CMOS Image Sensor Substrate Bias Voltage Control
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in reducing the voltage required for transferring signal charge from photodiodes to floating diffusion, limiting the dynamic range due to high gate voltage needs and increased dark current when varying reset gate voltage.
Innovation Solution
Applying a substrate bias voltage to the P-well region during charge transfer and varying it during storage periods to reduce readout voltage and expand dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high gate voltage is applied to transfer signal charge from photodiode to floating diffusion, then transfer efficiency is improved, but the voltage consumption increases and dynamic range is limited
Solution Approach 1:
The patent applies dynamic voltage control to the transfer gate, switching between a first voltage level during photoelectric conversion and a second voltage level during charge transfer. This dynamic adjustment optimizes both transfer efficiency and power consumption by applying high voltage only when needed for transfer, not during the entire operation cycle.
Solution Approach 2:
The patent changes the voltage parameter of the transfer gate based on operational phase. By switching the transfer gate voltage between two distinct levels corresponding to different operational phases (conversion vs. transfer), the system achieves efficient charge transfer while minimizing overall voltage consumption and expanding dynamic range.
2Adaptability or versatility
If the reset gate voltage is varied to expand dynamic range, then the voltage range is improved, but dark current increases
Solution Approach 1:
The patent dynamically adjusts the reset gate voltage between a first voltage level during photoelectric conversion and a second voltage level during charge transfer. This dynamic control allows the system to expand its operational voltage range and dynamic range while managing dark current by applying different voltages at different operational phases rather than continuously varying the voltage.
3Reliability
If a high voltage is applied to ensure complete electron transfer, then transfer completeness is improved, but the circuit complexity and power consumption increase
Solution Approach 1:
The patent uses dynamic voltage switching with two distinct voltage levels for the transfer gate based on operational phase. This approach ensures complete electron transfer by applying appropriate high voltage during transfer phase while simplifying overall control through a systematic two-level voltage scheme rather than complex continuous voltage control.
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
Enables efficient transfer of electrons with lower voltages and increased dynamic range by deflecting potentials in the photodiode and floating diffusion, reducing dark current and operational limitations.
Implementation Method 1
a photoelectric conversion element formed in the well region for receiving light and producing signal charge in accordance with an amount of the received light
Implementation Method 2
Applying a substrate bias voltage to the P-well region during charge transfer and varying it during storage periods to reduce readout voltage and expand dynamic range. Technical Efficacy: Enables efficient transfer of electrons with lower voltages and increased dynamic range by deflecting potentials in the photodiode and floating diffusion
Data Source
AI summary
A CMOS image sensor is disclosed which can achieve reduction of the voltage used to read out signal charge and can achieve expansion of the dynamic range. A P-well region is formed on a semiconductor substrate, and an embedded photodiode, a transfer transistor, an amplification transistor, a selection transistor, a reset transistor, a floating diffusion and so forth are provided in the P-well region. Signal charge of the photodiode is transferred to the floating diffusion by operation of the transfer transistor. A substrate bias voltage in the form of a negative voltage is applied to the P-well region in synchronism with the charge transfer operation of the transfer transistor to control the potential balance between the photodiode and the transfer gate portion to reduce the voltage for charge transfer. Further, during charge storage of the photodiode, the substrate bias voltage is varied to modify the angle of the sensitivity curve to achieve expansion of the dynamic range.