z. Koren, “On-Line Mapping of In-Field Defects in Image Sensor Arrays”, Proc. IEEE Int. Symposium on Defect and Fault Tolerance, pp 439 - 447, Washington, DC Sept 2006 dft06-defect-sim.pdf 198KB
Continued increase in complexity of digital image sensors means that defects are more likely to develop in the field, but little concrete information is available on in-field defect growth.  This paper presents an algorithm to help quantify the problem by identifying defects and potentially tracking defect growth.  Building on previous research, this technique is extended to utilize a more realistic defect model suitable for analyzing real-world camera systems.  Monte Carlo simulations show that abnormal sensitivity defects are successfully detected by analyzing only 40 typical photographs. Experimentation also indicates that this technique can be applied to imagers with up to 4% defect density, and that noisy images can be successfully diagnosed with only a small reduction in accuracy.  Extension to colour imagers has been accomplished through independent analysis of colour planes of images

M.L. La Haye, C. Jung, M.H. Izadi, G.H, Chapman, and K.S. Karim, “Noise Analysis of Fault Tolerant Active Pixel Sensors with and without Defects” Proc. SPIE Electronic Imaging, Sensors, Cameras, and Systems for Scientific/Industrial Applications VIII, v6068, 6068041-0411, San Jose, Jan 2006. ei06_noise.pdf  282KB
As the sizes of imaging arrays become larger both in pixel count and area, the possibility of pixel defects increases during manufacturing and packaging, and over the lifetime of the sensor.  To correct for these possible pixel defects, a Fault Tolerant Active Pixel Sensor (FTAPS) with redundancy at the pixel level has been designed and fabricated with only a small cost in area.  The noise of the standard Active Pixel Sensor (APS) and FTAPS, under normal operating conditions as well as under the presence of optically stuck high and low faults, is analyzed and compared.  The analysis shows that under typical illumination conditions the total noise of both the standard APS and FTAPS is dominated by the photocurrent shot noise.  In the worst case (no illumination) the total mean squared noise of the FTAPS is only 15.5% larger than for the standard APS, while under typical illumination conditions the FTAPS noise increases by less than 0.1%.  In the presence of half stuck faults the noise of the FTAPS compared to the standard APS stays the same as for the FTAPS without defects.  However, simulation and experimental results have shown that the FTAPS sensitivity is greater than two times that of the standard APS leading to an increased SNR by more than twice for the FTAPS with no defects.  Moreover, the SNR of a faulty standard APS is zero whereas the SNR of the FTAPS is reduced by less than half.

J. Dudas, C. Jung , G.H. Chapman, I. Koren, and Z. Koren, “Robust Detection of Defects in Solid-State Imaging Arrays” Proc. SPIE Electronic Imaging, Image Quality and System Performance III, v6059, 60590X1-0X12, San Jose, Jan 2006. ei06_defects.pdf 575KB
As digital imagers continue to increase in size and pixel density, the detection of faults in the field becomes critical to delivering high quality output.  Traditional schemes for defect detection utilize specialized hardware at the time of manufacture and are impractical for use in the field, while previously proposed software-based approaches tend to lead to quality-degrading false diagnoses.  This paper presents an algorithm that utilizes statistical information extracted from a sequence of normally captured images to identify the location and type of defective pixels.  Building on previous research, this algorithm utilizes data local to each pixel and Bayesian statistics to more accurately infer the likelihood of each defect, which successfully improves the detection time.  Several defect types are considered, including pixels with one-half typical sensitivity and permanently stuck pixels.  Monte Carlo simulations have shown that 35 ordinary images are sufficient to accurately identify all faults without false diagnoses.  Testing also indicates that noisy stuck pixels can be detected successfully with only minimal cost to performance.  More simulations show this algorithm can be extended to higher resolution imagers and those having up to 0.5% of all pixels being faulty

