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.
Highlights
In optical sensing and imaging applications, in order to improve sensitivity and signal to noise ratio (SNR), an active pixel sensor (APS) including a photodetector, generally a photodiode, and several transistors forming a multi-component circuit has been designed where an in-pixel amplifier, called a source follower, must be used
The brief discussion on these concepts can be found in [16]. We experimentally proved these concepts in single-pixel optical sensors based on PD-gated amorphous silicon (a-Si):H and LTPS THINFILM TRANSISTOR (TFT), respectively [4], [5]
The studied photosensitive TFT forms 1-T APS differing from the conventional APSs in that its high SNR is achieved by operating the 3-D dual-gate photosensitive TFT in the subthreshold region where the output current is an exponential function of the threshold voltage
Summary
In optical sensing and imaging applications, in order to improve sensitivity and SNR, an APS including a photodetector, generally a photodiode, and several transistors forming a multi-component circuit has been designed where an in-pixel amplifier, called a source follower, must be used. A device concept of a subthreshold-operated photodiode (PD)-gated transistor was proposed It achieves high gain with neither high field nor multi-transistor circuit. The gain derives from light-induced gate-modulation effect and to enable this, subthreshold operation must be conducted. It vertically integrates a PD with a transistor in a compact one-transistor (1-T) APS format, leading to high spatial resolution. Such a device concept has been implemented in a variety of material systems, making it a viable alternative technology for high gain optical sensors. The simulated output characteristics with and without light exposure is illustrated in Fig. 2 (c), where the output current elevates in response to the decrease of the light-induced threshold voltage
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