Transient Current Behavior of Vertically Integrated Amorphous Silicon Diodes
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0989-A12-03
Transient Current Behavior of Vertically Integrated Amorphous Silicon Diodes Gregory Choong1, Nicolas Wyrsch1, Christophe Ballif1, Rolf Kaufmann2, and Felix Lustenberger2 1 Institute of Microtechnology, Breguet 2, Neuchatel, 2000, Switzerland 2 CSEM, Badenerstrasse 569, Zurich, 8048, Switzerland ABSTRACT Monolithic image sensors based on Thin Film on CMOS (TFC) Technology are becoming more and more attractive as an alternative solution to conventional active pixel sensors (APS). Imager with high sensitivity, high dynamic coupled with low dark current values (10-100 pA/cm2 @ 104 V/cm) have been developed. However, issues such as light-induced degradation and image lag hinder the commercial development of a-Si:H based image sensors. The problem of image lag is caused by residual current due to the release of trapped charges after the switch off of the illumination. In this paper, we present a comprehensive study of the transient behavior of the photocurrent in a-Si:H photodiodes deposited on glass, as well as in corresponding diodes implemented in a TFC image sensor when illumination is switched off or periodically varied. The influence of the pixel architecture for two different cases is also analyzed: One setup reproduces the typical 3 transisor APS pixel architecture behavior, in which the bias voltage of the diode varies with the photogenerated charge while the second setup keeps a constant bias voltage applied to the diode by using a charge integrator. The influence of the light-induced defect creation on the performance of the sensors is also presented and discussed. INTRODUCTION During the last years, the development of TFC (Thin Film on CMOS) image sensor has attracted a lot of attention [1,2,4,5]. The vertical integration of a-Si:H photodiodes on top of a dedicated CMOS integrated circuit can enhance the performance of the resulting monolithic photodiodes. As the photodiode array does not share the die area with the readout electronics, this concept greatly improve the geometrical fill factor (ratio of pixel active area to total pixel area) and leads to an improvement of the sensitivity of the sensor. The latter combined with the high quantum efficiency in the visible spectral range of a-Si:H render this technology very attractive for visible light imagers. The dark current value Jdark is the key factor for high dynamic range image sensor and low light level detection and it must be as low as possible [13]. Jdark arises from thermal generation through bulk defect states. Optimization of a-Si:H n-i-p photodiodes (>1mm2) deposited on glass substrates in our laboratory led to Jdark values as low as 1 pA/cm2 (at ñ1 V for 1 µm thick detectors) while the corresponding diodes deposited on TFC readout chips exhibited Jdark values as low as 12 pA/cm2 for a reverse bias voltage V=-1 V. TFC devices (compared to c-Si CMOS sensors) usually suffer from image lag. This effect is caused by residual current due to the release of trapped charges resulting of an earlier exposition to light [11]. Moreover, light-soa
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