Gate-All-Around Nanosheet Transistors for Sub-3nm Logic Node Performance Scaling

Introduction to Gate-All-Around (GAA) Nanosheet Transistors

As semiconductor technology approaches the sub-3nm logic node, traditional FinFET architectures face increasing challenges in electrostatic control and power efficiency. Gate-all-around (GAA) nanosheet transistors have emerged as a leading solution to extend Moore’s Law by providing superior gate control, reduced leakage, and enhanced drive current. This article explores the electrical and thermal properties of GAA nanosheet transistors and their role in enabling continued performance scaling.

Evolution from FinFET to GAA Nanosheet Transistors

FinFETs revolutionized transistor design by introducing a 3D gate structure that improved electrostatic control compared to planar transistors. However, as dimensions shrink below 5nm, FinFETs encounter:

• Short-channel effects (SCEs): Increased leakage due to insufficient gate control.
• Mobility degradation: Narrower fin widths reduce carrier mobility.
• Parasitic capacitance: Higher contact resistance and fringe capacitance limit performance gains.

GAA nanosheet transistors address these limitations by wrapping the gate material entirely around the channel, improving electrostatic control and enabling further scaling.

Architecture of GAA Nanosheet Transistors

A GAA nanosheet transistor consists of multiple stacked horizontal silicon sheets, each fully enclosed by the gate. Key structural features include:

• Nanosheet channels: Typically 5–12nm thick, providing higher drive current per footprint.
• Inner spacer: Reduces parasitic capacitance between gate and source/drain regions.
• Gate stack: High-k dielectric (e.g., HfO₂) and metal gate for optimal threshold voltage tuning.

Fabrication Process

The fabrication of GAA nanosheet transistors involves:

1. Epitaxial growth: Alternating layers of Si and SiGe are deposited.
2. Patterning: Lithography and etching define the nanosheet stacks.
3. Selective etching: SiGe is removed to release the Si nanosheets.
4. Gate formation: Deposition of high-k dielectric and metal gate around the nanosheets.

Electrical Properties and Performance Advantages

GAA nanosheet transistors exhibit superior electrical characteristics compared to FinFETs:

• Enhanced gate control: Full gate wrap-around minimizes off-state leakage (I_off).
• Higher drive current (I_on): Wider effective channel width per footprint.
• Reduced DIBL: Lower drain-induced barrier lowering due to improved electrostatic integrity.

Performance Metrics at Sub-3nm Nodes

Recent research from IMEC and TSMC highlights the following metrics for GAA nanosheets at sub-3nm:

• Subthreshold swing (SS): ~65–70 mV/decade, nearing the Boltzmann limit.
• I_on/I_off ratio: >10⁵, ensuring efficient switching.
• Contact resistance: <50 Ω·µm with optimized metallization.

Thermal Challenges and Mitigation Strategies

The compact structure of GAA nanosheets introduces thermal management challenges:

• Self-heating effects: Higher power density increases local temperatures, degrading mobility and reliability.
• Thermal resistance: Nanosheet geometry limits heat dissipation pathways.

Thermal Optimization Techniques

To mitigate thermal issues, researchers propose:

1. Buried power rails: Reduces heat accumulation in the active region.
2. Advanced thermal interface materials (TIMs): Enhances heat dissipation to the substrate.
3. Nanosheet thinning: Improves thermal conductivity by reducing phonon scattering.

Comparative Analysis: GAA vs. FinFET at Sub-3nm

The table below summarizes key differences between GAA nanosheet and FinFET architectures at sub-3nm nodes:

Parameter	GAA Nanosheet	FinFET
Gate Control	Full surround (4x effective gate)	Tricgate (3x effective gate)
Drive Current (Ion)	Higher (~15% improvement)	Limited by fin width
Leakage Current (Ioff)	Lower (~30% reduction)	Higher due to SCEs
Thermal Resistance	Higher (requires mitigation)	Lower due to fin geometry

The Future of GAA: Forksheet and CFET Innovations

Beyond conventional GAA nanosheets, two emerging technologies promise further scaling:

Forksheet Transistors

A hybrid between FinFET and GAA, forksheets use a dielectric wall to separate nMOS and pMOS regions, enabling tighter pitch scaling.

Complementary FETs (CFETs)

CFETs stack nMOS and pMOS vertically, potentially doubling transistor density. Challenges include thermal management and process complexity.

The Path Forward for Moore’s Law

The adoption of GAA nanosheet transistors at sub-3nm nodes represents a critical step in sustaining Moore’s Law. However, continued innovation in materials (e.g., 2D channels), thermal management, and heterogeneous integration will be essential to push beyond 1nm nodes. As the semiconductor industry embraces these advancements, GAA nanosheets stand as a testament to human ingenuity in the face of physical limits.