The laser scanned photodiode: Theoretical and electrical models of the image sensor M. Fernandes a, * , M. Vieira a , R. Martins b a Electronics Telecommunications and Computer Department, ISEL, Lisbon, Portugal b Materials Science Department, FCT-UNL, Portugal Available online 17 April 2006 Abstract The laser scanned photodiode (LSP) presents a new concept of image sensor with application in fields where low cost, large area and design simplicity are of major importance. Over the past few years this type of sensor has been under investigation and development, where several structures have been tested and characterized. In this work we present the physical explanation of device operating prin- ciple, with recourse to numerical simulation applied to structures with different compositions of the doped layers. An electrical model for this type of device is presented, enabling a fast evaluation of the device characteristics by means of an electrical simulation program. Ó 2006 Elsevier B.V. All rights reserved. PACS: 85.60.Gz; 85.60.Bt; 78.66.Jg Keywords: Amorphous semiconductors; Silicon; Devices; Sensors 1. The LSP (laser scanned photodiode) working principle The typical method used for detecting images with solid state sensors is the use of an array of optical sensors, with the associated electronics for signal amplification and selec- tion. Presently, two main technologies are in use: the CCD (charge coupled device) and the CMOS [1], although the physical principle is common (charge photogeneration in semiconducting material), their structure is quite different, details can be found on the literature. One drawback com- mon to both sensor types is the small active area, typically fractions of one square inch, thus forcing the use of optical elements to focus the image on the sensor. Other inconve- nient is the complexity of the associated electronic circuit for amplification and control. The LSP uses a single large area p–i–n structure fabricated in the amorphous silicon technology [2]. As only two electrical contacts are present, the electronic front-end is composed of a single current to voltage converter and signal amplifier. The information about the local illumination conditions over the active area of the sensor is obtained by scanning the sensor in raster mode, with a low power focused laser beam, and recording the amplitude of the laser beam generated photocurrent. 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. 2. Device structure The devices investigated in this work are large area (4 · 4 cm 2 ) amorphous silicon p–i–n structures in the assembly glass/ZnO:Al/p-(Si:H)/i-(Si:H)/n-(Si:H)/Al. The semiconductor layers were fabricated by plasma enhanced chemical vapor deposition, at 13.56 MHz radio frequency, the deposition conditions are presented elsewhere [2]. The front transparent contact (ZnO:Al) was produced by rf- sputtering and the metal back contact (Al) by thermal evaporation. In order to decrease the conductivity of the doped layers methane is introduced in the reactor during the growth of one or both layers. This step is of major 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.077 * Corresponding author. E-mail address: mfernandes@deetc.isel.ipl.pt (M. Fernandes). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 1801–1804