Separated Unit Pixel Minimizing Charge Transfer Path
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Solution Overview
Problem
The separation type unit pixel experiences sensitivity reduction due to a lengthened charge transfer path from the photodiode to the floating diffusion area, which affects image sensor performance.
Innovation Solution
A separation type unit pixel design that includes a substrate with a photodiode junction of P-type and N-type diffusion areas, a gate electrode conductor, a floating diffusion area, and a sensitivity reduction preventing conductor. The sensitivity reduction preventing conductor is applied with a signal that is enabled when the transfer control signal is disabled, after a predetermined delay, to minimize the charge transfer path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the unit pixel is divided into two parts formed in different wafers to increase the photodiode area, then the photodiode area is increased, but the charge transfer path becomes lengthened causing sensitivity reduction
Solution Approach 1:
The patent applies reverse bias voltage to the P-type epitaxial substrate to dynamically control the depletion layer width. By changing the voltage parameter, the depletion layer extends to fill the charge transfer path, creating an electric field that actively transports charges along the lengthened path caused by the separated wafer structure, thereby maintaining sensitivity despite the increased photodiode area.
Solution Approach 2:
The patent introduces a depletion layer as an intermediary mechanism between the photodiode and floating diffusion area. This depletion layer, created by reverse biasing the P-type substrate, acts as an electric field mediator that enables efficient charge transport across the lengthened physical path, compensating for the sensitivity loss caused by the separated wafer structure.
2Length of stationary object
If DC bias is applied to the P-type epitaxial substrate to increase depletion area width, then the depletion area width increases, but the charge transfer path becomes lengthened
Solution Approach 1:
The patent uses reverse bias voltage as a controllable parameter to extend the depletion layer width into the charge transfer path region. By adjusting the voltage level, the depletion layer dynamically expands to occupy the space between the photodiode and floating diffusion area, transforming the harmful lengthened path into a useful electric field region for charge transport.
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 design minimizes the charge transfer path, thereby reducing sensitivity reduction and enhancing image sensor performance by optimizing the voltage profile and electric field distribution within the photodiode.
Implementation Method 1
The photodiode is constructed with junctions of N-types 25 and 28 and P-types 28 and 29, respectively to generate charges corresponding to incident light energy
Implementation Method 2
the electron-hole pairs may be moved to the P area 26 or the N area 25 by diffusion and drift
Implementation Method 3
the electron-hole pairs may be moved to the P area 26 or the N area 25 by diffusion and drift
Implementation Method 4
an electric field including a predetermined electric field due to fixed ionized charges and an electric field due to the DC bias exists
Implementation Method 5
the electron-hole pairs may be moved to the P area 26 or the N area 25 by diffusion and drift
Data Source
AI summary
Provided are a separation type unit pixel for preventing sensitivity reduction to prevent a depletion area from decreasing and a method of driving the unit pixel. The separation type unit pixel for preventing sensitivity reduction includes: a substrate; a photodiode constructed with a junction of a P-type diffusion area and an N-type diffusion area which are formed under a surface of the substrate in a vertical direction; a gate electrode conductor which is disposed on an upper portion of the surface of the substrate to be adjacent to the N-type or P-type diffusion area; a floating diffusion area formed to be adjacent to another surface of the gate electrode conductor; and a sensitivity reduction preventing conductor disposed on an upper portion of the photodiode area to cover the photodiode area.