Back-Gate Vertical Transistor Pixels for Direct Photocurrent Sensing
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in directly sensing photo current, which is essential for adjusting integration time without full array scanning, leading to inefficiencies, especially in applications like automotive or endoscopic imaging, due to the mixing of photo current with drain current from the reset transistor.
Innovation Solution
The implementation of a back-gate-modulated vertical source follower transistor that allows direct photocurrent sensing by using the substrate as the drain of the source follower transistor, eliminating the need for dedicated addressing transistors and reducing pixel wiring complexity, thereby enabling direct photo-diode current sensing without array scanning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a reset transistor is connected to the supply line in conventional CMOS image sensors, then the reset operation can be performed, but the photo current is mixed with the drain current and cannot be detected directly
Solution Approach 1:
The pixel circuit is segmented into multiple independent blocks: one for reset operation (reset transistor connected to supply line) and another for photo current detection (transimpedance amplifier with separate input). This allows both functions to operate independently without current mixing, resolving the contradiction between reliable reset operation and photo current detectability
2Area of stationary object
If multiple photo-diodes share the same circuitry, then the pixel size can be reduced, but additional connections are required which reduce the fill factor
Solution Approach 1:
The patent transitions from planar sharing of circuitry to three-dimensional stacking where multiple photo-diodes are vertically integrated with shared readout circuitry. This vertical integration eliminates the need for additional lateral connections, maintaining high fill factor while enabling multi-photo-diode functionality
3Adaptability or versatility
If the sensor array is read out several times to determine correct integration time, then photo current detection capability is achieved, but valuable time is consumed
Solution Approach 1:
The transimpedance amplifier provides continuous real-time photo current measurement that feeds back to the control circuit, enabling automatic integration time adjustment without requiring multiple readout operations. The system serves itself by using the detected photo current signal to autonomously determine and adjust the optimal integration time, eliminating time-consuming iterative reading
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
This solution enhances pixel fill factor, reduces noise, and improves sensitivity and performance by allowing direct photocurrent monitoring, enabling more efficient adjustment of exposure time without the need for full array scanning.
Implementation Method 1
Typical complementary metal-oxide-semiconductor (CMOS) image sensors sense light by converting impinging photons into electrons
Data Source
AI summary
Image sensor arrays may include image sensor pixels each having at least one back-gate-modulated vertical transistor. The back-gate-modulated vertical transistor may be used as a source follower amplifier. An image sensor pixel need not include an address transistor. The image sensor pixel with the back-gate-modulated vertical source follower transistor may exhibit high fill factor, large charge storage capacity, and has as few as two row control lines and two column control lines per pixel. This can be accomplished without pixel circuit sharing. The pixel may also provide direct photo-current sensing capabilities. The ability to directly sense photo-current may facilitate fast adjustment of sensor integration time. Fast adjustment of sensor integration time may be advantageous in automotive and endoscopic applications in which the time available for the correction of integration time is limited.