Guo Neng Lu

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.

Investigation of a buried double p-n junction structure implemented in CMOS technology for wavelength-sensitive detection

A buried double p–n junction (BDJ) structure in a CMOS process has been investigated to validate a wavelength-sensing method, which consists in using the ratio of the deep junction current to the shallow junction current for wavelength determination. Theoretically, an analytical BDJ device model has been established; and experimentally a chip incorporating BDJ sensing elements as well as electronic circuitry has been designed and measured. Both simulated and measured results, which are in good agreement, confirm the monotonic increase of the spectral response of the ratio. This allows the dominant wavelength of the incident light to be identified.

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.

Colorimetric characterization of a buried triple p–n junction photodetector

Abstract The operation of a buried triple p–n junction (BTJ) photodetector and its colorimetric characterization are described. Guidelines are given for the choice of the linear transformation between the device color space and the CIE 1931 standard system of color measurement. With a least squares fitting to the third order a mean color difference of 2.15 is obtained. The effect of the uniformity of the color space on the accuracy of the linear transformation is discussed.

A CMOS low-voltage, high-gain op-amp

A CMOS, self-biasing, single-supply op amp is presented. It is designed with regulated cascode transistors for gain enhancement and a common-mode feedback technique for bias stabilisation of complementary regulated cascodes. It enables supply voltage lowering to about 2|V/sub /spl tau//|+2|V/sub ds,sat/| with the maintain of high-gain operation. At V/sub dd/=1.8 V, the measured DC gain of the op-amp is 115 dB, with a unity-gain frequency of 8.6 MHz for a capacitive load of 20 pF.

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.

Design of a CMOS BDJ detector array for fluorescence imaging application

The Buried Double Junction (BDJ) detector [1], which can be used either as a wavelength-sensitive device and as a photodetector, can be implemented in standard CMOS IC technologies, with no requirement for additional post process step. It has recently been applied to fluorescence detection [2]. The wavelength-sensitive operation of the CMOS BDJ detector is based on the wavelength dependence of the Silicon absorption coefficient a(X) in the visible range. An absorption length defined as 1(X) varies monotonically, from about 0. 1pm to several micrometers when the wavelength of an incident monochromatic light changes from 0.4 jim to 0.8 jim.