Two color-sensitive detectors, based respectively on BDJ (Buried Double p-n Junction) and BTJ (Buried triple p-n Junction) 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 responses, thus allowing trichromatic color detetion. 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.
Two novel integrated optical detectors called BDJ (Buried Double p-n Junction) detector and BTJ (Buried Triple p-n Junction) 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 model, proposed an parameters extraction method and study influence of different parameters for BDJ detectors. Simulations and measurements have validated these models. More, we propose a design of BTJ detectors for developing new color imaging systems.
t, INTRODUCTIONAt present, the progress of' the integrated circuit technology allowed the development of Systems On Chip in standard CMOS technotogy. These systems find in particular applications in imaging domain. Imaging systems such connexionnist retinas were so studied [l]. These systems inctude an imager constituted by APS cells. An APS cell is constituted itselfofa photodetector and a circuit of preamplification. The most used photodetectors are photodiodes because of their weak dimensions and of their linearity according to the luminous ilux, but phototransistors can be also used [2]. Their advantages are. due to their current gain and their wide spectral response.To develop such systems, it is necessary to be able to simulate them by means of adapted tools of simulation. One of the most adapted language to describe mixed systems is VHDL-AMs. Models of each component of the system are needed especially photodetectors models. The modeling and the simulation of photodiodes by using VHDL-AMS were already presented [3] but complete models of phototransistors, which can be simulated, do not exist in literature. So to be able to simulate imaging systems containing APS cells including phototransistors, an electric model based on a physical approach was elaborated, it was written in the VHDL-AMS language. It allows the simulation of the spectral response of phototransistors sensibilities and the study of their Iinearity according to the power of the incident light. A phototransistor is a bipolar transistor which can be illuminated and where only ColIector and Emitter are connected to external circuit. The technology chosen currently to develop Systems On Chip is the standard CMOS technology. In this technology, it is possible to realize PNP bipolar transistor and so a phototransistor by using the P+ Drain diffusion for Emitter, the N Well for Base and the P Substrate for Collector (fig. l), indeed Substrate is usually P-type.m N Well Fig. I . Vertical structure of phototransistor. This defines the vertical structure. A lateral structure of phototransistor can be also realized by using the P+ Drain diffusions for Emitter and Collector and the N Well for Base (fig 2). ~ N Well P Subssrate I Fig 2. Lateral structure ofphototnnslstor. B. BehaviorThe phototransistor is a component known for a long time. Its behavior was qualitatively studied and is presented in numerous works [4]. As in the case of the photodiode, it bases on the photoelectric effect.When an incidental Iight consisted of photons, the energy of which is greater to the gap energy o f silicon, penetrates into the semiconductor, it is absorbed and electron-hole pairs are generated. In a P-N junction a depletion layer is created between both regions and an electric field appears in this zone. This electric field separates the electron-hole pairs generated by the incident Iight in the depletion layer and near the edges of this region. It leads to the creation of a photocurrent through the junction.In the phototransistor, they are two P-N junctions (Emitter-B...
We present here a new paradigm based on a centralized architecture to realize electronic artificial retina. This original architecture, named connectionnist retina, can execute in real time RBF and MLP neural networks applications. We demonstrate that this intelligent embedded system could be used for vision applications. We describe here the realized prototype system.
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