Mohamed Ben Chouikha

Color-sensitive photodetectors in standard CMOS and BiCMOS technologies

The authors present a novel technique for color detection by using a buried double pn junction (B.D.J.) and a buried triple pn junction (B.T.J.) structure. For the B.D.J. wavelength-dependent photocurrents I1 and I2 can be measured. The wavelength of monochromatic incident light can be identified from the ratio I2/I1. In the case of the B.T.J. with wavelength dependent photocurrents the three colorimetric components of the incident light can be extracted. These structures can be implemented in standard CMOS and BiCMOS technology respectively.

Modeling of BDJ and BTJ structures for color detection

Two color-sensitive detectors, based respectively on BDJ and BTJ structures, have recently been developed in standard VLSI processes. The BDJ structure implemented in a CMOS process can produce two photocurrents, and the photocurrent ratio is a monotone function of the wavelength. The BTJ structure realized in a BiCMOS process gives three band-pass spectral response, thus allowing trichromatic color detection. In order to obtain better insight into the behavior of these two structures, and to simulate their characteristics, we have established physical models for photocurrent calculations. The following approach has been adopted: i) calculating drift and diffusion photocurrent components which are produced in different depletion layers and neutral regions of silicon; ii) according to their contributions, determining photocurrents flowing through each buried junction. A computer program can be written for device simulations. The validity of these models has been verified through comparison between simulations and measurements. These models can also be used to study effects of parameters involved in the presented models.

CMOS linear array of BDJ color detectors

A linear array of 64 BDJ cells has been designed and fabricated in a 1.2 micrometers CMOS process. It is aimed to build anew, self-calibrated micro-spectrophotometer. Each cell contains a BDJ detector which can operate as booth light intensity-sensitive and wavelength-sensitive device, and makes use of MOS transistors working in the weak inversion mode to perform logarithmic current-voltage conversion. Measurement of the fabricated chip has been carried out. A large detection light intensity dynamic range and a low fixed pattern noise have been obtained.

Modeling and design of multiple buried junctions detectors for color systems development

Two novel integrated optical detectors called BDJ detector and BTJ detector have been developed in our laboratory. These two detectors have different applications: the BDJ detector elaborated in CMOS process can be used for wavelength or light flux detection while the BTJ detector based on a bipolar structure gives the trichromatics components of a light. To develop microsystems, we need simulation tools as SPICE model. So, we have elaborated a physical mode, proposed a parameters extraction method and study influence of different parameters for BDJ detectors. Simulations and measurements have validated these models. More, we prose a design of BTJ detectors for developing new color imaging systems.

Design and implementation of integrated BDJ detector in a standard CMOS technology

The integrated BDJ detector basically consists of two buried junctions for collection of photocarriers at different depths. It can operate not only as a photodetector, but also as a wavelength-sensitive detector, because the ratio of the deep junction photocurrent to the shallow junction photocurrent is wavelength-dependent. The BDJ detector can be implemented using a standard CMOS process. The optimum design for a particular application requires divers considerations, such as process parameters, detector sizing, on-chip interface electronics, temperature sensing and compensation, etc. In particular, some geometrical and electrical parameters have significant effects on the device behavior and performances, and detector size as well as on- chip circuitry should be properly defined to meet specifications. Also, temperature-dependence of characteristics may need to be compensated. Two examples concerning design of integrated BDJ detector for specific applications are shown. One is for detecting spectral changes of absorption, while the other is for building a self-calibrated microspectrophotometer.

Buried double p-n junction structure using a CMOS process for wavelength detection

Buried double p-n junction (BDJ) structure designed and fabricated in a standard CMOS process is presented. Under reverse-biasing conditions, it provides two measurable photo- generated junction currents I1 and I2, which have a linear dependence on the incident photon flux. Over the visible range, the ratio I2/I1 is a monotone- increasing function of the wavelength, which can serve as a reference curve for wavelength determination. The reference curve r((lambda) ) can be obtained by measurement or by calculation. A physically-based model is suggested for the simulation of photocurrents and the computation of the curve r((lambda) ). Two application examples of the BDJ detector are presented. For the development of microspectrophotometry, replacing photodiodes by BDJ detectors offers a solution to problems of wavelength calibration. In the case of colorimetric pH measurement, the BDJ detector is used to detect spectral changes of absorption.

Optoelectronic library for color sensor design

An optoelectronic PDK library including Buried Double pn Junction (BDJ) detector and optical stimuli is implemented for Spectre simulator by writing the behavioral models in Verilog-A, analog subset of Verilog-AMS language. In order to ensure accurate behavioral description of the BDJ detector, an improved physical model is proposed. To validate the accuracy of the model, measurements of dark and photogenerate currents are curried out on a CMOS test circuit and compared to simulation results. Under Cadence Virtuoso Custom Design Platform, the BDJ device can be connected to its readout circuits and to an optical stimulus in order to perform optoelectronic simulations. This affords a reliable solution for color sensor design.

Characterization of a color-sensitive photodetector implemented in a BiCMOS technology

The operation and the colorimetric characterization of a buried triple p-n junction (BTJ) tristimulus detector are presented. A method defining a linear transformation between the detector color space and the C.I.E. standard is proposed. With the least squares fitting to the third order a mean color difference of 2.15 CIELAB units, between the detector response and the C.I.E. specification is predicted. The temperature effects on the detector and the linear transformation accuracy are studied between minus 60 degrees Celsius and 60 degrees Celsius. The color shifts in the detector specifications due to a temperature variations are smaller than 0.5.

Contribution to quality assessment of digital halftoning algorithms

Many new proposals are continually published in the halftoning domain. Alas, the demonstration of the interest of the proposed methods is often limited to a few favourable tests for the proposed methods, and images showing the defects of the other halftoning methods. The halftoning community needs to be able to compare a halftoning method with the innovations that appear in this domain. A complete and measured evaluation of quality is necessary through to a well defined set of test images and metrics to evaluate the algorithm. This paper proposes a protocol for the quality assessment of digital halftoning algorithm that can be used to compare one algorithm to another. It discusses the assessment of halftoner quality. It analyzes the perceived image quality concepts and defines the technical criteria that a good halftoner must match. A first sketch of a simple quality assessment protocol is proposed. It is composed of test images and quality metrics. This protocol could be used to provide new proposed halftoning algorithms with objective results.

Modeling and simulation under SPICE of optoelectronic systems including BDJ detector

We have recently developed in our laboratory a new integrated photodetector called BDJ. This detector allows determination of the wavelength of a monochromatic light. It was used to realize colorimetric applications. To develop such optoelectronic systems or microsystems we need simulations of their electronic behavior. Simulators like SPICE give in their libraries models for electrical components but not for optoelectronic components like photosensors or optic system. So we have developed a SPICE model to simulate the BDJ detector behavior and the optic source response. This model was implemented under SPICE and to illustrate its use, we have chosen to simulate two colorimetric applications developed in our laboratory; the first one allows determination of iron concentration and the second of the pH of solutions. In these applications, the optic system is composed of a light source (in practice Leds), and of a tube containing a liquid sensitive to the incident light wavelength; the transmission coefficient of the liquid depends on iron concentration in the first case and on reactive concentration and pH in the second case. Behavioral models of this optic system were included in the BDJ detector SPICE model. So we can obtain photocurrents ratio versus iron concentration or pH and reactive concentration. This system was simulated with an electronic associated circuit. This circuit is a classic analog circuit including several operational amplifiers. The optoelectronic system with associated circuit was described and simulated under SPICE and gives good results in comparison with measurements.