Readout Circuit Charge Injection Noise Suppression
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Solution Overview
Problem
Existing photodiode readout circuits in hybrid sensors face limitations due to significant 1/f noise, especially at low frequencies, which affects the accuracy of charge measurement and dynamic range, and require memory storage for correlated double sampling, leading to increased noise power and longer cycle times.
Innovation Solution
A read circuit structure with a floating diffusion node, MOS-type transfer transistor, and buried collection diode, allowing for complete charge depletion and reduced noise, along with a charge recovery structure that enables close-timed correlated double readings, minimizing 1/f noise influence and eliminating the need for extensive memory storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If correlated double sampling is implemented using existing readout circuits, then noise suppression is achieved, but memory storage is required which increases device complexity and cycle time
Solution Approach 1:
The patent combines the correlated double sampling function with the existing readout circuit by utilizing the floating diffusion node and transfer transistor mechanism. The first read (reset level) and second read (signal level) are performed sequentially using the same circuit components, eliminating the need for separate memory storage. The floating diffusion node naturally holds the charge states during the sampling process, merging the sampling function into the readout path itself.
Solution Approach 2:
The readout circuit performs correlated double sampling on its own without requiring external memory assistance. The transfer transistor automatically transfers charges between the floating diffusion node and collection region, and the circuit inherently performs the subtraction operation by comparing the two read levels. This self-service approach eliminates the need for separate memory storage components.
2Measurement precision
If existing readout circuits are used, then charge integration is performed, but 1/f noise significantly affects measurement accuracy especially at low frequencies
Solution Approach 1:
The patent converts the harmful 1/f noise effect into a beneficial measurement approach by using correlated double sampling. The technique measures the reset level immediately before signal integration and the signal level after integration, then subtracts the two readings. This differential measurement eliminates low-frequency 1/f noise that affects both readings equally, while preserving the desired signal that changes during integration.
Solution Approach 2:
The readout circuit performs periodic correlated double sampling at the frame rate or line rate. Each sampling cycle consists of a reset phase followed by a signal integration phase, with both readings taken periodically. This periodic action allows the circuit to systematically reject 1/f noise by comparing readings taken at regular intervals, improving measurement precision despite the presence of low-frequency noise.
3Measurement precision
If conventional readout structures are used, then charge is accumulated in the photodiode, but complete charge depletion is not achieved leading to residual charge and measurement errors
Solution Approach 1:
The patent extracts residual charge from the photodiode by introducing a dedicated charge collection region and transfer transistor mechanism. The transfer transistor actively transfers charges from the floating diffusion node to the collection region, ensuring complete depletion of the photodiode. This extraction mechanism removes residual charges that would otherwise contaminate subsequent measurements, improving charge measurement accuracy.
4Adaptability or versatility
If hybrid sensor configuration is used with photodiodes in one substrate and read circuits in another, then specialized photodiode materials can be used, but charge transfer between substrates introduces additional noise and complexity
Solution Approach 1:
The patent introduces a floating diffusion node as an intermediary between the photodiode substrate and the readout circuit substrate. This intermediate structure receives charges from the photodiode through the hybridization contact and then transfers them to the collection region on the readout circuit substrate. The floating diffusion node acts as a buffer that isolates the charge transfer process from direct coupling, reducing noise and interference between the two substrates while enabling the use of specialized photodiode materials.
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
The solution significantly reduces 1/f noise, improves dynamic range, and allows for more accurate charge measurement with reduced cycle time, enhancing the effectiveness of charge integration and readout processes in hybrid sensors.
Implementation Method 1
a first doped region (4) forming with said substrate an injection junction (4) adapted to transfer to said substrate electrical charges coming from a charge source (2) external to said substrate
Implementation Method 2
a second doped region (6) forming with said substrate a collection diode (6) able to collect in said substrate charges injected by said injection diode and to accumulate said charges during a charge integration cycle
Implementation Method 3
a MOS-type transfer transistor (9) whose transfer gate is located between said buried collection diode and said charge recovery structure, above and electrically isolated from the substrate, and which is controllable to transfer the charges collected in the second doped region (6) to the charge recovery structure
Implementation Method 4
an electromagnetic radiation sensor composed of pixel reading circuits, each pixel comprising at least one photodiode for emitting an electrical signal representative of the radiation to which the pixel is exposed
Data Source
Figure 1a~2a
Figure 2b~2c
Figure 3~4
AI summary
The invention concerns a structure of a readout circuit, formed on a semiconductor substrate (1) of a first type, and intended to measure the charges received from an external charge source (2) external to the substrate (1) according to successive charge integration cycles, said structure comprising: an injection diode configured to inject, into the substrate (1), the charges received from the external charge source (2), a collector diode suitable for collecting, in the substrate (1), at least a portion of the charges injected by the injection diode and for accumulating said charges during an integration cycle, a charge recovery structure (7), configured to recover the charges accumulated in said collector diode, means for initialising the charge recovery structure (7) at the end of each integration cycle, by restoring the electrical potential of said charge recovery structure to an initial potential.