Solid State Technology and Devices Seminar: Designing a semiconductor photovoltaic diode array for a retinal prosthesis
Seminar | October 26 | 1-2 p.m. | Cory Hall, 521 Hogan Room
Theodore Kamins, Stanford University
Several groups are investigating silicon devices to be implanted near the retina for patients with age-related macular degeneration (AMD) and retinitis pigmentosa (RP), in which the photoreceptors degrade while other parts of the retina remain mostly functioning. One class of these devices, subretinal photodiode arrays, aims to replace the degraded photoreceptors and couple to the functioning portions of the eye. Most implementations rely on a power source external to the eye, requiring complex surgical procedures and creating paths for serious infections. In our approach the device operates in the photovoltaic mode, so that the external light provides both the signal and the power source. Each pixel in the two-dimensional photodiode array independently converts pulsed infrared light into electrical current that stimulates the retinal neurons.
After a brief discussion of the overall system, this talk will emphasize the design, fabrication, and optoelectronic performance of the photodiode array. To enhance charge injection, each pixel contains multiple photodiodes connected in series to provide a photovoltage sufficient to stimulate the neurons, but which is limited by the ionization threshold of the fluid in the eye. An active and a return electrode in each pixel provide localized current flow to improve spatial resolution. For improved resolution, the pixel pitch must decrease. However, as the lateral dimensions scale, the extent of the vertical field also decreases, limiting coupling to the retinal cells. Three-dimensional structures offer the possibility of overcoming this limitation and improving spatial resolution
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