G.H. Chapman, I. Koren, Z. Koren. J. Dudas, and C. Jung, “On-Line Fault Identification in Fault-Tolerant Imagers”, Proc. IEEE Defect and Fault Tolerance Conf., pp 149-157, Monteray, CA Oct. 2005. dft05_pixel.pdf  497KB
Detection of defective pixels that develop on-line is a vital part of fault tolerant schemes for repairing imagers during operation. This paper presents new algorithms for the identification of stuck low and high pixels in both regular and fault tolerant APS systems. These algorithms do not require specialized illuminations but instead operate on a sequence of regular images. Unlike previous techniques simulations show these new methods find all faulty pixels and yet do not wrongly identify good pixels as faulty pixels

C. Jung, M.H. Izadi, M.L. La Haye, G. H. Chapman, and K.S. Karim, “Noise Analysis of Fault Tolerant Active Pixel Sensors”, Proc. IEEE Defect and Fault Tolerance Conf., pp 140-148, Monteray, CA Oct. 2005. dft05_noise.pdf  259KB
As digital imagers grow in pixel count and area, the ability to correct for pixel defects becomes more important.  A fault tolerant Active Pixel Sensor (APS) has previously been designed and fabricated that can correct for stuck high and stuck low defects.  Analyses of the pixel noise for a standard APS and a fault tolerant APS are presented that consider reset noise, photocurrent shot noise, dark current shot noise, transistor thermal noise, transistor flicker noise, operational amplifier noise, and feedback resistor thermal noise.  Under worst case conditions (no illumination), the noise of the fault tolerant APS is 1.106× more than a standard APS.  At a typical illumination level, the fault tolerant APS noise is nearly unchanged to that of a standard APS.  Previous research has shown that the fault tolerant APS is more sensitive than a standard APS, thus the overall signal-to-noise ratio of the fault tolerant APS should be greater than the standard APS except under very low light conditions

C. Jung, G.H. Chapman, M.L. La Haye, S. Djaja, D.Y.H. Cheung, H. Lin, E. Loo, Y. Audet, “Fault Tolerant Photodiode and Photogate Active Pixel Sensors”, Proc. SPIE Electronics Imaging, Sensors and Camera Systems for Scientific and Industrial Applications VI, v 5677, pp. 78-89, San Jose,  Jan. 15, 2005 ei05.pdf  464KB
As the pixel counts of digital imagers increase, the challenge of maintaining high yields and ensuring reliability over an imager’s lifetime increases. A fault tolerant active pixel sensor (APS) has been designed to meet this need by splitting an APS in half and operating both halves in parallel. The fault tolerant APS will perform normally in the no defect case and will produce approximately half the output for single defects. Thus, the entire signal can be recovered by multiplying the output by two. Since pixels containing multiple defects are rare, this design can correct for most defects allowing for higher production yields. Fault tolerant photodiode and photogate APS’ were fabricated in 0.18-micron technology. Testing showed that the photodiode APS could correct for optically induced and electrically induced faults, within experimental error. The photogate APS was only tested for optically induced defects and also corrects for defects within experimental error. Further testing showed that the sensitivity of fault tolerant pixels was approximately 2-3 times more sensitive than the normal pixels. HSpice simulations of the fault tolerant APS circuit did not show increased sensitivity, however an equivalent normal APS circuit with twice width readout and row transistors was 1.90 times more sensitive than a normal pixel.

