SRAM Cell Layout With GAA Transistors for Read Stability
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Solution Overview
Problem
Existing SRAM technologies face challenges in achieving high-speed, low-power operation and stability as device densities shrink with scaling IC technology nodes, leading to tradeoffs between SRAM speed and power consumption, and limited design flexibility.
Innovation Solution
The implementation of gate-all-around (GAA) transistors with optimized layouts, where pull-down transistors and pass-gate transistors have a channel width ratio and adjusted gate lengths to enhance the β ratio, improving SRAM performance by increasing the on-current of pull-down transistors relative to pass-gate transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If device density is increased through scaling, then functional density increases, but manufacturing complexity increases
Solution Approach 1:
The patent implements gate-all-around (GAA) transistors that extend vertically into the third dimension, allowing multiple gates to surround the channel from all directions. This vertical stacking approach increases functional density without proportionally increasing lateral manufacturing complexity, as the gates are formed through sequential deposition and etching processes rather than requiring proportionally more lithography steps.
2Area of moving object
If SRAM cell size is reduced, then chip footprint decreases, but read stability deteriorates
Solution Approach 1:
The GAA transistor structure utilizes vertical channel stacking to maintain adequate channel volume and control resistance despite reduced lateral cell dimensions. The multi-layer gate structure provides enhanced electrostatic control over the channel, compensating for the reduced planar area and maintaining read stability in scaled-down SRAM cells.
Solution Approach 2:
The patent optimizes the β ratio by adjusting the channel width ratio between pull-down and pass-gate transistors, and by tuning gate lengths. These parameter changes enhance the on-current of pull-down transistors relative to pass-gate transistors, improving read stability even as cell area is reduced through scaling.
3Quantity of substance
If transistor scaling continues, then functional density increases, but design flexibility decreases
Solution Approach 1:
The GAA transistor structure segments the gate into multiple independent gates that can be controlled separately. This segmentation allows for differentiated control of pull-down and pass-gate transistors within the same SRAM cell, enabling optimized β ratios and improved read stability while maintaining scalability. The segmented gate structure provides additional design knobs for tuning performance without increasing lateral footprint.
Data Source
AI summary
A memory semiconductor includes a first memory cell including a first pull-up (PU) transistor and a first pull-down (PD) transistor sharing a first gate structure, a second memory cell including a second PU transistor and a second PD transistor sharing a second gate structure, a first active area shared by the first PD transistor and the second PD transistor, and a second active area shared by the first PU transistor and the second PU transistor. The first gate structure has a first portion for the first PU transistor and a second portion for the first PD transistor. The second gate structure has a third portion for the second PU transistor and a fourth portion for the second PD transistor. A distance between the second portion and the fourth portion is greater than a distance between the first portion and the third portion.