Biomedical Engineering Reference
In-Depth Information
V
CC
(a)
h
IT
bR
i
ds
G
D
S
D
Epilayer
bR
S
GaAs substrate
ITO
P
(b)
FET
BR
FIGURE 14.6
(a) Schematic (left) and equivalent circuit
(right) of a monolithically integrated
BR/GaAs MODFET photoreceiver. (b) Photo-
micrography of a fabricated chip illustrating a
series of MODFET devices and the selectively
deposited and oriented dried BR film.
term refers to the human retina's ability to preprocess image information prior to inter-
pretation by the brain, especially with respect to motion detection and edge discrimina-
tion. The two closely related effects are common in the human visual field, which is
frequently characterized by changing levels of intensity, brought about either by a chang-
ing physical environment, or facilitated by actual head and eye movement.
Implementation of these properties into a conventional semiconductor device is not easily
accomplished.
The first attempt to employ BR in an artificial retina was made by Miyasaka et al. in
1992, who fabricated an 8
8 patterned array (64 pixels) of transparent SnO
2
electrodes on
a glass substrate. Purple membrane was applied to the array in an oriented
Langmuir-Blodgett film, which was coated with an electrolyte gel. The array was capped
with a gold-coated glass plate, and each individual pixel was connected to an external
amplifier-detector circuit. When the array was illuminated, a current flowed through the
external circuit connecting the transparent electrodes and the system exhibited differential
responsivity behavior, that is, the array did not generate a signal under constant illumina-
tion, but only in response to changes in illumination [85].
An artificial retina architecture capable of higher resolution was built and tested by
Chen and Birge in 1993 [84]. In this case, a 256
×
256 array of charge-sensitive semicon-
ductor elements (CID array) was used, covered with a thin poly(vinyl alcohol)-based BR
film (Figure 14.7). An x-y addressable CID imaging array is capable of producing an elec-
trical output proportional to the light incident upon each addressed pixel location—the
need for individually addressing each pixel is eliminated, making the design much sim-
pler. The oriented BR-based film deposits charge on the array surface upon illumination,
and the CID spatially samples the protein-induced photocurrent at discrete sites by apply-
ing the appropriate x-y address. Although the prototype worked as expected, cation con-
tamination of the semiconductor surface made the device unreliable. The problem was
ultimately resolved by utilizing chemically modified BR, in which the inorganic cations
[86] were replaced by organic analogs [87].
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