A new transistor made from semiconducting vertical nanowires of gallium antimonide (GaSb) and indium arsenide (InAs) could rival today’s best silicon-based devices. The new transistors are switched on and off by electrons tunnelling through an energy barrier, making them highly energy-efficient. According to their developers at the Massachusetts Institute of Technology (MIT) in the US, they could be ideal for low-energy applications such as the Internet of Things (IoT).

Electronic transistors use an applied voltage to regulate the flow of electricity – that is, electrons – within a semiconductor chip. When this voltage is applied to a conventional silicon transistor, electrons climb over an energy barrier from one side of the device to the other, and it switches from an “off” state to an “on” one. This type of switching is the basis of modern information technology, but there is a fundamental physical limit on the threshold voltage required to get the electrons moving. This limit, which is sometimes termed the “Boltzmann tyranny” because it stems from the Boltzmann-like energy distribution of electrons in a semiconductor, puts a cap on the energy efficiency of this type of transistor.

Highly precise process

In the new work, MIT researchers led by electrical engineer Jesús A del Alamo made their transistor using a top-down fabrication technique they developed. This extremely precise process uses high-quality, epitaxially-grown structures and both dry and wet etching to fabricate nanowires just 6 nm in diameter. The researchers then placed a gate stack composed of a very thin gate dielectric and a metal gate on the sidewalls of the nanowires. Finally, they added point contacts to the source, gate and drain of the transistors using multiple planarization and etch-back steps.

The sub-10 nm size of the devices and the extreme thinness of the gate dielectric (just 2.4 nm) means that electrons are confined in a space so small that they can no longer move freely. In this quantum confinement regime, electrons no longer climb over the thin energy barrier at the GaSb/InAs heterojunction. Instead, they tunnel through it. The voltage required for such a device to switch is much lower than it is for traditional silicon-based transistors.

Steep switching slope and high drive current

Researchers have been studying tunnelling-type transistors for more than 20 years, notes Yanjie Shao, a postdoctoral researcher in nanoelectronics and semiconductor physics at MIT and the lead author of a study in Nature Electronics on the new transistor. Such devices are considered attractive because they allow for ultra-low-power electronics. However, they come with a major challenge: it is hard to maintain a sharp transition between “off” and “on” while delivering a high drive current.

When the project began five years ago, Shao says the team “believed in the potential of the GaSb/InAs ‘broken-band’ system to overcome this difficulty”. But it wasn’t all plain sailing. Fabricating such small vertical nanowires was, he says, “one of the biggest problems we faced”. Making a high-quality gate stack with a very low density of electronic trap states (states within dielectric materials that capture and release charge carriers in a semiconductor channel) was another challenge.

After many unsuccessful attempts, the team found a way to make the system work. “We devised a plasma-enhanced deposition method to make the gate electric and this was key to obtaining exciting transistor performance,” Shao tells Physics World.

The researchers also needed to understand the behaviour of tunnelling transistors, which Shao calls “not easy”. The task was made possible, he adds, by a combination of experimental work and first-principles modelling by Ju Li’s group at MIT, together with quantum transport simulation by David Esseni’s group at the University of Udine, Italy. These studies revealed that band alignment and near-periphery scaling of the number of conduction modes at the heterojunction interface play key roles in the physics of electrons under extreme confinement.

The reward for all this work is a device with a drive current as high as 300 uA/m and a switching slope less than 60 mV/decade (a decade, in this context, is a power of 10 difference between off and on states), meaning that the supply voltage is just 0.3 V. This is below the fundamental limit achievable with silicon-based devices, and around 20 times better than other tunnelling transistors of its type.

Potential for novel devices

Shao says the most likely applications for the new transistor are in ultra-low-voltage electronics. These will be useful for artificial intelligence and Internet of Things (IoT) applications, which require devices with higher energy efficiencies. Shao also hopes the team’s work will bring about a better understanding of the physics at surfaces and interfaces that feature extreme quantum confinement – something that could lead to novel devices that benefit from such nanoscale physics.

The MIT team is now developing transistors with a slightly different configuration that features vertical “nano-fins”. These could make it possible to build more uniform devices with less structural variation across the surface. “Being so small, even a variation of just 1 nm can adversely affect their operation,” Shao says. “We also hope that we can bring this technology closer to real manufacturing by optimizing the process technology.”

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