The Future beyond MOSFETs

Introduction

The metal–oxide–semiconductor field-effect transistor (MOSFET) has been a cornerstone of complementary metal–oxide–semiconductor (CMOS) technology, driving the evolution of integrated-circuit products for over six decades.

However, as the physical gate length of MOSFETs has been scaled down to sub-20 nanometers, the challenges of maintaining low power consumption while downscaling transistors have become increasingly daunting.

The limitations of MOSFETs in terms of power consumption and energy efficiency have prompted the exploration of ‘beyond MOSFET’ transistors, aiming to break the energy-efficiency bottleneck.

This article will provide a comprehensive assessment of the existing and future CMOS technologies, discussing the potential of ‘trans-resistors’ as the next frontier in semiconductor technology.

The Evolution of Transistors: A Historical Perspective

Three major electronic devices—the vacuum tube, the bipolar junction transistor (BJT), and the metal–oxide–semiconductor field-effect transistor (MOSFET)—actively marked the evolution of electronics throughout its history.

While the first two devices played crucial roles in advancing modern computing, it was the rise of the MOSFET, particularly CMOS technology, that ignited the explosive development of information technology.

The continuous scaling of MOSFETs has driven important metrics such as cost, performance, and energy consumption towards efficiency, making them extremely favorable in low-power applications.

However, the road to scaling has not been without challenges, with short-channel effects (SCEs) plaguing CMOS technology as the device size entered the sub-1-micrometer regime.

Why is the transistor called ‘transistor’?

We’ve been studying transistors since 12^th standard and using them daily in our smartphones, laptops, smartwatches, TVs and what not. We are surrounded by transistors. But have you ever thought how transistor got its name?

The fundamental aspects of a transistor—‘trans’ (arising from ‘transfer’ or modulation) and ‘resistor’ (arising from resistance of a channel)—capture the approach to manipulate the information state or carrier, which is the ‘resistor’.

Thus, ‘trans’ captures the approach to manipulate the information state or carrier—which is the ‘resistor’. Commercialized MOSFETs achieve ‘trans’ through electric-field effect via a static gate capacitor, and BJTs realize ‘trans’ via p–n junction barrier modulation. Both devices implement ‘resistor’ using thermionic emission over a barrier.

5 Innovations in Transistor Technologies

The future of ‘trans-resistors’ beyond MOSFETs holds promise for driving future materials, device physics, and topology, as well as heterogeneous integration.

Let’s explore briefly about some beyond MOSFET technologies:

NC FET (Negative Capacitance FET): Utilizes the negative capacitance state of ferroelectric materials to construct a negative Cgox, targeting to overcome the unity upper limit of gate efficiency. It aims to address the challenges of power consumption and energy efficiency by introducing a negative capacitance state.

SG FET (Suspended-Gate FET): Introduces a nanoelectromechanical (NEM) switch into the gate stack, utilizing non-equilibrium-state switching transients to realize an abrupt increase of charge density and drain current, resulting in an ultrasmall subthreshold swing (SS). However, the memory nature of the NEM switch and phase-change memory cell introduces hysteresis, limiting SG FETs to memory applications.

Mott-G FET (Mott-Gated FET): Employs a phase-change memory cell connected in series with the source to achieve an abrupt change of resistance of the ‘resistor’ and hence the drain current during the memory-state switching. This also introduces hysteresis in the I–V curve.

SB FET (Schottky Barrier FET): Involves charge carriers tunnelling through the Schottky barrier between the metallic source and the semiconducting or vacuum channel, allowing for the modulation of resistance and drain current.

Superlattice FET: Utilizes a multi-quantum well in the source region to form an artificial resonant tunnelling band, which is narrow enough to filter the thermionic emission of high-energy carriers, aiming to improve device performance.

Transistors go beyond MOSFETs

Three-Dimensional Integration and Beyond-Moore Pathways

The future of ‘trans-resistors’ beyond MOSFETs also encompasses three-dimensional integration and beyond-Moore pathways.

Practical approaches to increase device density and reduce interconnect delay and power dissipation include actively considering three-dimensional integration, such as wire bonding, flip-chip-based 3D packaging, through-Si-via (TSV)-based 3D die/wafer stacking, and monolithic 3D integration (m-3D).

Heterogeneous 3D integration exploits the integration of dissimilar systems, incorporating diverse materials, devices, and functionalities both vertically and laterally. This approach has the potential to create highly powerful and energy-efficient system-on-chip and heterogeneous system-of-chips configurations.

heterogenous Integration — Heterogenous integration is taking moore’s law ahead

Conclusion

This article delves into the challenges and opportunities of FET scaling. It also discusses the future of ‘trans-resistors’ beyond MOSFETs and explores potential pathways like three-dimensional integration and beyond-Moore technologies.

Pushing transistor boundaries, ‘trans-resistors’ beyond MOSFETs mark a paradigm shift, addressing power consumption and improving energy efficiency. These innovations will drive advancements in materials, device physics, and topology, leading to revolutionary progress in semiconductor technology.

Reference: Nature