Tunnel FET

A Tunnel Field-Effect Transistor (Tunnel FET or TFET) is a type of semiconductor device that operates on the principle of quantum mechanical tunnelling rather than traditional thermionic emission used in conventional MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). It is regarded as one of the most promising candidates for low-power and high-efficiency electronic applications, particularly in next-generation nanoelectronics and energy-efficient integrated circuits.

Introduction and Concept

In traditional MOSFETs, current flows when electrons gain sufficient thermal energy to overcome the potential energy barrier between the source and the channel. However, in Tunnel FETs, carriers move across a p–i–n (p-type–intrinsic–n-type) structure via band-to-band tunnelling (BTBT) — a quantum process that allows electrons to pass directly through the energy barrier rather than going over it.
This tunnelling mechanism enables TFETs to achieve steeper subthreshold swings and lower operating voltages than conventional MOSFETs, leading to significant reductions in power consumption.

Structure of Tunnel FET

A Tunnel FET resembles a MOSFET in general layout but differs in doping configuration and operating mechanism. The typical structure consists of:

• Source: Heavily doped p-type (for n-channel TFET) or n-type (for p-channel TFET).
• Channel: Lightly doped or intrinsic semiconductor region controlled by the gate.
• Drain: Heavily doped n-type (for n-TFET) or p-type (for p-TFET).
• Gate Oxide: Thin insulating layer separating the gate electrode from the channel.
• Gate Electrode: Controls the electric field that modulates tunnelling probability.

The p–i–n structure is crucial since tunnelling occurs between the valence band of the source and the conduction band of the channel when appropriate gate voltage is applied.

Working Principle

The operation of a Tunnel FET is based on band-to-band tunnelling (BTBT):

1. OFF-State: When no gate voltage is applied, the conduction band of the channel and the valence band of the source are misaligned, creating a large energy barrier. This prevents carrier tunnelling, and the device remains off with minimal leakage current.
2. ON-State: When a positive gate voltage (for n-TFET) is applied, the energy bands bend sufficiently to align the valence band of the source with the conduction band of the channel. This allows electrons to tunnel directly through the barrier from source to channel, resulting in current flow.
3. Tunnelling Current: The current depends exponentially on the tunnelling probability, which is influenced by factors such as bandgap energy, gate voltage, and the electric field across the junction.

This tunnelling-based operation allows TFETs to exhibit subthreshold swing (SS) values below the thermionic limit of 60 mV/decade achievable by MOSFETs at room temperature.

Key Features and Advantages

• Low Subthreshold Swing (SS): TFETs can achieve SS < 60 mV/decade, enabling faster switching at lower voltages.
• Ultra-Low Power Consumption: Operating at low supply voltages (typically below 0.5 V) significantly reduces both dynamic and static power dissipation.
• Reduced Leakage Current: Due to the tunnelling mechanism and band alignment, TFETs exhibit extremely low off-state leakage currents.
• Compatibility with CMOS Technology: TFETs can be fabricated using materials and processes similar to those used for CMOS devices, facilitating hybrid integration.
• Scalability: Suitable for nanoscale device architectures, addressing the limitations of conventional MOSFET scaling.

Limitations and Challenges

Despite their promise, TFETs face several challenges that limit large-scale commercial implementation:

• Low ON-State Current (ION): The tunnelling current is typically weaker than thermionic emission, leading to reduced drive strength and slower switching.
• Material Limitations: Achieving efficient tunnelling requires materials with narrow bandgaps or heterojunctions, which can complicate fabrication.
• Ambipolar Conduction: TFETs may exhibit unwanted conduction in both polarities (n-type and p-type), leading to leakage and reduced performance.
• Complex Fabrication: Maintaining precise doping gradients and interface quality is critical for reliable operation, increasing manufacturing complexity.
• Temperature Sensitivity: Tunnelling behaviour may vary with temperature, affecting device stability in diverse environments.

Types of Tunnel FETs

Depending on material configuration and design, TFETs can be classified as:

1. Silicon-based TFETs: Use conventional silicon substrates; easier to fabricate but limited by low tunnelling efficiency.
2. III–V Compound TFETs: Utilise narrow bandgap materials such as InAs, GaAs, or GaSb for improved tunnelling performance.
3. Heterojunction TFETs: Employ interfaces between materials with different bandgaps to enhance tunnelling probability and current.
4. Graphene and 2D Material TFETs: Incorporate two-dimensional materials like graphene, MoS₂, or WSe₂ for superior electrostatic control and reduced short-channel effects.
5. Vertical TFETs: Designed with tunnelling occurring in a vertical direction to reduce footprint and improve device density.

Comparison between MOSFET and Tunnel FET

Applications of Tunnel FETs

Due to their low power operation and compact size, TFETs are well-suited for several emerging applications:

• Ultra-Low Power Electronics: Ideal for portable, battery-operated devices such as smartphones, sensors, and IoT systems.
• Energy-Efficient Processors: Potential replacement for CMOS in low-voltage logic circuits.
• Biomedical Devices: Suitable for wearable and implantable electronics requiring minimal energy consumption.
• Non-Volatile Memory Devices: Used in designing steep-slope transistors for fast and energy-efficient memory access.
• Flexible and 3D Electronics: Integration in flexible substrates and stacked architectures due to small form factor.

Future Prospects

Research on Tunnel FETs continues to advance in materials engineering, device design, and fabrication techniques. The focus areas include:

• Developing heterojunction TFETs with higher ON-current using III–V or 2D materials.
• Optimising gate engineering and source–drain doping profiles to enhance tunnelling efficiency.
• Integrating TFETs with complementary CMOS circuits to create hybrid low-power systems.
• Exploring vertical and nanowire architectures for better electrostatic control and scaling advantages.