Hydrogenated Amorphous Silicon Speed Sensor Based on the Flying Spot Technique
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ABSTRACT In the past we have developed a transient technique, called the Flying Spot Technique (FST). FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper, we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analyzed through a p.i.n structure being the transient transverse photovoltage dependent on the movement of the object (position and velocity). Assuming that the transport properties of the material and the geometry of the device are known and using a triangulation method we show that it is possible to map the movement of the object. Details concerning material characterization, simulation and device geometry are presented. 1.
INTRODUCTION
The p.i.n devices based on hydrogenated amorphous silicon (a-Si:H) are fundamental elements of many different types of radiation sensors. Most of these devices are based on either the transverse or the lateral photovoltaic effect [1 , 2 ]. It is known that when a junction is locally illuminated by a light spot, a lateral photovoltage is set up in addition to the transverse one. This lateral photovoltage varies in magnitude and polarity
as the light spot moves along the structure. The current division in the lateral direction (dependent on the light spot velocity and position) directly influences the potential profile along the junction and leads to a transient photovoltage during the light spot movement. In the past we have developed a transient technique, the FST [3 ], based on this lateral phototoeffect. This technique is a powerful tool to characterize solar cells, optical sensors and other optical devices. It allows to infer the ambipolar diffusion length and the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the device were known. In this paper we propose to apply FST together with the well known triangulation principle in the detection of moving objects. 2. 2.1
PHYSICAL MODEL Main principles of the FST
It is known that illumination of a semiconductor junction leads to the appearance of a photovoltage effect between the two sides (transverse direction). If the illumination is not spatially uniform, an additional effect arises resulting in a lateral photovoltage parallel to the junction, Vphl in addition to the usual transverse one, Vpht. 839 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
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Charge movement, potential distribution and device configuration of a p.i.n. structure illuminatedperpendicularly with localized monochromatic light.
Figure 1 shows the charge movement, potential distribution and geometrical configuration of a p.i.n. structure illuminated perpendicularly with localized monochromatic light. Let A
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