【Semiconductor】🟢 04. GAA Structure

— The Completed Form of Electric-Field Control

topics: [“GAA”, “Nanosheet”, “MOSFET”]

🧭 Introduction

Planar MOSFETs reached a dead end due to SCE,
and FinFETs restored electric-field control through geometry.

So what does the next step,
Gate-All-Around (GAA), truly represent?

In this article, GAA is positioned as:

Not an extension of scaling technology,
but the “completed solution” to electric-field control as a design problem

🧩 Positioning of GAA

GAA is often described as
“the next-generation structure after FinFET.”

However, from a physical standpoint, GAA is:

Not an improved FinFET
but
A structure that operates at a higher design level

If FinFET is a structure that increased gate enclosure,
then GAA is:

A structure designed from the outset with 100% enclosure as a premise

🧱 Structural Characteristics

The basic structure of GAA is as follows:

Channel: nanowire / nanosheet
Gate: fully surrounds the channel
Source and drain: connected vertically

This configuration results in:

Channel potential constrained on all sides
Extremely limited paths for drain electric-field intrusion
Subthreshold characteristics approaching ideal behavior

🔒 Why GAA Is Close to a “Final Form”

The essence of SCE has consistently been:

Electrodes other than the gate
dominating the channel potential

In GAA structures:

The gate is adjacent to the channel in all directions
There is virtually no escape path for the electric field
The potential distribution is almost entirely defined by gate boundary conditions

In principle, this means:

There is very little room left to further strengthen gate control

In this sense, GAA represents:

A structure extremely close to the theoretical limit
of electric-field control

📐 Extension of Weff in GAA

In FinFETs, Weff was determined by:

Fin height
Fin width
Number of fins

In GAA, Weff becomes even more explicitly defined by:

Sheet width
Number of stacked sheets

Conceptually:

Weff ≒ Sheet width × Number of sheets

The critical point here is that:

Weff has become purely an accumulated structural quantity

🛠 Where Did Design Freedom Move?

In the GAA era:

Gate length
Gate oxide
Electric-field control

are already close to being “fully optimized.”

As a result, design freedom has shifted almost entirely to:

Sheet width
Sheet thickness
Number of stacked sheets
Sheet-to-sheet spacing

These are three-dimensional structural parameters.

This signifies that:

Electrical characteristic design has fully transitioned
into structural design

🎯 The Design Philosophy Reached by GAA

GAA is not:

A means to achieve further scaling
but rather
The definitive solution to making electric-field control fully valid

In this structure, discussions such as:

“How to suppress SCE”
“How to further reduce voltage”

are no longer the central theme.

Instead, the focus shifts to the next set of physical constraints:

Quantum effects
Thermal transport
Manufacturing variability
Mechanical stability

📝 Summary

✅ GAA is not an extension of FinFET, but a structure at a different level
✅ Full gate enclosure brings electric-field control close to its theoretical limit
✅ Weff has become a completely structure-dependent parameter
✅ The design battlefield has shifted from electric fields to 3D geometry

GAA can be described as:

The structure in which MOSFETs
have fully answered the challenge of electric-field control

📘 Edusemi-v4x | Advanced Node Technologies (FinFET, GAA, CFET)

GitHub Pages (Public Educational Material, Japanese)
https://samizo-aitl.github.io/Edusemi-v4x/f_chapter1_finfet_gaa/
GitHub (Source Management, Markdown Manuscripts)
https://github.com/Samizo-AITL/Edusemi-v4x/tree/main/f_chapter1_finfet_gaa

Planar MOSFET → FinFET → GAA → CFET
This article corresponds to Special Chapter 1,
which systematically explains the evolution of electric-field control structures.

This article is positioned as an introduction that starts from
the physical limits of planar MOSFETs (SCE) and explains
why structural transitions were inevitable.
It is recommended to read it together with the subsequent FinFET, GAA, and CFET articles.