Phosphorus-Turned-Metal Tackles Resistance Issues of 2D Semiconductors

March 20, 2023 by Jake Hertz

KAUST researchers created a metal when stacking blue phosphorous, which effectively eliminated the contact issues of 2D semiconductors—a promising discovery for future transistors.

As progress slows down with Moore’s law, researchers have begun investigating new ways to harness performance gains out of semiconductors. One emerging branch in the semiconductor field is two-dimensional (2D) semiconductors, a material structure that promises improved performance and efficiency over traditional solutions.

However, before this technology can become a reality, some technical challenges must still be solved. This week, researchers at King Abdullah University of Science and Technology (KAUST) announced a research paper describing a new material, ultrathin blue phosphorous semiconductors, that overcome existing challenges for 2D semiconductors, like contact resistance. 


FET based on a newly-discovered blue form of ultrathin phosphorous

FET based on a newly-discovered blue form of ultrathin phosphorous. Image courtesy of KAUST


In this article, we’ll discuss the strengths of 2D semiconductors and how the team at KAUST leveraged the properties of the stacked phosphorous material to unlock higher transistor performance.


The State of 2D Semiconductors

In recent years, 2D materials have garnered attention for their promise in applications, including electronics, optoelectronics, and quantum computing.

Composed of a single layer of atoms arranged in a crystalline lattice structure, 2D semiconductors are incredibly thin and possess unique electronic and optical properties.


A 2D material has a thickness of a single atom

A 2D material has a thickness of a single atom. Image courtesy of Ossila


One of the key advantages of 2D semiconductors is their high carrier mobility, which refers to the ease with which electrons and holes can move through the material. This property makes them attractive for use in high-performance transistors, where high mobility equates to fast switching speeds and low conduction losses.

Beyond this, another important property of 2D semiconductors is their tunable bandgap energy, the energy required to excite an electron from the valence band to the conduction band. Importantly, the bandgap energy of a semiconductor also determines the color of light that the material is capable of absorbing and emitting. Hence, with tunable bandgap energy, 2D semiconductors are easily optimized for high performance in specific applications, such as light-emitting diodes (LEDs) or solar cells.


KAUST Makes a Surprising Discovery With Blue Phosphorous

This week, researchers at KAUST published a new paper in Nature that describes an advance in 2D semiconductor materials.

One prevailing roadblock to 2D semiconductor adoption is contact resistance. 2D semiconductors connect to the outside world via metal contacts, which by nature are three-dimensional, unlike the 2D nature of the semiconductor. Because of this, the point of contact between the 2D semiconductor and the 3D metal creates current crowding that leads to increased contact resistance. This resistance minimizes the power efficiency of 2D semiconductors, which ultimately limits their viability.

To address this challenge, the KAUST researchers experimented with a newly discovered two-dimensional blue phosphorene as a single electroactive material. Alone, the blue phosphorene is a semiconductor; however, when the material is stacked into a bilayer, it becomes a metal. Hence, by stacking layers of blue phosphorous, the KAUST researchers harnessed the benefits of 2D materials without the contact resistance that arises from the mismatch between 2D materials and 3D metals.

By experimenting with two different transport directions—armchair and zigzag—the researchers built transistors that demonstrated exceptional performance characteristics.


Armchair and zigzag device results

Armchair and zigzag device results. Image courtesy of Nature. (Click to enlarge)


Specifically, the resulting device demonstrated a high Ion/Ioff ratio of up to 2.6 × 104 and a transconductance of up to 811 μS/μm. According to the researchers, these values significantly outperform existing 2D FETs.