High Voltage Tolerant Word-Line Driver Using Thin Gate Oxide Transistors
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Solution Overview
Problem
Designing memory peripheral circuits, such as word-line drivers, to withstand high voltages required for certain memory technologies like STT-MRAM is challenging due to the need for thick gate oxide transistors, which increase process complexity and memory size.
Innovation Solution
A high-voltage tolerant word-line driver is developed using only thin gate oxide transistors, decoupling read and write operations, and incorporating a fast bootstrap circuit, stack-transistor with level-shifted gate control, and a stack-transistor to ground, which reduces the need for high voltage level-shifters and minimizes transistor size, allowing operation with three different power supplies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thick gate oxide transistors are used to withstand high voltage, then high voltage tolerance is improved, but process complexity increases
Solution Approach 1:
The word-line driver is divided into multiple stages (first stage, second stage, third stage) with different transistor types. The first stage uses thick gate oxide transistors for high voltage switching, while the second and third stages use thin gate oxide transistors for control functions, thereby segmenting the voltage stress across different transistor types and reducing overall process complexity
Solution Approach 2:
Thin gate oxide transistors are used as intermediary control elements that manage the switching of thick gate oxide transistors. The control logic stage uses thin gate oxide transistors to generate control signals that drive the high voltage switches, acting as an intermediary between the control circuitry and the high voltage power supply
2Reliability
If thick gate oxide transistors are used to withstand high voltage, then high voltage tolerance is improved, but memory size increases
Solution Approach 1:
The driver circuit is segmented into multiple functional stages, allowing thin gate oxide transistors to be used in control stages where high voltage tolerance is not required. This reduces the area occupied by control transistors while maintaining high voltage capability in the power switching stage
Solution Approach 2:
Different regions of the circuit are assigned different transistor types based on local requirements. The high voltage switching nodes use thick gate oxide transistors for durability, while the control logic and signal processing regions use thin gate oxide transistors for compactness, optimizing area usage throughout the circuit
3Reliability
If high voltage level-shifters are used to drive word-line, then high voltage operation is achieved, but device area increases
Solution Approach 1:
The level-shifting function and high voltage switching are merged into a single integrated stage. The first stage simultaneously performs level shifting from low voltage control signals to high voltage word-line driving levels, eliminating the need for separate level-shifter circuits and reducing overall device area
Solution Approach 2:
The thick gate oxide transistors in the first stage serve multiple functions: they act as high voltage switches, level-shifters, and power drivers all at once. This multi-functionality reduces the number of dedicated circuits needed, thereby minimizing the total area occupied by the word-line driver
Data Source
AI summary
Described is a word-line driver which is operable to switch a voltage level of a word-line to one of: first power supply, second power supply, or third power supply wherein the voltage level of the second power supply is higher than the voltage level of the first power supply, and wherein transistors of the word-line driver have same gate oxide thicknesses.