Non Linear Optical Gain in Bulk Barrier Amorphous Silicon Phototransistor
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NON LINEAR OPTICAL GAIN IN BULK BARRIER AMORPHOUS SILICON PHOTOTRANSISTOR D. Caputo, G. de Cesare, A. Nascetti, F. Palma Department of Electronic Engineering, University of Rome "La Sapienza" via Eudossiana 18, 00184, Rome, Italy ABSTRACT In this work we report studies on the non linear dependence of the optical gain with the incident power in an amorphous silicon bulk barrier phototransistor based on a n-i-p-i-n structure. The optical gain shows a quasi-hyperbolic dependence on the illumination intensity. The non-linear behavior was predicted by an analytical device model which takes into account the properties of both material and structure, which lead to the amplification mechanism of the device. INTRODUCTION Hydrogenated amorphous Silicon (a-Si:H) is widely investigated for large area photosensitive applications for both its efficient absorption of visible light and its suitability in device realization on various large-area substrates. Examples of applications are in the photovoltaic field [1], in medical imaging [2] and in the detection of the different spectral components of the light [3, 4]. In application as imager, a large dynamic range is usually required in order to detect both low intensity radiation and bright illumination. If the response of the device or, more in general, of the whole detection system is linear with the incident light intensity, failure in the image reconstruction can occur either due to saturation of output charge or due to noise limits. A solution of this problem can be that presented in [5], where the integration time of each individual pixel (constituted by a p-i-n diode) is changed according to the local illumination intensity. In this realization, the pixel for the light detection is integrated on a CMOS technology (TFA, Thin Film on Asic technology), which performs the driving circuitry. An alternative solution can be a device, which shows a non linear-gain with illumination intensity. At this aim, in this paper we show that a bulk barrier amorphous silicon phototransistor (BBPT) [6] meets this requirements and then can be suitable for large-area high-dynamic-range imagers. DEVICE STRUCTURE AND OPERATION The BBPT is a n-i-δp-i-n stacked structure of amorphous silicon layers grown on a glass substrate coated by Transparent Conductive Oxide (TCO). This layer represents the bottom electrode of the device (the transistor collector electrode), while a 3000 Å thick aluminum film evaporated on the top n-layer represents the top electrode (the transistor emitter electrode). A sketch of the band diagram of the device structure is reported in figure 1, where the thickness
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300 Å
4200 Å
60Å 500 Å 300 Å base φ'be δp
i
i
φbe
n
incident radiation n collector
emitter Vbias
Figure 1. Band diagram of an a-Si:H BBPT in equilibrium condition and forward bias condition. The lowering of the barrier due to the absorbed illumination is qualitatively depicted. Estimated thickness of the investigated device are also shown. of the different layers of a manufactured device are also sh
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