Light Filtering Properties in a-SiC:H Multilayer Structures: A SPICE model
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0910-A24-01
Light Filtering Properties in a-SiC:H Multilayer Structures: A SPICE model J. Martins, M. Vieira, M. Fernandes, P. Louro, and A. Fantoni DEETC, ISEL, R. Conselheiro Emidio Navarro, 1, Lisboa, 1949-014, Portugal ABSTRACT A SPICE model of a-SiC:H/a-Si:H p-i-n/p-i-n detector with light-bias and applied voltage controlled spectral sensitivity is presented. The equivalent electric circuit is composed of two series connected diodes, representing the p-i-n structures, with two current sources in parallel, representing the photogeneration for different steady state RGB illumination. Electrical simulations were performed for different transducer configurations and illumination conditions, and compared with the experimental data. The influence of the series and parallel resistances on sensor parameters is analyzed. A physical model supported by the electrical simulation gives insight into the methodology used for image representation and color discrimination. INTRODUCTION Early work on characterization and modeling of image sensors electrical behavior [1, 2, 3] gave some insight for the development of the existing design. The large-area p-i-n structures used for pattern detection of a B/W image were modeled by a consistent non-planar array of photodiodes. At the moment, the results from different light wavelengths of the image to be detected are the main concern. Both models will be merged in order to achieve image pattern and color detection. The circuit includes elements trying to capture several aspects of the sensor structure and the observed data: diodes for p-i-n structures; current sources discriminating image light wavelength cases of study; extra parallel and series resistances; and current dependent current sources for some transmission and feedback aspects. Adjusting the values of some parameters or the type of the simulation analysis, it was possible to explain some of the observed singularities of the sensor, aiming to explain color detection. The simulation procedure has two main phases: the circuit construction and the simulation process. In the first the input parameters and the array dimension are defined in order to include the image area and the surrounding dark region. In the second a current distribution (m×n matrix) is obtained. Electrical simulations were performed for different transducer configurations and illumination conditions and compared with the experimental data. The influence of the model parameters on the sensor electrical behavior is analyzed. Image pattern detection in addition to light filtering is introduced. A physical model supported by the electrical simulation gives insight into the methodology used for image representation and color discrimination. EXPERIMENTAL CONFIGURATION AND CHARACTERIZATION The Laser Scanned Photodiode sensor consists of 4x4 cm2 glass/ITO/p-i-n a-SiC:H multilayer structure (front diode) which faces the incident illumination representing the image optical bias, followed by a a-SiC:H(-p)//Si:H(-i)/SiC:H(-i’)//Si:H (-n)/ITO heterostructure (back diode)
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