M.L. La Haye, G.H. Chapman, C. Jung, D.Y.H. Cheung, S. Djaja, and Y. Audet, “Characteristics of Fault-Tolerant Photodiode and Photogate Active Pixel Sensor (APS)”, Proc. IEEE Intern. Symposium on Defect and Fault Tolerance in VLSI Systems, pg. 58-66, Cannes, France Oct. 2004. dft04l.pdf  392KB
Reliability and manufacturing costs due to defects is a significant problem with image sensors and the ability to recover from a fault would alleviate some of these costs.  A fault-tolerant APS has been designed by splitting the APS pixel into two halves operating in parallel, where the photo sensing element has been divided in two and the readout transistors have been duplicated while maintaining a common row select transistor.  This split design allows for a self correcting pixel scheme such that if one half of the pixel is faulty, the other half can be used to recover the entire output signal.  The fault tolerant APS design has been implemented in a 0.18μm CMOS process for both a photodiode based and photogate based APS.  Test results show that the fault tolerant pixels behave as expected where a non-faulty pixel behaves normally, and a half faulty pixel, where one half is either stuck low or high, produces roughly half the sensitivity.  The ratios of a normally operating pixel to a faulty pixel show an average value of 2.03 (stuck low) and 1.89 (stuck high) for the photodiode based APS, and 1.73 (stuck low) and 1.77 (stuck high) for the photogate based APS.  Preliminary results indicate that the sensitivity of a redundant pixel is approximately three times that of a traditional pixel for the photodiode APS and approximately twice that for the photogate APS.

G.H. Chapman, D.Y.H. Cheung, S. Djaja, Y. Audet, C. Jung, and B. Wai, “Fault Tolerant Active Pixel Sensors for Large Area Digital Imaging Systems”, Proc. SPIE Photonics West, Optoelectronic Integrated Circuits VIII , v 5356, pg 142-153, San Jose,  Jan. 2004 photonw04opto.pdf  357KB
Digital imaging detectors are growing larger in silicon area and pixel count, which increases fabrication time defects, reducing yield, hence increasing costs and limiting area.  In harsh environments, like high radiation conditions, what used to work might fail with time.  Fault tolerant Active Pixel Sensors have been created by splitting the photodiode and readout transistors into two parallel operating halves with only a small area cost.  These offer standard operation normally, but produce a recoverable image of half illumination sensitivity for single defects.  The single-defect case can be compensated by a multiplication of two, whereas the double-defect case is much less likely but can be corrected via software.  This paper presents the experimental and simulation results obtained from the fault-tolerant APS' fabricated in CMOS 0.18-micron technology, disregarding the effects of interpolation.  Test results suggest that after compensation, the percentage differences between the normally operating fault tolerant APS and the single-defect optically stuck-high and stuck-low cases are 0.5% and 1.5% respectively, which falls within experimental errors.  Combining these fault tolerant APS' with a software interpolation technique results in a system where initial simulations show the production of almost defect free images under error conditions with hundreds of dead pixels.

D.Y.H. Cheung, G.H. Chapman, S. Djaja and Y. Audet, “Implementation and Testing of Fault-Tolerant Photodiode-based Active Pixel Sensor (APS)”,Proc. IEEE Intern. Symposium on Defect and Fault Tolerence in VLSI Systems, pg. 53-60, Boston, MA, Nov. 2003. dft03aps.pdf 755KB
The implementation of imaging arrays for System-On-a-Chip (SOC) is aided by using fault- tolerant light sensors.  Fault-tolerant redundancy in an Active Pixel Sensor (APS) is obtained by splitting the photodiode and readout transistors into two parallel operating devices, while keeping a common row select transistor.  This creates a redundant APS that is self-correcting for most common faults. Simulations suggest that, by combining hardware fault-tolerance capability with software correction, Active Pixel Sensor arrays could be virtually immune to defects.    To test this concept in hardware, a fault-tolerant photodiode APS was designed and fabricated using a CMOS 0.18µm process.  Testing included both fully functional APS’, and those in which various failure modes and mechanisms are introduced (equivalent to stuck low and stuck high faults). Test results show that the output voltage for the stuck high case and the stuck low case varies linearly with light intensity. For the stuck low case, the sensitivity is 0.57 of that for a non-defective redundant APS, and the stuck high case is 0.40. These deviate from the theoretical value of 0.5 by +14% and -20% respectively.

