Mesa Photoelectric Conversion Structure for Dark Current Suppression
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Solution Overview
Problem
Photoelectric conversion elements suffer from image quality deterioration due to interface generation noise, such as dark current and afterimages, and inability to accurately detect reset potential, which hampers correlated double sampling and overall image quality.
Innovation Solution
A photoelectric conversion element with a mesa portion having a compound semiconductor material with band gap energy larger than the photoelectric conversion layer, a first electrode for reading charge, and a transfer gate facing the mesa portion to control charge transfer, reducing dark current and afterimages, and enabling accurate reset potential detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a pn junction is directly connected to a sense node above the photoelectric conversion layer, then the structure is simple, but image quality deteriorates due to interface generation noise such as dark current
Solution Approach 1:
An intermediate layer with band gap energy between the photoelectric conversion layer and the pn junction is introduced. This intermediate layer acts as a mediator that reduces the band offset at the interface, thereby suppressing interface generation noise such as dark current while maintaining structural feasibility.
Solution Approach 2:
The band gap energy parameter of the intermediate layer is specifically selected to be between that of the photoelectric conversion layer and the pn junction material. By changing this parameter, the band offset is reduced, which suppresses dark current generation at the interface.
2Object-affected harmful factors
If a material with larger band gap energy is disposed above the photoelectric conversion layer to suppress dark current, then dark current is suppressed, but a band offset forms at the interface serving as a transfer barrier causing afterimages
Solution Approach 1:
The band gap energy of the intermediate layer is precisely controlled to be between that of the photoelectric conversion layer and the pn junction material. This parameter optimization reduces the band offset at both interfaces, simultaneously suppressing dark current while preventing afterimage generation by enabling smooth charge transfer.
Solution Approach 2:
The intermediate layer is positioned specifically at the interface region where band offset problems occur. By localizing this layer with appropriate band gap energy, the quality of charge transfer is improved at the critical interface region without affecting other parts of the device.
3Ease of operation
If a pn junction is always connected to the sense node, then charge reading is straightforward, but reset potential before optical signal input cannot be accurately acquired preventing correlated double sampling
Solution Approach 1:
The charge reading function is segmented into two distinct phases: a reset phase where the intermediate layer allows measurement of reset potential before optical signal input, and a signal reading phase where photocurrent is read. This segmentation enables correlated double sampling by separating the reset potential measurement from the signal measurement.
Solution Approach 2:
The intermediate layer structure enables preliminary measurement of the reset potential before the optical signal is input. This preliminary action allows the system to capture the baseline potential, which is then used for correlated double sampling to accurately detect the optical signal by subtracting the reset potential.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively suppresses dark current and afterimages, allowing for improved image quality and accurate detection of reset potential, enhancing the correlated double sampling process.
Implementation Method 1
a photoelectric conversion layer including a compound semiconductor material
Data Source
AI summary
There is provide a photoelectric conversion element and an imaging device, in which image quality is capable of being improved. The photoelectric conversion element includes: a photoelectric conversion layer including a compound semiconductor material; a mesa portion disposed on a part of an upper surface side of the photoelectric conversion layer and including a compound semiconductor material having band gap energy larger than the band gap energy of the photoelectric conversion layer; a first electrode disposed on the mesa portion and configured to read charge photoelectrically converted in the photoelectric conversion layer via the mesa portion; and a transfer gate disposed to face a part of the upper surface side of the photoelectric conversion layer and at least a part of a sidewall of the mesa portion.