Modeling the Laser Scanned Photodiode S-shaped J-V Characteristic
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0989-A19-03
Modeling the Laser Scanned Photodiode S-Shaped J-V Characteristic Miguel Fernandes1, Manuela Vieira1, and Rodrigo Martins2 1 DEETC, ISEL, Rua Conselheiro Emidio Navarro, 1, Lisbon, 1949-014, Portugal 2 Materials Science Dept., FCT-UNL, Campus da FCT-UNL, Quinta da torre, Lisbon, 2825 Monte da Caparica, Portugal ABSTRACT The devices analyzed in this work present an S-shape J-V characteristic when illuminated. By changing the light flux a non linear dependence of the photocurrent with illumination is observed. Thus a low intensity light beam can be used to probe the local illumination conditions, since a relationship exists between the probe beam photocurrent and the steady state illumination. Numerical simulation studies showed that the origin of this S-shape lies in a reduced electric field across the intrinsic region, which causes an increase in the recombination losses. Based on this, we present a model for the device consisting of a modulated barrier recombination junction in addition to the p-i-n junction. The simulated results are in good agreement with the experimental data. Using the presented model a good estimative of the LSP signal under different illumination conditions can be obtained, thus simplifying the development of applications using the LSP as an image sensor, with advantages over the existing imaging systems in the large area sensor fields with the low cost associated to the amorphous silicon technology. INTRODUCTION The Laser Scanned Photodiode (LSP) concept enables the fabrication of large area image sensors with application in fields where low cost, and design simplicity are of major importance [1,2]. The device is a large area p-i-n amorphous silicon based structure deposited over a glass substrate by a PECVD technique with two semitransparent electrical contacts. In order to decrease the conductivity of the doped layers methane is introduced in the reactor during the growth of the n and p layers. This step is of major importance since the lateral currents in the doped layers degrade the sensor performance. The LSP technique relies on the fact that the photocurrent of the device does not change linearly with the light intensity, and thus the local illumination conditions over the active area of the structure can be evaluated by measuring the current generated by a low power light beam. In order to extract the image information the low power modulated light beam scans the active area of the device, in raster mode, and the photocurrent generated in each position is measured at the electrical contacts by a lock-in amplifier. By recording the photocurrent measured on each point of the sensor an image of the light pattern captured by the sensor is obtained without the need of any additional signal processing. In this paper a model for p-i-n heterojuntion photodiodes based on the modulated barrier photodiode is presented, and the results from simulation are compared with the experimental data.
EXPERIMENT The devices investigated in this work are large area (4 x 4 cm2) amorphous silic
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