Adjustable Threshold a-Si/SiC:H Color Detectors
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p
50A
DI
300 A a-SiC:H
2000 A * r1
cell
S
D2
!n
a-SiC:H A500
n
6500 A
i
9000 A 400A (a)
2
n
a-Si:H
(b)
P~
iA
50A 300 A
ii a-SiC:H $ ... 2000 A n ! ... i2 700 A S100 A 700 A i2 nisn 2000 A
i3
i3
p
pA
a-Si:H
9000 A 400A
(c)
Fig. 1. (a) General structure of the ATCD. (b) ATCI) using a tn-i-n as the inner cell. (c) ATCD using a n-i-p-i-n as the inner cell. 785 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
As we can see, the ATCD is made from a front p-i-n diode (DI), an inner cell, and a rear n-t-p diode (D2). The metal electrode is evaporated on the top and connected to ground. Light penetrates through the glass and the transparent conductive oxide (TCO): due to the different composition and thickness of the layers, blue light is absorbed in Dl, green light mainly in the inner cell, red light in D2. Apart from the presence of the inner cell, the device operation is that of a back-to-back diode. A positive value of the external voltage forward biases the diode DI (equivalent to a resistance) and reverse biases the diode D2 (equivalent to a photocurrent source). In this case, the photocurrent is correlated to the integral absorption of red light in the intrinsic layer of the diode D2, and the current flowing in the external circuit is proportional to the intensity of the red color (this has been proved over three orders of magnitude of the incident red light power). Similarly, a negative value of the external voltage forward biases the diode D2 (equivalent to a resistance) and reverse biases the diode DI (equivalent to a photocurrent source). In this case, the photocurrent is correlated to the integral absorption of blue light in the intrinsic layer of the diode Dl, and the current flowing in the external circuit is proportional to the intensity of the blue color (this has been proved over three orders of magnitude of the incident blue light power). The role of the inner cell is to add a third non-linear symmetric I-V characteristic which affects the device operation in a way which depends on the wavelength of the incident light. In the present devices, the X=450nm component does not reach the inner cell, whereas the absorption of the ?L=550nm component is strong in it. Thus, a blue light illumination leaves the inner cell in the dark, while under green light illumination the inner cell is filled with photogenerated carriers. Under illumination the I-V characteristic of the inner cell tends to a very low resistance, due to the high rate of generation of carriers. On the contrary, in the dark (or under blue light illumination) the high-impedance, non-linear IV characteristic of the inner cell adds to the other two. Now. let us consider a negative value of the external bias tsee Fig.la). In the case of II I 2 green light the operation of the front diode Dl ........... 2000 as photocurrent source is not affected by the .... ... 1500 A inner cell which behaves as a short circuit, .......... whereas in the case of blue light the current E photogenerated by DI is limited b
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