Amorphous and microcrystalline glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si 1 - x C x :H)/Al imagers with different n-layer resistivities were produced by plasma-enhanced chemical vapor deposition technique (PE-CVD). The transducer is a simple, large area p-i-n photodiode; an image projected onto the sensing element leads to spatially confined depletion regions that can be readout by scanning the photodiode with a low-power modulated laser beam. The essence of the scheme is the analog readout and the absence of semiconductor arrays or electrode potential manipulations to transfer the information coming from the transducer. The effect of the image intensity on the sensor output characteristics (sensitivity, linearity, blooming, resolution, and signal-to-noise ratio) are analyzed for different material composition. The results show that the responsivity and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of one order of magnitude in the image intensity and on the spatial resolution is achieved with a responsivity of 0.2 mW/cm 2 by decreasing the n-layer conductivity by the same amount. In a 4 x 4 cm 2 laser-scanned photodiode (LSP) sensor, the resolution was less than 100 μm and the signal-to-noise (S/N) ratio was about 32 dB. A physical model supported by electrical simulation gives insight into the methodology used for image representation.

Laser-scanned p-i-n photodiode (LSP) for image detection

REVIEW ARTICLE: Microdisplacement transducers

A historical outline traces the development from the initial experiments using hybrid technologies to present day integrated arrays in volume production, and to the latest developments. The theory of operation of photodiode arrays is explained, showing how sensitive scanned arrays are made possible by the charge integration technique, and describing and comparing the various scanning methods which have been proposed and developed, including serial multiplexing, bucket brigades, CCD, CID etc. The optical and electrical characteristics of arrays are discussed, pointing out the effects of various main limiting factors on array performance. The main fields of application of arrays are presented, together with the considerations relevant to designing systems of various types.

https://iopscience.iop.org/article/10.1088/0022-3735/8/5/001/pdf

Silicon photodiode arrays

Bias sensitive multispectral structures for imaging applications

Results on the optimization of an pinpii 'n type a- Si:H based three color detector with voltage controlled spectral sensitivity are presented. The sensor element consists of a glass/ITO/p-i-n a-SiC:H multilayer structure which faces the incident illumination, followed by a-SiC:H(-p)/a-SiC:H(-i)/Si:H(-i ')/SiC:H (- n ')/ITO heterostructure, that allows the optically addressed readout. Results show that this approach leads to regionally different collection parameters resulting in multispectral photodiodes, coding for red (R), blue (B), and two green (G) components. In the polychromatic operation mode different sensitivity ranges are selected by switching between different biases so that the basic colors can be resolved with a single device. Positive bias is needed under blue irradiation and moderated reverse bias under green. The threshold voltage between green and red sensitivity depends on the thickness of the bottom a-SiC:H (-i) layer, and corresponds to the complete confinement of the absorbed green photons across the pinpin sequence. The various design parameters are discussed and supported by a 2D numerical simulation.

Multispectral Structures for Imaging Applications

Studies were carried out on a self-scanned image sensor comprising a linear integrated-circuit array of photodiodes and metal-oxide-semiconductor (MOS) transistors. It was found that the maximum scanning rate is about 2.5×105bits/s, and that the value is mainly restricted by the photo-induced current and the junction capacitance of the photodiode.

http://ieeexplore.ieee.org/iel5/5/31129/01449527.pdf

Y. Ishihara

Papers

A self-scanned image sensor employing MOS structure