7T SRAM Bitcell Transmission Gate Read Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional SRAM bit cells face decreasing reliability and performance due to reduced read static noise margin as memory devices shrink, and existing solutions like 8T SRAMs increase area and power consumption, while write-back schemes raise power consumption further.
Innovation Solution
A 7T SRAM bit cell design with a transmission gate that selectively couples storage nodes during operations, reducing read disturbance and half-select issues by using a single bit line and controlling inverter cross-coupling, thereby improving noise margins and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional 6T SRAM bit cells are used, then the structure is simple and area is small, but read static noise margin decreases and read disturbance increases as memory devices shrink
Solution Approach 1:
The bit cell is segmented into two inverters with asymmetric cross-coupling: one inverter has its output directly coupled to the other inverter's input, while the second inverter uses a transmission gate to selectively couple its output to the first inverter's input. This segmentation allows independent control of read and write paths, improving read stability without significantly increasing complexity.
Solution Approach 2:
The transmission gate introduces dynamic control to the cross-coupling mechanism. During read operations, the transmission gate is configured to isolate the storage node from the bit line to prevent read disturbance. During write operations, it enables proper signal coupling. This dynamic behavior adapts the circuit's connectivity based on operational mode, enhancing reliability without permanent structural complexity.
2Reliability
If 8T SRAM bit cells are used to improve read stability, then read static noise margin improves, but area and power consumption increase
Solution Approach 1:
The transmission gate serves multiple functions: it acts as a selective coupler during read operations to prevent read disturbance, enables write signal propagation during write operations, and provides control for half-select immunity. This multi-functionality replaces the need for separate dedicated transistors for each function, achieving 8T-like reliability with only 7 transistors total, reducing area compared to conventional 8T designs.
Solution Approach 2:
The transmission gate combines the functions of read protection and write enablement into a single controllable element. By merging these functions that would otherwise require separate transistors, the design achieves improved read stability without proportionally increasing the transistor count and area, as demonstrated by the 7T configuration versus 8T alternatives.
3Reliability
If write-back schemes are used to improve write reliability, then write accuracy improves, but power consumption increases
Solution Approach 1:
The transmission gate is pre-configured based on the operational mode (read or write) before the actual operation begins. During write operations, it is set to enable proper signal coupling in advance, ensuring write accuracy without requiring post-write verification or correction cycles. This preliminary configuration eliminates the need for power-intensive write-back schemes while maintaining write reliability.
Solution Approach 2:
The asymmetric cross-coupled inverter structure with transmission gate control provides self-correcting behavior during write operations. The circuit inherently maintains proper signal levels and state stability through its asymmetric feedback paths, eliminating the need for external write-back correction mechanisms. This self-service capability ensures write accuracy without additional power consumption from correction cycles.
4Area of stationary object
If memory devices are shrunk to increase density, then area efficiency improves, but read static noise margin and reliability decrease
Solution Approach 1:
The bit cell employs local quality differentiation through asymmetric cross-coupling: one inverter path has direct coupling while the other has transmission gate control. This local differentiation optimizes the read path for stability by isolating the storage node during read operations, maintaining strong noise margins even in scaled devices. The asymmetric structure allows tailored optimization of specific circuit regions without increasing overall area.
Solution Approach 2:
The transmission gate provides dynamic adaptability that allows the bit cell to maintain optimal electrical characteristics across different operating conditions and scales. During read operations, it dynamically isolates the storage node to prevent disturbance and maintain noise margins. This dynamic control compensates for the reduced noise margins inherent in scaled devices, maintaining reliability without increasing area.
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
Systems and methods relate to a seven transistor static random- access memory (7T SRAM) bit cell (fig. 5, 5009 which includes a first inverter having a first pull-up transistor (512), a first pull¬ down transistor (514), and a first storage node (510), and a second inverter having a second pull-up transistor (522), a second pull¬ down transistor (524), and a second storage node (520). The second storage node is coupled to gates of the first pull-up transistor and the first pull-down transistor. A transmission gate (540) is configured to selectively couple the first storage node to gates of the second pull-up transistor and the second pull-down transistor during a write operation, a standby mode, and a hold mode, and selectively decouple the first storage node from gates of the first pull-up transistor and a first pull-down transistor during a read operation. The 7T SRAM bit cell can be read or written through an access transistor (552) coupled to the first storage node (510).