Polysilicon Shield Biasing for NAND Flash Body Effect Control
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Solution Overview
Problem
The increasing complexity of NAND flash memory design due to shrinking process geometries leads to challenges such as reduced transistor spacing and increased threshold voltage due to the body effect, which requires higher chip voltages and larger charge pump areas, making it difficult to efficiently transfer high programming voltages to word lines during memory operations.
Innovation Solution
The use of polysilicon shields biased at specific voltages next to transistors to counteract the body effect and prevent field punch-through, reducing the increase in threshold voltage and increasing junction breakdown voltage, thereby allowing for reduced maximum chip voltage, smaller transistor junction spacing, and smaller die size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If process geometries are shrunk to reduce cost per bit, then storage density is improved, but transistor spacing is reduced and threshold voltage increases due to body effect
Solution Approach 1:
A polysilicon shield structure is introduced as an intermediary element between the transistor gate and the substrate. This shield acts as a mediator that blocks the body effect from directly affecting the transistor threshold voltage, allowing the transistor to maintain stable operation even as process geometries are shrunk and transistor spacing is reduced.
Solution Approach 2:
The invention changes the electrical parameters of the polysilicon shield by biasing it to a first voltage that is less than a second voltage applied to the transistor gate. This parameter change creates a voltage differential that counteracts the body effect, enabling the transistor to maintain a stable threshold voltage despite reduced spacing and scaled dimensions.
2Stability of the object's composition
If threshold voltage increases due to body effect, then transistor stability is improved, but maximum chip voltage must be increased and charge pump area increases
Solution Approach 1:
The polysilicon shield serves as a protective intermediary that isolates the transistor from the substrate's body effect. By introducing this intermediate structure with appropriate voltage biasing, the system maintains transistor threshold voltage stability without requiring increased chip voltage or enlarged charge pump circuits.
Solution Approach 2:
The invention applies a voltage bias parameter change to the polysilicon shield, setting it to a first voltage less than the second voltage applied to the transistor gate. This parameter adjustment creates a protective voltage barrier that prevents body effect from increasing the threshold voltage, thereby eliminating the need for higher chip voltages or larger charge pump areas.
3Manufacturing precision
If polysilicon shield is biased to reduce body effect, then threshold voltage increase is reduced, but junction breakdown voltage must be maintained
Solution Approach 1:
The invention carefully controls the voltage parameter of the polysilicon shield, biasing it to a first voltage that is less than the second voltage applied to the transistor gate. This specific parameter relationship ensures that the shield effectively reduces body effect and controls threshold voltage while maintaining adequate junction breakdown voltage for reliable operation.
Solution Approach 2:
The polysilicon shield is strategically positioned and biased to provide localized protection against body effect at the transistor junction. By applying the shield voltage specifically at this critical location with the appropriate voltage differential, the system achieves precise threshold voltage control without compromising the overall junction breakdown voltage reliability.
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 reduces the threshold voltage increase, minimizes the need for high chip voltages and large charge pump areas, and allows for more compact memory array designs by controlling the polysilicon shield voltages based on the type of memory operation, enhancing the efficiency of memory operations and reducing the die size.
Implementation Method 1
reduces an increase in a threshold voltage of the transistor due to a body effect
Implementation Method 2
prevent field punch-through between the source junction and the neighboring junction
Data Source
AI summary
Methods for reducing an increase in the threshold voltage of a transistor due to the body effect and increasing the junction breakdown voltage for junctions of the transistor are described. The transistor may comprise an NMOS transistor that transfers a programming voltage (e.g., 24V) to a word line of a memory array during a programming operation. In some cases, a first poly shield may be positioned within a first distance of a gate of the transistor and may comprise a first polysilicon structure that is directly adjacent to the gate of the transistor. The first poly shield may be arranged in a first direction (e.g., in the channel length direction of the transistor). The first poly shield may be biased to a first voltage greater than ground (e.g., 10V) during the programming operation to reduce an increase in the threshold voltage of the transistor due to the body effect.