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, 11 (1), 55-67
eCollection

Vertically Integrated Photo Junction-Field-Effect Transistor Pixels for Retinal Prosthesis

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Vertically Integrated Photo Junction-Field-Effect Transistor Pixels for Retinal Prosthesis

Samir Damle et al. Biomed Opt Express.

Abstract

Optoelectronic retinal prostheses transduce light into electrical current for neural stimulation. We introduce a novel optoelectronic pixel architecture consisting of a vertically integrated photo junction-field-effect transistor (Photo-JFET) and neural stimulating electrode. Experimental measurements demonstrate that optically addressed Photo-JFET pixels utilize phototransistive gain to produce a broad range of neural stimulation current and can effectively stimulate retinal neurons in vitro. The compact nature of the Photo-JFET pixel can enable high resolution retinal prostheses with the smallest reported optoelectronic pixel size to help restore high visual acuity in patients with degenerative retinal diseases.

Conflict of interest statement

SD: Nanovision Biosciences, Inc. (F,P); YLiu: Nanovision Biosciences, Inc. (E), NOW: Nanovision Biosciences, Inc. (F,C,P, R) YLo: Nanovision Biosciences, Inc. (F, C, P,R)

Figures

Fig. 1.
Fig. 1.
(A) Design of a Photo-JFET pixel from a vertical etched mesa. A dielectric passivation layer deposited on the sidewall of the silicon mesa induces a channel along the vertical wall of the middle layer of the mesa. The sidewall view illustrates the pair of back-to-back p/n diodes and the junction-field-effect transistor (JFET) realized as an NPN or PNP stack. (B) Scanning electron microscope micrograph (SEM) of a 13 µm Photo-JFET single pixel test structure. The mesa sidewall and outer rim are coated in SiO2 layer while electrical contact is made through a center opening in the passivation. (C) SEM micrograph of prototype array with 40 µm pixels paired with 10 µm electrodes (light gray) spaced at 45 µm pitch, a theoretical prosthetic acuity of 20/188 (D) SEM micrograph of prototype array with 13 µm pixels paired with 10 µm electrodes (light gray) spaced at 15 µm pitch
Fig. 2.
Fig. 2.
Theoretical performance characteristics of a vertical silicon Photo-JFET using typical device parameters for a range of retinal irradiance. (a) Calculated gate to source voltage (VGS) for a range of irradiance conditions following Eq. (1). Marked on the graph are the possible values of the threshold voltage to turn on the JFET channel for ideal case of minimum threshold, bright indoor retinal irradiance, and 200µW/mm2 supplemented NIR. (b) Calculated total current (ITot) using the identified threshold voltages in (a). (c) Calculated gain achieved over the irradiance range. For simplicity, we have ignored the subthreshold current in (b) and (c), thus underestimating the total current and gain in subthreshold regime.
Fig. 3.
Fig. 3.
I-V Characteristics of Photo-JFET pixels for 13 µm diameter pixels under illumination with visible (518 nm) and NIR (850 nm) light.
Fig. 4.
Fig. 4.
(A) Voltage dependence of responsivity for NIR illumination (850 nm) of a 13 µm Photo-JFET pixel and (B) Responsivity of a 13 µm diameter pixel vs. NIR irradiance (850 nm) at 2 V bias.
Fig. 5.
Fig. 5.
Overview of the in vitro experiment to demonstrate retinal stimulation with Photo-JFET pixels. (A) diagram of the circuit to connect Photo-JFET pixels on chip to neural stimulation electrodes and the instrumentation used to record stimulated action potentials from retinal neurons (B) 10x microscope image of Rd10 retina atop of PEDOT/IrOx electrode on transparent substrate with a glass micropipette electrode used to loose patch RGC to record action potentials (E) Spiking behavior recorded from RGC in response to electrical stimulation driven by Photo-JFET pixels (D) Comparison of spikes elicited by Photo-JFET stimulation under dark and NIR illumination
Fig. 6.
Fig. 6.
Comparison of measured current produced by 13 µm Photo-JFET pixels versus theoretical current of passive photodiode pixels at 13 µm and 40 µm diameter under the same illumination conditions with NIR light (850 nm). A responsivity of 0.4 A/W is assumed for passive photodiodes based on reported values [21].

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