2-Layer Transistor Pixel | Technology | Sony Semiconductor Solutions Group

Overview

The 2-Layer Transistor Pixel is the world’s first^*1 stacked CMOS image sensor technology with a pixel structure that separates photodiodes and pixel transistors on different substrate layers, as opposed to the conventional style of having the two on the same substrate. This new structure approximately doubles^*2 saturation signal level^*3 relative to conventional image sensors, widens dynamic range and reduces noise. This pixel structure will enable pixels to significantly improve their imaging properties even at smaller pixel sizes.

*1) As of an announcement on Dec 16, 2021
*2) Comparison between our conventional back-illuminated CMOS image sensor and 1μm square conversion when this technology is applied.
*3) Maximum number of electrons that a single pixel can store.

Movie illustration of 2-Layer Transistor Pixel

Text version of the video

Technical Features

Photodiode and pixel transistor separated on different layers and stacked one atop the other

The 2-Layer Transistor Pixel technology is an advancement in the stacked CMOS image sensor, which consists of a pixel chip made up of back-illuminated pixels stacked atop a logic chip where signal processing circuits are formed. In conventional stacked CMOS image sensors, photodiodes and pixel transistors sit alongside each other on the same substrate. Using its proprietary stacking technology, SSS has created a structure in which the photodiodes for converting light to electrical signals and the pixel transistors on a separate substrate for controlling the signals one atop the other.

Improved photodiode capacity and amp transistor size

The new structure has improved the capacity of the photodiode while approximately doubling^*2 the saturation signal level^*3 relative to conventional image sensors by stacking the photodiodes and pixel transistors on separate substrate layers. In turn, this will widen dynamic range—the range of dark-to-bright tones that can be captured by a camera.

Additionally, the photodiode-free substrate for pixel transistors can provide extra space for the pixel transistors other than transfer gates (TRG), such as reset transistors (RST), select transistors (SEL) and amp transistors (AMP). This has created larger areas available for the amp transistors, leading to a success in substantially reducing the noise to which nighttime and other dark-location images are prone.

Applications

Capture images close to naked-eye view even in settings with a combination of bright and dim illumination
Making low-noise images possible even in low-light (e.g., indoor, nighttime) settings

Capture images close to naked-eye view even in settings with a combination of bright and dim illumination

Image sensors for mobile cameras, for their small sizes, cannot take in as much incident light as full-size image sensor of SLR cameras, and the range of brightness for capturing images is limited. For example, a high-contrast, backlit setting will result in an image with overexposure and underexposure, quite different from the way it looks to the naked eye.
The new technology, with approximately double^*2 the saturation signal level relative to conventional image sensors, has succeeded in widening the dynamic range—the range of dark-to-bright tones that can be recorded by a camera. This will enable the image sensor to produce images that reduce overexposure and underexposure in settings with a combination of bright and dim illumination (e.g., backlit settings), without increasing the size of the image sensor, achieving images closer to the way they are perceived through the human naked eye.

Simulated image (SDR) of conventional stacked CMOS image sensor

Simulated image (SDR) of 2-Layer Transistor Pixel stacked CMOS image sensor

Making low-noise images possible even in low-light (e.g., indoor, nighttime) settings

Adjusting the exposure to a dimly lit environment to take brighter images can cause grainy noise.
By increasing the size of the amp transistors, the 2-Layer Transistor Pixel technology has succeeded in substantially reducing noise which can affect images taken in dark settings. High-quality, low-noise indoor, nighttime, and other dark-location images are now possible thanks to this technology, without having to increase the size of the image sensor.

Shot with conventional stacked CMOS image sensor

Shot with 2-Layer Transistor Pixel stacked CMOS image sensor

2-Layer Transistor Pixel stacked CMOS image sensor makes it possible to shoot night sky and human skin with low noise.

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