Image Sensor Booster Circuit Reduces RTS Noise
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Solution Overview
Problem
Image sensors face challenges in reducing Random Telegraph Signal (RTS) noise, particularly due to electronic trap/de-trap phenomena in source follower transistors, leading to inconsistent output voltages across pixels.
Innovation Solution
The image sensor incorporates a booster mechanism that pre-boosts the floating diffusion node voltage before the transfer transistor is turned on, and enables the booster only after the selection transistor is activated, optimizing the timing of signal transitions to minimize noise and stabilize output voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a source follower transistor is used in a pixel with multiple photodiodes, then the output voltage of the first channel tends to be relatively high, but Random Telegraph Signal (RTS) noise is generated due to electronic trap/de-trap in the source follower transistor
Solution Approach 1:
The booster transistor is activated in advance (before the transfer transistor transfers the signal) to pre-boost the floating diffusion node voltage. This preliminary action ensures that when the signal is transferred, the source follower transistor operates from an elevated voltage baseline, preventing electron trapping in the dielectric layer and eliminating RTS noise while maintaining high output voltage.
Solution Approach 2:
The invention dynamically changes the voltage parameter of the floating diffusion node by introducing a controllable booster transistor. The booster transistor's gate voltage is adjusted based on the operational phase (reset, transfer, or output), thereby changing the operating conditions of the source follower transistor to prevent electron trapping while maintaining signal integrity.
2Stability of the object's composition
If the booster is enabled continuously to maintain high voltage levels, then output voltage remains stable, but additional noise may be introduced and power consumption increases
Solution Approach 1:
The booster transistor is activated periodically based on the pixel operational phase rather than continuously. It is enabled during the reset phase to pre-boost the floating diffusion node, disabled during signal transfer, and re-enabled only when needed for output. This periodic activation maintains voltage stability when necessary while minimizing noise and power consumption during other phases.
Solution Approach 2:
The invention introduces dynamic control of the booster transistor through phase-dependent gating signals. The booster's activation state changes dynamically according to the operational phase (reset, transfer, or output), allowing the system to adapt voltage levels to specific operational requirements rather than maintaining a fixed high voltage state throughout.
3Speed
If the transfer transistor is turned on to transfer the signal, then the voltage level is provided to the floating diffusion node, but electron trapping occurs causing RTS noise
Solution Approach 1:
The booster transistor is activated in advance of the transfer transistor activation to pre-boost the floating diffusion node voltage. This preliminary boosting ensures that when the transfer transistor turns on to transfer the signal, the elevated voltage baseline prevents electron trapping in the source follower transistor's dielectric layer, thereby enabling fast signal transfer without RTS noise.
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 approach effectively reduces RTS noise by controlling the voltage levels and electron trapping/de-trapping in the dielectric layer, resulting in more consistent and accurate output values across pixels, improving overall image sensor performance.
Implementation Method 1
a photoelectric conversion unit configured to receive light to generate an electric charge
Data Source
AI summary
An image sensor includes a photoelectric conversion unit configured to receive light to generate an electric charge and provide the electric charge to a first node, a transfer transistor configured to provide a voltage level of the first node to a floating diffusion node in response to a first signal, a booster configured to increase a voltage level of the floating diffusion node in response to a second signal, a source follower transistor configured to provide the voltage level of the floating diffusion node to a second node, and a selection transistor configured to provide a voltage level of the second node to a pixel output terminal in response to a third signal. After the selection transistor is turned on, the booster is enabled, and before the transfer transistor is turned on, the booster is disabled.