Variable Capacitance Floating Diffusion for Solid-State Imaging
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Solution Overview
Problem
Conventional solid-state imaging devices face a tradeoff between increasing signal electric charge treatment amount and maintaining low dark state random noise, as existing techniques either increase capacitance, which lowers conversion gain, or add circuit elements that complicate pixel size reduction.
Innovation Solution
A solid-state imaging device with a photodiode, a reading transistor, a floating diffusion, a reset transistor, and a transistor that selectively adds capacitance to the floating diffusion, allowing for dynamic control of capacitance to optimize signal transfer and noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If capacitance of floating diffusion is increased to treat more signal electric charges, then signal electric charge treatment amount is improved, but conversion gain decreases and dark state random noise increases
Solution Approach 1:
The patent applies dynamics by making the floating diffusion capacitance variable rather than fixed. A control transistor selectively connects the floating diffusion to a capacitance node, allowing the capacitance value to be dynamically adjusted between a first value (for high signal charge treatment) and a second value (for low noise). This dynamic adjustment resolves the contradiction by enabling the system to adapt capacitance based on operating conditions.
Solution Approach 2:
The patent changes the capacitance parameter of the floating diffusion from a fixed value to a variable value. By controlling the capacitance node connection state, the capacitance parameter can be switched between different values to optimize either signal charge treatment amount or conversion gain depending on the operational requirements, thus resolving the tradeoff between these two parameters.
2Object-affected harmful factors
If capacitance of floating diffusion is increased to reduce dark state random noise, then noise reduction is improved, but conversion gain decreases
Solution Approach 1:
The patent uses dynamic control of the capacitance node connection to adjust floating diffusion capacitance based on operational needs. When low noise is prioritized, the capacitance is increased by connecting to the capacitance node. When high conversion gain is needed, the capacitance is reduced by disconnecting. This dynamic adjustment allows optimization of noise performance without permanently sacrificing conversion gain.
Solution Approach 2:
The capacitance parameter of the floating diffusion is made variable through control transistor switching. By changing the capacitance parameter between different values, the system can optimize for low dark state random noise when needed while maintaining high conversion gain in other operating conditions, resolving the contradiction between noise reduction and conversion gain.
3Quantity of substance
If variable capacitance is added to floating diffusion to optimize signal treatment, then signal electric charge treatment is improved, but device complexity increases
Solution Approach 1:
The control transistor serves multiple functions: it acts as a switch for variable capacitance control, a reset element for the floating diffusion, and a connection controller for the capacitance node. This multi-functionality reduces the need for separate dedicated components, thereby limiting the increase in device complexity while achieving variable capacitance for optimized signal electric charge treatment.
Solution Approach 2:
The patent merges the capacitance control function with the existing reset transistor functionality. The same transistor structure and control mechanism are used to achieve both variable capacitance adjustment and reset operations, consolidating multiple functions into a single component to minimize the increase in device complexity.
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 configuration enables increased signal electric charge treatment while maintaining low dark state random noise by selectively adding capacitance, thereby overcoming the tradeoff between signal treatment and noise levels.
Implementation Method 1
a photodiode configured to photoelectrically convert incident light
Data Source
AI summary
A solid-state imaging device includes a plurality of unit pixels. Each unit pixel has a photodiode, a reading transistor, a floating diffusion, a capacitance adding transistor, and a reset transistor. The reading transistor reads signal electric charges from the photodiode. The floating diffusion accumulates the signal electric charges read from the reading transistor. The capacitance adding transistor selectively adds capacitance to the floating diffusion. The reset transistor resets an electric potential of the floating diffusion.