Image Sensors in Tfa Technology - Status and Future Trends
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dark current is mainly determined by thermal generation within the i-layer. The dynamic range amounts to > 100 dB for low levels of negative bias and can easily be extended to higher illumination levels. The temperature behavior is determined by the dark current which rises exponentially with temperature (figure 2) in the range between 300 K and 350 K, whereas the photocurrent remains constant. A temperature increase of 50 K decreases the dynamic range of the detector by 20 dB which yields a dark current doubling temperature of about 15 K. Noise in pin detectors consists of shot noise and flicker noise and is discussed in [4]. 10-4 1000Ix
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Fig. 2 Temperature dependence of dark and photocurrent of a pin photodiode
Fig. 1 I/V characteristics of a pin photodiode in the dark and under illumination
Since in many sensor applications readout speed is an important parameter, the transient behavior of the photodetector is of fundamental interest. The photocurrent rise and decay after switching on and off illumination of a pin diode are depicted in figure 3 a,b. Steady-state conditions are reached within a few gts after switching on illumination irrespective of the incident illumination level. However, the decay after switching off illumination exhibits a more complicated behavior. After an initial reduction of the current within 10 ýts a quasi stationary plateau occurs during which the transient current decreases only slowly. In the ms range the decay turns into a steeper decrease. The significant intermediate plateau is attributed to thermal emission of trapped carriers into the extended states and subsequent field assisted extraction and is linked to the continuous density of states within the band gap. 10l
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Tine [s] b) Tine [s] Fig. 3 Photocurrent transients of a pin photodiode after switching (a) on and (b) off illumination for different illumination levels at a bias voltage of -1.5 V
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Thin Film Detector Structures for Color Applications Besides simple b/w detection amorphous silicon multilayers are also capable of color recognition. In the past a variety of two terminal color devices have been developed [5-9]. Common to all of these color detectors is that they exploit the wavelength dependence of the absorption coefficient in amorphous silicon and the carrier generation profile inside the device. The special feature of the color detectors is the voltage controlled shift of the main collection region which is obtained by appropriate variation of the drift length. An example of a three color device based on a pin structure with the intrinsic absorber subdivided into three i-layers is depicted in figure 4. The electric field profile in conjunction with the generation profile allows collection of carriers generated by strongly absorbed radiation for low values of revers
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