Avalanche Photodiode Pixel Circuit for Charge Discharge Reliability
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Solution Overview
Problem
Existing photodetectors face reliability issues due to charge accumulation leading to dielectric breakdowns, particularly in fine avalanche photodiode arrays, where the extension of the reset period increases current flow and shortening it prevents charge discharge, compromising circuit reliability.
Innovation Solution
A photodetector design incorporating an avalanche photodiode with a first semiconductor layer of one conductivity type and a second semiconductor layer of opposite conductivity type, along with a first transistor of opposite channel polarity for reset and a second transistor in a half-ON state during exposure, to manage charge discharge and prevent breakdowns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the reset period is extended to allow charge discharge, then reliability is improved, but current flow increases causing energy loss and heat generation
Solution Approach 1:
The patent divides the single reset function into two separate transistors: a first transistor (reset transistor) dedicated to resetting the photodiode voltage, and a second transistor (transfer transistor) for transferring charge to the floating diffusion. This segmentation allows independent optimization of each transistor's operation, enabling the reset period to be extended for reliability without proportionally increasing overall current consumption, as each transistor operates in a specialized manner.
Solution Approach 2:
The patent implements dynamic control of transistor gate voltages during different operation phases. The first transistor is activated during the reset period to discharge accumulated charge, while the second transistor is activated during the transfer phase. This dynamic switching optimizes current flow timing, allowing extended reset periods for reliability while minimizing energy loss by ensuring current flows only when necessary for charge discharge and transfer.
2Loss of energy
If the reset period is shortened to reduce current flow, then energy loss is reduced, but charge cannot be discharged leading to dielectric breakdown
Solution Approach 1:
By segmenting the charge discharge and transfer functions into separate transistors, the patent enables the reset period to be optimized independently from the transfer period. The first transistor handles charge discharge during a sufficiently long reset period to prevent dielectric breakdown, while the second transistor handles charge transfer in a separate phase, allowing energy-efficient operation without compromising reliability.
Solution Approach 2:
The first transistor performs preliminary charge discharge action during the reset period before the second transistor transfers the charge. This preliminary action ensures that excessive charge is removed to prevent dielectric breakdown, while the separate transfer action by the second transistor can be timed efficiently to minimize overall current flow and energy loss.
3Device complexity
If a single transistor is used for both reset and transfer, then device complexity is reduced, but reliability deteriorates due to charge accumulation
Solution Approach 1:
The patent applies segmentation by using two separate transistors instead of one, with the first transistor dedicated to resetting the photodiode and the second transistor dedicated to transferring charge to the floating diffusion. This functional segmentation prevents charge accumulation issues that would occur with a single transistor, as each transistor operates in its optimized role, thereby improving reliability despite increased device complexity.
Solution Approach 2:
While the patent uses two transistors, each transistor is designed with multi-functionality in mind. The first transistor primarily resets but can also influence charge distribution, while the second transistor transfers charge but also affects the overall circuit state. This multi-functionality approach allows the system to achieve high reliability through specialized roles while maintaining reasonable complexity by avoiding unnecessary additional components.
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 reliability of photodetectors by effectively discharging excessive charge via the second transistor, reducing the likelihood of dielectric breakdowns and allowing for a longer exposure period without compromising circuit integrity.
Implementation Method 1
at least one avalanche photodiode including a first semiconductor layer of a first conductivity type and a second semiconductor layer of a second conductivity type
Implementation Method 2
detects a photon
Data Source
AI summary
A photodetector includes: at least one avalanche photodiode including a first semiconductor layer of a first conductivity type and a second semiconductor layer of a second conductivity type; a first transistor connected to the first semiconductor layer and including a channel of the second conductivity type that has polarity opposite to polarity of the first conductivity type; and a second transistor connected to the first semiconductor layer and including a channel of the first conductivity type.