Active pixel sensor readout circuit using indium–tin–zinc-oxide thin-film transistors for image sensor applications

Abstract

In this study, we describe an active pixel sensor (APS) readout circuit using indium–tin–zinc-oxide (ITZO) thin-film transistors (TFTs). The APS readout circuit with a pixel size of 90 μm was fabricated using ITZO TFTs with a channel length of 2 μm, and its performance was experimentally evaluated. The ITZO TFTs were found to have good low-frequency noise performance with an extracted Hooge’s parameter (α H) of 3.5 × 10−3, and the APS exhibits a high DC voltage gain of ∼0.81 with satisfactory linearity in a wide output voltage range of ∼6.4 V as well as manageable response times of less than 15 μs. The obtained results show the potential of our APS circuits to be applied to TFT-based image sensors with enhanced image quality.

Organic photoconductive film–stacked active pixel sensor pixel circuits using indium–tin–zinc-oxide thin-film transistors

“…The layout of the APS pixel shown in Fig. 1(c) was designed with a size of 50 × 50 μm 2 , which is smaller than that in our previous study (90 × 90 μm 2 ) 27) and was achieved using more narrow wirings with a width of 2.5 μm. The TFTs used had a channel width of 10 μm and a channel length of 2 μm and are the same size as that in our previous study.…”

“…The TFT circuits in the APS were fabricated by the following procedure including the same TFT fabrication processes as reported in the previous literature. 27) A 50 nm thick Mo alloy was deposited on a glass substrate by DC magnetron sputtering to form gate electrodes, and a 200 nm thick silicon oxide was deposited by sputtering as a gate insulator. A 50 nm thick indium-tinoxide (ITO) was then deposited by DC magnetron sputtering to form pixel electrodes, and a 30 nm thick ITZO semiconductor layer was deposited by sputtering and patterned.…”

“…25,26) On the basis of previous reports, we have experimentally fabricated a prototype APS readout circuit using ITZO TFTs with a pixel size of 90 μm and confirmed its excellent properties. 27) However, the response to light irradiation has not yet been investigated because the examination was conducted using only a readout circuit without any photoconversion layers.…”

“…In this study, we demonstrate the signal readout capabilities of TFT-based APS circuits combined with an OPF. A more miniaturized APS pixel (50 μm) than in the previous study (90 μm) was fabricated, using TFTs with the same field-effect mobility (∼24 cm 2 V −1 s −1 ) and the same size (channel width/length of 10 μm/2 μm) as those previously reported, 27) by combining the APS pixel with a blue-sensitive OPF to replace the blue-sensitive layer in our three-layerstacked image sensor. 14) The blue-sensitive OPF has more improved properties than that used in our previous sensor, such as an external quantum efficiency (EQE) of ∼59% and a dark current density as low as <100 pA cm −2 .…”