Y. Audet and G.H. Chapman, "Design of a Self-Correcting Active Pixel Sensor", Intern. Symposium on Defect and Fault Tolerence in VLSI Systems, pg 18-27, San Francisco, CA. Oct. 2001 dft01yf2.pdf 127 KB
Digital cameras are growing ever larger in silicon area and pixel count, which increases the occurance of defects at fabrication time, or dead pixels that develop over their lifetime.  An Active Pixel Sensors self-correcting for most common faults is created by spliting the photodiode and readout transistors into two parallel portions with only a small area cost.  Simulations show operation is the same a single large device when no faults.  When one half of the redundant pixel is stuck at low, output over a wide current range is reduced by 1.98 to 2.01.  For one half stuck at high faults output, after offset removal, is reduced by a factor of 1.85 to 1.92. Hence self-correction of the pixel can be done with good accuracy via a simple shift circuit and with high accuracy with digital processing.  Variation in transistor threshold voltages between the pixel halves of even 10% only causes modification of factors by 2-4%, hence giving a small effect.

I. Koren, G.H. Chapman, and Z. Koren, "Advanced Fault-Tolerance Techniques For a Color Digital Camera-On-A-Chip", IEEE Intern. Symposium on Defect and Fault Tolerence in VLSI Systems, pg 3-10, San Francisco, CA. Oct. 2001 dft01k.pdf  385KB
Color digital imagers contain red, green and blue subpixels within each color pixel. Defects that develop either at fabrication time or due to environmentally induced errors over time can cause a single color subpixel (e.g., R) to fail, while leaving the remaining colors intact. This paper investigates seven software correction algorithms that interpolate the color of a pixel based on its nearest neighbors. Using several measurements of color error, all seven methods were investigated for a large number of digital images. Interpolations using only information from the single failed color (e.g., R) in the neighbors gave the poorest results. Those using all color measurements and a quadratic interpolation formula, combined with the remaining subpixel colors (e.g., G and B) produced significantly better results. A formula developed using the CIE color coordinates of tristimulus values (X, Y, Z) yielded the best results.

I. Koren, G.H. Chapman, and Z. Koren, "A Self-Correcting Active Pixel Camera", Proceedings IEEE Intern. Symposium on Defect and Fault Tolerence in VLSI Systems, Yamanashi, Japan, pg 56-64 Oct. 2000. dft00.pdf  392KB
Digital cameras on-a-chip are becoming more common and are expected to be used in many industrial and consumer products. With the size of the CMOS active pixel-array implemented in such chips increasing to 512x512 and beyond, the possibility of degradation in the reliability of the chip over time must be a factor in the chip design. In digital circuits, a commonly used technique for reliability or yield enhancement is the incorporation of redundancy (e.g., adding redundant rows and columns in large memory ICs). Very limited attempts have been directed towards fault-tolerance in analog circuits, mainly due to the very high level of irregularity in their design. Since active pixel arrays have a regular structure, they are amenable to reliability enhancement through a limited amount of added redundancy. The purpose of this paper is to investigate the advantages of incorporating some fault-tolerance methods, including redundancy, into the design of an active pixel sensor array.

G.H. Chapman and Y. Audet, "Creating 35 mm Camera Active Pixel Sensors", Proceedings Intern. Symposium on Defect and Fault Tolerence in VLSI Systems, Albuquerque, NM, pg 22-30, Nov. 1999. dft99p.pdf   98KB
A 36x24 mm Active Pixel Sensor imaging area device is studied which would be ideal for use with standard 35 mm cameras. By applying multichip methods to Active Pixel Sensors, the 39x30 mm system contains on board all the control circuitry and A/D converters, so the system outputs digital data. The large area requires a redundancy of design for a high yield. This starts with the Active Pixel cell, which able to withstand several defects and still be repairable, which CCD cells are not. The whole system is targeted at preventing bad rows or columns. By using spares in the row and column circuitry, as well as spare to A/D converters the chip yield is only limited by a relatively small logic and control block. With repairs the yield of this 11.7 sq. cm system goes from almost nil to more than 80%-93% with modest defect densities of 1.5 to 0.5 per sq. cm. By being a retrofit for current 35 mm cameras, and having larger photodiode pixels than current APS's this CMOS device would be nearly as sensitive as CCD's but at much lower production costs and much higher yields.

