Mesa Photodiode Array Doping for MTF Improvement
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Solution Overview
Problem
Mesa photodiodes based on CdxHg1-xTe material face challenges in achieving high modulation transfer function (MTF) due to limitations in faithfully transcribing incident photon distributions, leading to suboptimal image fidelity and increased risk of minority carriers being recombined in neighboring photodiodes.
Innovation Solution
The introduction of a second doped region with a higher doping density between photodiodes, forming a potential barrier to prevent minority carriers from diffusing to neighboring photodiodes, combined with a cadmium concentration gradient to enhance the potential barrier and guide carriers to the PN junction, improves the MTF by ensuring incident photons generate currents only in the intended photodiode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photodiodes are arranged in an array with small pitch to increase resolution, then the array can detect finer details, but minority carriers diffuse to neighboring photodiodes causing recombination and reducing image fidelity
Solution Approach 1:
The patent applies local quality by creating a doped region specifically in the zone between photodiodes with different doping characteristics than the bulk material. This localized modification creates potential barriers only where needed (at the interfaces between photodiodes) without affecting the overall photodiode structure or requiring changes to all regions of the material.
Solution Approach 2:
The patent changes the doping parameter (carrier concentration) in the intermediate zones between photodiodes. By modifying the doping level and type in these specific regions, potential barriers are created that prevent minority carrier diffusion to neighboring photodiodes, thereby improving image fidelity without requiring larger pitch between photodiodes.
2Quantity of substance
If the useful layer thickness is increased to improve photon absorption, then more photons are detected, but the diffusion length of minority carriers increases allowing them to reach neighboring photodiodes
Solution Approach 1:
The patent introduces localized doped regions with altered electrical properties specifically in the zones between photodiodes. These regions create potential barriers that act as electrical boundaries, preventing carrier diffusion horizontally between photodiodes while allowing the useful layer to maintain sufficient thickness for effective photon absorption.
Solution Approach 2:
The doped regions between photodiodes serve as intermediary structures that mediate between the need for thick useful layers (for photon absorption) and the need to prevent carrier diffusion. These intermediary zones with modified doping characteristics create potential barriers that block carrier diffusion paths without interfering with vertical photon absorption in the useful layer.
3Ease of manufacture
If standard doping is used throughout the useful layer to simplify manufacturing, then the manufacturing process is easier, but neighboring photodiodes interfere with each other reducing MTF
Solution Approach 1:
The patent implements local quality by applying different doping characteristics to specific zones within the useful layer. The zones between photodiodes receive modified doping (different concentration or type) compared to the bulk material, creating localized potential barriers that prevent carrier diffusion while maintaining overall manufacturing simplicity through a systematic doping pattern.
Solution Approach 2:
The patent segments the useful layer into distinct functional zones: photodiode regions with standard doping for photon detection, and intermediate zones with modified doping for carrier isolation. This segmentation allows each zone to perform its specific function optimally while maintaining a relatively simple overall manufacturing process that follows a repeating pattern.
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 enhances the modulation transfer function of the photodiode array, preventing minority carrier recombination in neighboring photodiodes and improving image fidelity, particularly beneficial for arrays with a pitch lower than the minority carrier diffusion length.
Implementation Method 1
A PN junction forms a photodiode. On the side of said upper wall, the pad has a region having an N or P doping, of a different type from that of the doping in the rest of the useful layer, thus forming a PN junction.
Implementation Method 2
The first doped zone comprises: at least one first region having a first doping density, located at least under each of said pads; and at least one second region, located between two neighboring pads, and having a second doping density higher than the first doping density.
Data Source
AI summary
An array of mesa photodiodes, including a useful layer of CdxHg1-xTe wherein pads are formed. The array includes a first doped zone having a first N or P doping; and second doped zones having a second P or N doping of a different type from that of the first doping, and each extending on an upper region of a pad. The first doped zone includes at least one first region having a first doping density, located at least under each of the pads; and at least one second region, located between two neighboring pads, and having a second doping density higher than the first doping density, each second region being separated from the closest second doped zone by at least one portion of the first region.