CCD Charge Multiplication Register Buried Channel Implant
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Solution Overview
Problem
The gain performance of CCD devices deteriorates over time due to the accumulation of trapped charge at the semiconductor surface, which affects the charge-to-voltage conversion and video chain electronics, leading to reduced sensitivity.
Innovation Solution
An additional register with modified doping in the CCD imager is introduced, where the depleted charge density is increased beneath a high voltage electrode, preventing electrons from interacting with the semiconductor surface and reducing trapped charge buildup, thereby maintaining gain performance over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If high voltage is applied to multiplication electrodes to achieve charge multiplication through impact ionisation, then gain is improved, but trapped charge accumulates at the semiconductor surface over time, causing gain performance to deteriorate
Solution Approach 1:
A buried channel implant is formed in the semiconductor substrate prior to forming the multiplication electrodes. This preliminary doping action creates a region of modified electrical characteristics that prevents trapped charge accumulation before the device begins operation, thereby maintaining gain performance stability over time while preserving the high gain capability.
Solution Approach 2:
The buried channel implant acts as an intermediary layer between the semiconductor surface and the multiplication electrodes. This intermediate doped region mediates the interaction between high voltage fields and the semiconductor surface, preventing direct charge trapping at the surface while allowing impact ionisation to occur in the multiplication region.
2Device complexity
If conventional doping is used in the multiplication register, then device complexity is reduced, but electron trajectories allow electrons to encounter the semiconductor surface, causing trapped charge buildup
Solution Approach 1:
Instead of uniformly doping the entire multiplication register region, a localized buried channel implant is formed only in specific areas where electrons are most likely to encounter the surface. This selective local doping modifies the electrical characteristics only where needed, maintaining low device complexity while effectively preventing trapped charge buildup at critical locations.
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 approach effectively prevents the degradation of gain performance and increases the charge capacity of the multiplication register, allowing for higher gains while maintaining performance stability over usage time.
Implementation Method 1
at least one of the electrodes is a high voltage electrode which has a voltage applied high enough in comparison to a preceding DC electrode so that the fields experienced by electrons cause impact ionisation
Implementation Method 2
In the region beneath this electrode doping is provided in a semiconductor layer so that the region has a higher depleted charge density than under the previous electrode
Data Source
AI summary
A CCD device includes multiplication elements arranged so as to multiply charge by clocking with a high voltage. An additional region beneath the high voltage electrode is so doped in relation to at least the preceding electrode as to have a higher depleted charge density than under the preceding electrode. This assists in preventing high energy electrodes from encountering the silicon surface of the semiconductor.