APS Thesis

Jozsef Dudas, "Characterization and Avoidance of In-Field Defects in Solid-State Image Sensors", MASc. Thesis, School of Engineering Science, Simon Fraser University, 2008 jdudas-Final-Thesis.pdf 3,3438K
   As solid-state image sensors become ubiquitous in sensing, control and photography products, their long-term reliability becomes paramount. This thesis experimentally examines the nature of in-field faults and demonstrates two combined hardware-software approaches for detecting and mitigating them. Characterization experiments found that most tested commercial cameras developed hot pixels that create image bright spots and degrade dynamic range. Faults appear spatially point-like and uniformly distributed, and they develop continually over time. Silicon displacement damage, induced by terrestrial cosmic rays, is the likely cause.
   A fault tolerant active pixel sensor is developed to isolate hot defects to a portion of the pixel, enabling software algorithms to correct the faults without sacrificing dynamic range. Experimentally-emulated hot pixels can be corrected within ±5% error.
   A new statistical software approach is developed to identify and calibrate stuck and abnormal-sensitivity faults from only regular photographs. Monte Carlo simulations verify the detection accuracy in complex environments.

Cory Jung, "Characterization of Fault Tolerant and Duo-Output Active Pixel Sensors" MASc. Thesis, School of Engineering Science, Simon Fraser University, 2007 cory_jung_thesis_final.pdf 3,268K
 The Fault Tolerant Active Pixel Sensor (FTAPS) corrects for defects by operating two pixel halves in parallel and retaining half sensitivity when affected by a point defect. Photodiode and photogate FTAPS devices were fabricated in 0.35µm technology and behaved as expected with and without defects. The photodiode FTAPS was almost twice as sensitive as a standard Active Pixel Sensor (APS) and 0.18µm technology pixels were about half as sensitive as 0.35µm technology pixels. The photodiode FTAPS noise was calculated to be greater than a standard APS, however the FTAPS signal-to-noise ratio remained on par with the standard APS due to increased sensitivity. A detection algorithm was developed for identifying standard APS and FTAPS defects from statistical analysis of the images taken. A Duo-output Active Pixel Sensor (DAPS) performed background subtraction, however crosstalk from charge collection of the parasitic n+ diffusion degraded its performance.

Desmond Y.H. Cheung, "CMOS Active Pixel Sensor for Fault Tolerance and Background Illumination Subtraction", MASc. Thesis, School of Engineering Science, Simon Fraser University, 2005 Desmond-MASc-Thesis-final.pdf  3,599KB
    As the CMOS active pixel sensor evolves, its weaknesses are being overcome and its strengths start to surpass that of the charge-coupled device.  This thesis discusses two novel APS designs.  The first novel APS design was a Fault Tolerance Active Pixel Sensor (FTAPS) to increase a pixel’s tolerance to defects.  By dividing a regular APS pixel into two halves, the reliability of the pixel is increased, resulting in higher fabrication yield, longer pixel life time, and reduction in cost.  Photodiode-based FTAPS pixels were designed, fabricated in CMOS 0.18 micron technology, and tested.  Experimental results demonstrated that the reliability of the pixel is increased and information that would have been lost without fault tolerance is recovered.
     The second novel APS pixel was designed to eliminate background illumination when a detector attempts to locate a desired laser signal.  This pixel design, namely the Duo-output APS (DAPS), consists of an extra output path, such that a signal can be selectively readout to one of the two paths at different time of a cycle.  During one half of a given cycle while the foreground signal is turned on, the sensor detects both the background and foreground levels.  During the other half of the cycle, the foreground is off, thus only the background level is detected.  The difference of the two outputs is the desired foreground signal without the background noise.  DAPS pixels were designed, fabricated in CMOS 0.18 micron technology, and tested.  Testing results identified design changes that will improve the background subtraction.

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