Subthreshold Operation of Photodiode-Gated Transistors Enabling High-Gain Optical Sensing and Imaging Applications

Abstract

In optical sensors and imagers, high gain that leads to high sensitivity and high signal to noise ratio (SNR) is often desirable. One popular approach is avalanche photomultiplication initiated by impact ionization in an avalanche photodiode or similar devices and the other approach is active pixel sensor (APS) with in-pixel amplifier. However, the former requires high electric field which induces high shot noise and the latter needs a multiple-transistor pixel circuit which compromises the fill factor and consequently, reduces the SNR. This work proposes and summarizes our recent efforts taken to achieve high gain optical sensors through subthreshold operation of photodiode-gated transistors.

Electronic global-shutter one-thin-film-transistor active pixel sensor array with a pixel pitch of 50 μm and photoconductive gain greater than 100 for large-area dynamic imaging

“…Despite all aforementioned efforts, the trade-off between SNR, fill factor, and spatial resolution still remains. To resolve the trade-off between SNR and resolution, we have proposed one-TFT APS concept for large area imaging applications [17][18][19][20]. This work presents an electronic global-shutter one-Thin-Film-Transistor active pixel sensor array with a pixel pitch of 50 μm and photoconductive gain greater than 100 for large-area dynamic imaging.…”

Amorphous silicon 3‐D one‐transistor active pixel sensor enabling large area imaging

“…Our previous work on one-TFT APS concept has attempted to resolve SNR-resolution tradeoff potentially for large area imaging applications. [7][8][9] Here, we report a 256 Â 256 one-TFT active pixel image sensor array for high-resolution, high-sensitivity, largearea imaging.…”

“…Such a 3-D dualgate photosensitive TFT combines a PD, a storage capacitor, and a switch, making the fill factor reach nearly 100%. Figure1Cillustrates its equivalent device circuit model and if it works in the subthreshold region, high gain can be also achieved [7][8][9]. Additionally, since both top and bottom TFTs are now vertically integrated, the pixel operation such as resetting becomes very quick, allowing for large-area dynamic imaging.…”

Design and Numerical Verification of a Gate-Controlled Lateral Thyristor for Low-Light Level Detection

“…However, the photocurrent at low-light level is insufficient to set conventional phototransistors to the forward amplification operation region and thus the conversion gain deteriorates abruptly when the incident optical power decreases [9]. Some improved phototransistors, such as punchthrough enhanced phototransistors and gate-controlled lateral phototransistors, have been proposed to improve the sensitivity to light, but the improvement in conversion gain results in high dark current [12], [13].…”