MOSFET Schmitt Trigger Hysteresis Using Gate Dopant Thresholds
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Solution Overview
Problem
Conventional Schmitt trigger circuits in advanced image and radiation sensors exhibit unreliable hysteresis characteristics due to fabrication process variations, leading to inconsistent pixel performance and high power consumption, which is unacceptable for large arrays of pixels requiring accurate and uniform hysteresis.
Innovation Solution
The implementation of Schmitt trigger circuits with two MOSFETs having identical source/drain/channel configurations but different polysilicon gate dopants, allowing for accurate and uniform hysteresis without the need for comparator or differential amplifier circuits and DC bias voltage, achieved by using standard and non-standard n-channel transistors with distinct threshold voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional Schmitt trigger circuits are used, then the circuit can be implemented with standard components, but the hysteresis characteristics vary due to fabrication process variations leading to unreliable performance
Solution Approach 1:
The patent changes the threshold voltage parameter of MOSFETs by modifying the polysilicon gate dopant concentration. Specifically, it uses a first MOSFET with a first dopant concentration and a second MOSFET with a second dopant concentration, creating fixed threshold voltage differences that are insensitive to fabrication variations. This parameter change approach directly addresses the reliability issue by making hysteresis characteristics dependent on dopant ratios rather than absolute voltage values.
Solution Approach 2:
The patent applies local quality by creating MOSFETs with different polysilicon gate dopant concentrations in specific locations within the circuit. The first MOSFET has a first dopant concentration while the second MOSFET has a second dopant concentration, allowing each transistor to have tailored electrical characteristics suited for its specific function in establishing hysteresis thresholds.
2Reliability
If comparator or differential amplifier circuits are used to enhance hysteresis accuracy, then accurate and uniform hysteresis characteristics are achieved, but the circuit area and power consumption increase significantly
Solution Approach 1:
The patent extracts the hysteresis function from complex comparator or differential amplifier circuits and implements it using only two MOSFETs with different polysilicon gate dopant concentrations. By removing the unnecessary complex circuitry and keeping only the essential dopant-difference mechanism, the patent achieves accurate hysteresis with minimal chip area occupation.
Solution Approach 2:
The MOSFETs with different dopant concentrations inherently provide the hysteresis characteristics through their own threshold voltage differences, without requiring external comparator or differential amplifier circuits. The dopant structure itself serves the dual purpose of transistor switching and hysteresis definition, eliminating the need for separate accuracy-enhancing components.
3Reliability
If comparator or differential amplifier circuits are used to achieve accurate hysteresis, then uniform hysteresis characteristics are obtained, but DC bias voltage is required causing continuous power consumption
Solution Approach 1:
The patent enables the Schmitt trigger to operate with periodic switching action driven by the input signal, rather than requiring continuous DC bias voltage. The MOSFETs switch between on and off states based on the input signal crossing the hysteresis thresholds, creating periodic operation that consumes power only during transitions rather than continuously.
Solution Approach 2:
The dopant-difference structure provides uniform hysteresis characteristics inherently through the fixed threshold voltage relationship between the two MOSFETs, without requiring external DC bias voltage for calibration or maintenance. The uniformity is built into the device structure itself, eliminating the need for continuous power consumption to maintain hysteresis accuracy.
4Productivity
If larger pixel arrays are implemented, then the sensor capability is improved, but the power consumption and area requirements for each pixel become unacceptable
Solution Approach 1:
The patent extracts only the essential hysteresis function from complex Schmitt trigger circuits, implementing it with minimal components (two MOSFETs with different dopants). This extraction eliminates unnecessary circuitry that would consume power and occupy area, allowing each pixel to use minimal resources while maintaining required functionality for large-scale arrays.
Solution Approach 2:
By changing the approach to hysteresis implementation from voltage-based to dopant-concentration-based, the patent creates a structure where hysteresis is determined by fixed physical properties rather than active circuit elements. This parameter change dramatically reduces the active power consumption per pixel, making large-scale arrays feasible.
Data Source
AI summary
A Schmitt trigger's hysteresis is established by standard and non-standard MOSFETs having different (lower/higher) threshold voltages. For example, a standard n-channel transistor having a relatively low threshold voltage (e.g., 1V) sets the lower trigger switching voltage, and a non-standard n-channel transistor (e.g., an n-channel source/drain and a polysilicon gate doped with a p-type dopant) exhibits a relatively high threshold voltage (e.g., 2V) that sets the higher trigger switching voltage. An output control circuit generates the Schmitt trigger's digital output signal based on the on/off states of the two (non-standard and standard) MOSFETs, whereby the changes digital output signal between two values when the analog input signal falls below the lower threshold voltage (i.e., when both MOSFETs are turned on/off) and rises above the higher threshold voltage (i.e., when both MOSFETs are turned off/on). Self-resetting and other circuits utilize the Schmitt trigger to facilitate, e.g., high dynamic range image sensor pixels.