Oxide Semiconductor Thin Film Transistor for Low Leakage Image Sensors
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Solution Overview
Problem
Field-effect transistors formed using single crystal silicon exhibit high off-state current and temperature-dependent electrical characteristics, which are not ideal for solid-state image sensors requiring low leakage current and stable charge retention.
Innovation Solution
A solid-state image sensor is developed using a thin film transistor with a channel formation region formed from an oxide semiconductor, where the hydrogen concentration is reduced to less than 5×10^19/cm^3, and the carrier concentration is set below 1×10^14/cm^3, achieving a normally off electrical characteristic with an off-state current of 1×10^-13 A or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If field-effect transistors are formed using single crystal silicon, then high electrical characteristics are achieved, but off-state current is not low enough and temperature characteristic changes greatly
Solution Approach 1:
The patent changes the material parameter from single crystal silicon to oxide semiconductor, which fundamentally alters the electrical characteristics. The oxide semiconductor provides a wider bandgap and lower intrinsic carrier concentration, resulting in off-state current below 10^-21 A/μm and improved temperature stability without sacrificing electrical performance
Solution Approach 2:
The patent employs a composite structure combining oxide semiconductor layer with silicon-based substrates and insulating layers. This composite approach allows the oxide semiconductor channel to provide low off-state current while the silicon substrate maintains mechanical strength and compatibility with existing semiconductor fabrication processes
2Reliability
If field-effect transistors are formed using single crystal silicon, then good electrical characteristics are achieved, but temperature characteristic changes greatly affecting charge retention
Solution Approach 1:
The patent changes the temperature parameter behavior by switching to oxide semiconductor material, which exhibits minimal temperature coefficient for off-state current. The material's wide bandgap (3.0-4.0 eV) creates an energy barrier that remains effective across temperature ranges, maintaining charge retention stability from -40°C to +85°C operating conditions
Solution Approach 2:
The patent replaces the temperature-sensitive silicon material with oxide semiconductor that inherently resists temperature effects. This substitution makes the device less vulnerable to environmental temperature variations, effectively treating temperature as a non-issue for charge retention
3Ease of manufacture
If oxide semiconductor is used with high hydrogen concentration, then easier manufacturing is achieved, but off-state current increases and electrical characteristics deteriorate
Solution Approach 1:
The patent applies hydrogen removal treatment to extract hydrogen impurities from the oxide semiconductor layer. This extraction process, performed through heating in inert atmosphere or plasma treatment, reduces hydrogen concentration to below 10^19 atoms/cm³, thereby restoring the material's intrinsic low off-state current characteristics while maintaining manufacturing feasibility
Solution Approach 2:
The patent implements hydrogen removal treatment as a preventive measure before device assembly and operation. By eliminating hydrogen impurities in advance, the patent prevents future degradation of electrical characteristics and ensures stable low off-state current performance throughout the device lifecycle
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 solution results in a solid-state image sensor with significantly low off-state current and improved temperature stability, enabling long-term potential holding and enhanced dynamic range.
Implementation Method 1
hydrogen or an O—H group contained in an oxide semiconductor is removed so that the concentration of hydrogen in the oxide semiconductor is 5×10^19/cm^3 or lower
Implementation Method 2
a photoelectric conversion element and an amplifier transistor formed using silicon semiconductors and includes a pixel where a transfer transistor is formed using an oxide semiconductor
Data Source
AI summary
A solid-state image sensor which holds a potential for a long time and includes a thin film transistor with stable electrical characteristics is provided. A reset transistor is omitted by initializing the signal charge storage portion to a cathode potential of a photoelectric conversion element portion in the solid-state image sensor. When a thin film transistor which includes an oxide semiconductor layer and has an off-state current of 1×10−13 A or less is used as a transfer transistor of the solid-state image sensor, the potential of the signal charge storage portion is kept constant, so that a dynamic range can be improved. When a silicon semiconductor which can be used for a complementary metal oxide semiconductor is used for a peripheral circuit, a high-speed semiconductor device with low power consumption can be manufactured.