Multi-Layer Semiconductor Body for Steep Switching in Flash Memory
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Flash memory devices suffer from wide threshold voltage distribution and low refresh margin due to low slope in sub-threshold voltage range, leading to increased read and program/erase times.
Innovation Solution
A cell string with a semiconductor body formed from multiple layers of materials with different energy band gaps, utilizing positive feedback for steep switching characteristics, and a reading method that applies specific voltages to control electrodes to enhance current flow and reduce turn-on voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single-layer semiconductor body is used, then the device structure is simple, but the threshold voltage distribution is wide and refresh margin is low
Solution Approach 1:
The semiconductor body is divided into multiple layers (first semiconductor layer, second semiconductor layer, third semiconductor layer) with different energy band gaps. This segmentation allows each layer to contribute differently to the overall device performance, enabling steep switching characteristics and reduced threshold voltage distribution while maintaining structural organization.
Solution Approach 2:
The patent employs composite semiconductor materials with different energy band gaps (e.g., Si/SiGe/Si structure) to form the multi-layer semiconductor body. This composite approach enables exploitation of material-specific properties to achieve both low off-state current and high on-state current, improving refresh margin without excessive complexity.
2Loss of time
If the slope in sub-threshold range is low, then the device operation is stable, but the threshold voltage distribution becomes wide and read time increases
Solution Approach 1:
The patent changes the energy band gap parameter across different semiconductor layers to achieve steep switching characteristics. By selecting materials with appropriate band gaps (e.g., Si with 1.1 eV, SiGe with smaller band gap), the device achieves high slope in sub-threshold range, reducing both read time and threshold voltage distribution width.
3Power
If adjacent semiconductor layers have the same energy band gap, then the material selection is flexible, but the turn-on current is insufficient
Solution Approach 1:
Different semiconductor layers are assigned different energy band gap characteristics tailored to their specific functions. The first layer (contacting gate insulating film) uses material with larger band gap for low off-state current, while the second layer uses material with smaller band gap for high turn-on current. This local optimization of material properties achieves both high power performance and controlled material selection.
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
The solution achieves a small threshold voltage distribution and improved refresh margin, resulting in shorter read and verify times, and potentially shorter program times, with a very high slope in the I-V characteristic curve.
Implementation Method 1
using positive feedback to obtain steep switching characteristic
Implementation Method 2
adjacent layers are made of materials having different energy band gaps to increase a turn-on current
Data Source
AI summary
Provided are a cell string and a reading method for the cell string. The cell string includes a semiconductor body formed on a surface of an insulating layer, first and second semiconductor regions formed at respective ends of the semiconductor body and are formed by being doped with different types of impurities, two or more control electrodes which are separated from each other to be electrically isolated, and a gate insulating film stack which is formed between the semiconductor body and the control electrodes, wherein the semiconductor body is configured to include at least two layers, and adjacent layers of the semiconductor body have different energy band gaps, wherein the semiconductor body is formed by an intrinsic semiconductor or a semiconductor being doped with impurities, and wherein the first and second semiconductor regions are doped with impurities of which concentration is higher than that of the semiconductor body.