Trench Transistor Dopant Gradient for Threshold Control

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Solution Overview

Problem

Trench transistor structures experience electric field concentration at the top and bottom regions of trenches, leading to localized decreases in threshold voltage, which result in parasitic transistor effects and compromised subthreshold characteristics.

Innovation Solution

A semiconductor device with a trench structure where the dopant concentration at the bottom and surface of the trench is higher than the intermediate side walls, with dopant ions of the same conductivity type, to increase threshold voltage and prevent parasitic transistor effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of moving object

If a trench structure is formed in the channel region to increase effective channel width, then the transistor width is increased without increasing device size, but electric field concentration occurs at the top and bottom regions of the trenches causing localized threshold voltage decrease

Engineering Contradiction:

Improveeffective channel widthVSAvoidsubthreshold characteristics

Core Design Contradiction:

Area of moving objectVSReliability

Solution Approach 1:

The patent applies different dopant concentrations to different regions of the channel: higher concentration at the top and bottom surfaces, intermediate concentration at the trench side walls, and lower concentration in the bulk channel. This local quality variation compensates for electric field concentration at the trench regions, preventing localized threshold voltage decrease while maintaining the increased effective channel width benefit

Inventive Principle:

Principle #3Local quality

2Area of moving object

If the trench depth is increased to further increase effective channel width, then more channel area is utilized, but variations in transistor performance increase due to manufacturing tolerances

Engineering Contradiction:

Improveeffective channel widthVSAvoidtransistor performance variation

Core Design Contradiction:

Area of moving objectVSManufacturing precision

Solution Approach 1:

The patent modifies the dopant concentration parameter across different spatial regions of the channel. By increasing dopant concentration at the top and bottom surfaces and using intermediate concentrations at the trench regions, the invention compensates for manufacturing variations in trench depth, thereby reducing transistor performance variation while maintaining increased effective channel width

Inventive Principle:

Principle #35Parameter changes

3Ease of operation

If constant voltage is applied to the gate electrode, then the gate is biased properly for transistor operation, but electric field concentration at the trench regions causes parasitic transistor effects

Engineering Contradiction:

Improvegate bias controlVSAvoidparasitic transistor effects

Core Design Contradiction:

Ease of operationVSObject-generated harmful factors

Solution Approach 1:

The patent introduces local quality variations through spatially differentiated dopant concentrations in the channel region. Higher dopant concentration at the top and bottom surfaces counteracts the electric field concentration that occurs under constant gate bias, preventing parasitic transistor effects while maintaining proper gate control

Inventive Principle:

Principle #3Local quality

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 configuration enhances subthreshold characteristics while maintaining appropriate driving capability and preventing breakdown voltage decreases, reducing variations in transistor performance due to trench depth variations.

Implementation Method 1

the dopant concentration of the channel region at a bottom portion of the trench and the dopant concentration of the channel region at a surface of the substrate bounding the trench are both higher

Methodology Applied

Scientific EffectIon implantation: Ion Implantation

Data Source

PatentUS8809944B2Semiconductor device including a transistor with gate in a trench and a doped region under the trench to modify the threshold voltage

Publication Date: 2014.08.19 RENESAS ELECTRONICS CORP

AI summary

A semiconductor device includes a transistor with a substrate on which source and drain regions, both of a first conductivity type, and a channel region of a second conductivity type between the source and drain are formed, and a gate electrode formed in the channel region to bury a trench formed so the depth thereof changes intermittently in the width direction of the gate. In the channel region, each on a surface of the substrate and in a bottom portion of the trench, there are formed a second high-concentration region and a first high-concentration region, and the dopant concentration of the second conductivity type is higher than the dopant concentration of the second conductivity type in portions sideward from the trench. The dopant concentration of the second conductivity type in the first high-concentration region is higher than the dopant concentration of the second conductivity type in the second high-concentration region.