CMOS Active Pixel With Capacitor Transistor For Wide Dynamic Range
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Solution Overview
Problem
Conventional CMOS active pixels suffer from reduced sensitivity and operating range due to a small depletion region and weak electric field, especially at low voltage conditions, leading to decreased quantum efficiency and dynamic range.
Innovation Solution
A CMOS active pixel design that includes a photodiode with a P-I-N structure, a capacitance node, reset transistor, floating diffusion node, driving transistor, select transistor, and a capacitor transistor, where the capacitor transistor is gated by a control signal to increase the voltage of the floating diffusion node above the external power voltage, enhancing the sensitivity and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the size of the depletion region is increased to improve quantum efficiency, then the sensitivity is improved, but the device complexity and voltage requirements increase
Solution Approach 1:
The photodiode is segmented into distinct P-type and N-type impurity layers with different concentration profiles. The P-type layer has a lower concentration (1E16 to 1E18 atoms/cm³) while the N-type layer has a higher concentration (1E19 to 1E21 atoms/cm³), creating an optimized depletion region structure that improves sensitivity without requiring excessive voltage
Solution Approach 2:
Different regions of the photodiode are given different impurity concentrations to optimize local properties. The P-type region uses low concentration to maximize depletion width and electric field, while the N-type region uses high concentration to provide strong field for carrier collection, achieving both sensitivity and operational efficiency
2Reliability
If the external power voltage is increased to maintain the depletion region, then the quantum efficiency is improved, but the operating range decreases when voltage is low
Solution Approach 1:
The impurity concentration parameters are optimized to achieve the desired depletion region characteristics at lower voltages. By adjusting the concentration ranges of P-type (1E16 to 1E18 atoms/cm³) and N-type (1E19 to 1E21 atoms/cm³) impurity layers, the pinning voltage is reduced, enabling reliable operation across a wider voltage range including low voltage conditions
3Ease of manufacture
If the N type impurity of the floating diffusive layer enters the N- type impurity layer, then the manufacturing is simplified, but the size of the depletion region decreases
Solution Approach 1:
The N-type floating diffusive layer is designed with a localized high concentration region (1E19 to 1E21 atoms/cm³) that is spatially separated from the P-type impurity layer. This localization prevents unwanted diffusion into the P-type region while maintaining the electrical functionality of the floating diffusive node, preserving both manufacturability and depletion region integrity
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 design achieves high sensitivity at low light intensities and a wide dynamic range by maintaining the depletion region and increasing the electronic potential of the floating diffusion node, thereby improving the overall performance of the image sensor.
Implementation Method 1
A pixel of the image sensor detects the light generated from the object and converts the detected light to an electric value
Implementation Method 2
A strong electric field can be generated in the depletion region. Thus, the pairs of electrons and holes generated by photons incident on the depletion region are not recombined but separated from one another
Data Source
AI summary
Provided is an optical image receiving device having a high and rapid sensitivity and a wide dynamic range manufacture in a CMOS process. The image receiving device includes a capacitor transistor for a special purpose in addition to a general structure of three transistors and a light receiving portion. The capacitor transistor has first and second source/drain ports connected to the capacitance node and the floating diffusion node, respectively, and is gated in response to activation of a predetermined capacitor control signal. In the CMOS optical image receiving device, the floating diffusion node is pumped over an external power voltage. Thus, the electronic potential of the floating diffusion node in the initialization state is much higher than the maximum voltage of the light receiving portion. Thus, the CMOS active pixel has a very high sensitivity in a region where the intensity of light is weak. Furthermore, since the sensitivity decreases in a region where the intensity of light is strong, the dynamic range thereof can be increased very large.