DRAM Precharge Circuitry for Vertical Access Device Charge Dissipation
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Solution Overview
Problem
In DRAM memory systems, trapped charge in vertical access devices can lead to unintended data state changes due to accumulated positive charge, causing '1' to be read as '0', as the p-n junction becomes forward biased and charge is pumped into the capacitor.
Innovation Solution
Implementing a precharge operation where digit lines are grounded for an adjustable period to discharge accumulated charge from the bodies of vertical access devices, preventing charge accumulation and ensuring accurate data retention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If vertical access devices are used in DRAM memory cells, then device integration and memory density are improved, but trapped charge accumulation occurs causing unintended data state changes
Solution Approach 1:
The patent applies preliminary action by performing a precharge operation that grounds the digit line before the main read operation. This preliminary grounding discharges trapped positive holes from the access device body, preventing them from being pumped into the capacitor and causing false data state changes. The precharge operation prepares the device in advance to avoid reliability issues during subsequent operations.
2Ease of operation
If digit lines are precharged to reference voltage, then sense amplifier operation is improved, but trapped charge discharge is insufficient causing data errors
Solution Approach 1:
The patent segments the precharge operation into two distinct phases: first grounding the digit line to discharge trapped charge from the access device body, then charging to the reference voltage for sense amplifier operation. This segmentation allows both objectives to be achieved separately - complete charge discharge during grounding, and proper sense amplifier preparation during the reference voltage phase.
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 method effectively minimizes charge accumulation in vertical access devices, maintaining the integrity of stored data states by ensuring the p-n junction is reversed biased, thus preventing unintended data state changes during memory operations.
Implementation Method 1
trapped charge in vertical access devices can lead to unintended data state changes due to accumulated positive holes
Implementation Method 2
causing a stored '0' to be read as '1', as the p-n junction becomes forward-biased and charge is pumped into the capacitor
Implementation Method 3
ensuring the p-n junction is reversed-biased, thus avoiding unintended data changes during memory operations
Data Source
AI summary
Examples described include precharge operations and circuitry for performing precharge operations. Digit lines may be driven to ground during a portion of example precharge operations. By driving the digit lines to ground, charge accumulating in bodies of vertical access devices may be discharged to the digit lines in some examples. To drive the digit lines to ground, a dynamic reference may be used where the reference is ground during one portion of the precharge operation and another value, which may be between two supply voltages (e.g. VCC/2), during another portion of the precharge operation.