Researchers at the Massachusetts Institute of Technology (MIT) have developed a new type of transistor that utilises semiconducting vertical nanowires made of gallium antimonide (GaSb) and indium arsenide (InAs). This innovation has the potential to challenge the dominance of silicon-based transistors, which have been the cornerstone of modern electronic devices. The new transistors operate via a tunnelling mechanism whereby electrons move through an energy barrier, promising significant energy efficiencies, particularly for low-energy applications like the Internet of Things (IoT).
Traditionally, electronic transistors function by applying voltage to regulate electron flow within semiconductor chips. In silicon transistors, this process requires electrons to surmount an energy barrier, thus switching the device from an "off" state to an "on" state. This is limited by what is referred to as the “Boltzmann tyranny,” a physical limitation tied to the energy distribution of electrons within the semiconductor material. Consequently, this constrains the overall energy efficiency of such devices.
The MIT team, led by electrical engineer Jesús A del Alamo, employed a top-down fabrication technique to create their new transistors. This highly precise fabrication process involves the use of high-quality, epitaxially-grown structures, along with both dry and wet etching methods to produce vertical nanowires with diameters as small as 6 nanometres. Subsequently, the researchers integrated a gate stack comprised of a thin dielectric layer and a metal gate onto the nanowires, also introducing point contacts for the source, gate, and drain through meticulous planarisation and etch-back techniques.
Due to the sub-10 nm scale and the reduced thickness of the gate dielectric—measuring just 2.4 nm—the electrons within the devices are confined similarly to a quantum field, inhibiting their capacity to move freely. In this quantum confinement scenario, electrons can tunnel through much thinner barriers than those needed for conventional transistors, resulting in significantly lowered voltage requirements for switching the state of the device.
Yanjie Shao, a postdoctoral researcher at MIT and the lead author of a study published in Nature Electronics, highlighted the long-standing interest in tunnelling-type transistors but acknowledged the challenges inherent in producing devices with both a steep switching slope and high drive current. He stated, “We believed in the potential of the GaSb/InAs ‘broken-band’ system to overcome this difficulty.” The team encountered hurdles, particularly in the fabrication of small vertical nanowires and creating a high-quality gate stack with minimal electronic trap states, which can hinder performance.
The researchers ultimately developed a plasma-enhanced deposition method for the gate dielectric, a breakthrough they credit with achieving enhanced transistor performance. Shao explained, "After many unsuccessful attempts, we found a way to make the system work," signifying the resilience and ingenuity of the research team. The characterisation of the tunnelling transistors involved both experimental work and advanced theoretical modelling, contributing to a deeper understanding of the complex behaviours at play within these nanowire structures.
The end product boasts impressive specifications, including a drive current reaching up to 300 microamperes per millimetre and a rapid switching slope of less than 60 mV/decade. These metrics indicate that the voltage required for operation is a mere 0.3 V, significantly lower than traditional silicon counterparts, and approximately 20 times superior to similar tunnelling transistors available in the market.
Shao anticipates that the primary applications for these novel transistors will be in ultra-low-voltage electronic devices, potentially revolutionising sectors such as artificial intelligence and IoT that are heavily reliant on energy-efficient components. The ongoing work aims to further optimise the fabrication processes and to introduce a new configuration involving vertical “nano-fins” to enhance uniformity across devices while mitigating structural variability, a critical consideration given the extreme scaling of these technologies. "Being so small, even a variation of just 1 nm can adversely affect their operation," Shao noted, emphasizing the precision required in this nascent field.
Source: Noah Wire Services