Semiconductor Storage Device Reference Cell Read Voltage Control
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Solution Overview
Problem
Existing semiconductor storage devices face reading errors due to threshold voltage fluctuations, which cannot be satisfactorily prevented without expanding the threshold voltage window, leading to issues like leakage current and deteriorated data retention characteristics.
Innovation Solution
A semiconductor storage device with a reference cell system that determines read voltages based on data stored in adjacent memory cells, using a control circuit to select appropriate read voltages and prevent external reading or writing from the reference cell, thereby correcting read voltages to mitigate threshold voltage fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the threshold voltage window is expanded to prevent reading errors due to Vth fluctuation, then reading reliability is improved, but leakage current increases and data retention characteristics deteriorate
Solution Approach 1:
The patent applies preliminary action by writing data to a reference cell before reading from the target memory cell. This reference cell is used to determine the appropriate read voltage in advance, allowing the system to compensate for Vth fluctuations without expanding the threshold voltage window. The reference cell's state is used to select between multiple read voltages, preventing reading errors while maintaining proper retention characteristics.
Solution Approach 2:
The patent changes the parameter of read voltage based on the state of the reference cell. By determining the reference cell's state (erased or programmed) and using this information to select from multiple predetermined read voltages, the system dynamically adjusts the read voltage parameter to match the actual threshold voltage conditions, thereby preventing reading errors without requiring a wider Vth window.
2Measurement precision
If multiple read voltages are used to compensate for Vth fluctuation, then reading accuracy is improved, but device complexity increases
Solution Approach 1:
The patent uses a reference cell that copies the same structure and characteristics as the target memory cell. This reference cell is subjected to the same Vth fluctuation conditions, allowing it to serve as an accurate indicator for selecting the appropriate read voltage. The reference cell's state directly reflects the conditions in the target cell, enabling precise voltage selection without complex analysis circuits.
Solution Approach 2:
The patent implements feedback by using the state of the reference cell to determine which read voltage to apply to the target memory cell. The reference cell's state (read through the sense amplifier) provides feedback information about the actual threshold voltage conditions, which is then used to select the appropriate read voltage from multiple predetermined options, ensuring accurate reading.
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 reading errors caused by threshold voltage fluctuations without expanding the threshold voltage window, improving data retention and reliability by adjusting read voltages dynamically based on the state of adjacent memory cells.
Implementation Method 1
A semiconductor storage device capable of storing information by injecting electrons into a floating gate (FG) or extracting the electrons has been developed such as a flash memory
Implementation Method 2
Memory cell has its threshold voltage increased when electrons are injected into the floating gate and has its threshold voltage decreased when the electrons are extracted from the floating gate
Implementation Method 3
FG-FG coupling is a phenomenon that when a potential of an FG is varied by injection or extraction of electrons into/from the FG, a potential of an adjacent FG is varied as well due to parasitic-capacitance between the FGs to fluctuate a threshold voltage of a memory cell
Data Source
AI summary
A semiconductor storage device which includes a memory array including a plurality of memory cells for storing data by using a difference in a threshold voltage and at least one reference cell for storing data indicative of a state of a corresponding memory cell by using a difference in a threshold voltage, a control circuit for determining a read voltage based on data stored by a reference cell corresponding to a memory cell adjacent to a memory cell to be read, a read unit for executing reading from a memory cell to be read by using a determined read voltage, and a write unit for executing writing, when executing writing to a memory cell to be written to bring the memory cell into a written state, data indicating that the memory cell is in the written state to a reference cell corresponding to the memory cell.