Biomedical Engineering Reference
In-Depth Information
become evident. Either the photosensitive area must be reduced, resulting in increased
noise, or the pixel itself must become larger, which decreases resolution. These conflicts
drive photosensor array development toward 3D architectures. The same multilayered
approach is evident in nearly all biological vision systems. A 3D-sensor array has been
proposed, where three circuit layers with different functions are stacked vertically into one
chip (75). The first layer only consists of a photosensor array, the second layer contains
analog circuits to amplify the light-induced signal, and the third layer consists of circuits
to digitize the signal and read out the data. Each pixel in the sensor array is directly con-
nected with its processing circuits in parallel via the vertical interconnection. As the sig-
nals from individual pixels are processed and transferred in parallel, real-time and
high-performance image processing can be realized. Even though multichannel designs
are limited to relative small arrays at the current stage, rapidly developing thin-film tran-
sistor technology can allow improved readout capabilities for future bR photoreceptor
arrays. This technology will allow circuits to be constructed in many thin-3D layers, while
retaining its flexible characteristics.
17.4
Performance Analysis
The performance of the proposed bR photoreceptor array is evaluated at both the pixel
level and overall array level. The key characteristics concerning individual pixels involve
SNR, linearity and dynamic range, spectral response, and response time. Pixel uniformity
and overall responsivity under mechanical bending are applied for array performance
testing.
17.4.1
Noise and Signal-to-Noise Analysis
17.4.1.1 An Overview of Noise Sources
The smallest signal that a photodetector can detect is often limited by the amount of noise
that is present. Typically, lower noise levels offer better sensitivity. Hence, noise charac-
terization plays an important role in quantifying detector performance.
Shot noise and thermal noise are the two predominant noise sources in all photodetec-
tion systems (76). Shot noise is caused by random fluctuations in the arrival of discrete
particles, such as individual electrons. In semiconductors, shot noise is present when
charge carriers are generated and recombined at randomly varying rates. The power of
shot noise, in terms of current, is given by the Schottky formula
2
(17.7)
i
2
qI
f
ph
where q is the charge of one electron (1.6
10 19 C), I ph is the constant photocurrent, and
f is the measurement bandwidth.
Thermal noise is dominant when photocurrents are small and it is generated by thermal
fluctuations of moving electrons as they collide with atoms and other electrons in a con-
ductive material. The power of thermal noise, given in terms of current, is described by
the Johnson formula, also known as the Nyquist formula
4
KT
f
2
(17.8)
i
n
R
 
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