A transistor normally requires several fundamentally different materials—a semiconductor to carry current, an insulating dielectric to control it, and metallic contacts through which current enters and leaves. Integrating all of these components becomes increasingly difficult as semiconductor devices approach the atomic scale.
A research team led by Director JO Moon-Ho of the Center for Van der Waals Quantum Solids within the Institute for Basic Science (IBS) has now demonstrated a fundamentally different approach. Instead of fabricating the semiconductor, dielectric, and contact materials separately and then assembling them into a device, the researchers started from a single layered crystal and chemically converted selected atomic layers and regions into the different materials needed for a working transistor.
Atomically thin two-dimensional semiconductors have long been considered promising candidates for future electronics as conventional silicon devices approach their physical scaling limits. Yet despite more than two decades of research, one of the greatest obstacles to practical 2D electronics has not been the performance of the semiconductor itself, but the difficulty of integrating it with other components.
A transistor requires a highly uniform gate dielectric as well as low-resistance electrical contacts, but the surfaces of 2D semiconductors contain few sites where other materials can readily attach. This makes it difficult to deposit conventional dielectric materials uniformly. Direct deposition of metals can also damage the atomically thin semiconductor or create large contact barriers that reduce device performance. Mechanical transfer methods, in which separate 2D materials are peeled, positioned, and stacked one by one, have provided an alternative, but such processes are difficult to scale uniformly over large areas and remain poorly suited to mass production.
The IBS team instead used the crystal itself as the starting material for all of the required electronic functions.
The researchers began with multilayer tungsten diselenide (WSe₂), a 2D semiconductor. Its electrical properties can be changed by replacing some of the atoms surrounding the tungsten layer. By preserving the tungsten framework while selectively replacing these surrounding atoms, the researchers were able to change the electronic character of individual regions without rebuilding the entire crystal.
The upper layers of WSe₂ were first oxidized into insulating tungsten oxide (WOₓ). The oxide could then be further converted through vapor-phase reactions into tungsten disulfide (WS₂), an n-type semiconductor, or tungsten ditelluride (WTe₂), a semimetal. Regions that were not converted retained the original WSe₂ semiconductor characteristics. In this way, semiconducting, insulating, and semimetallic functions could be created directly within different layers and locations of the same parent crystal.
The tiny gaps between neighboring atomic layers helped the researchers control how far the reaction could spread. By adjusting the reaction temperature and gas pressure, they could stop the conversion after a chosen number of layers, achieving control at roughly 0.7 nanometers per WSe₂ monolayer.
Director JO Moon-Ho said, "One of the central challenges in two-dimensional semiconductor research has not simply been finding better semiconductors, but securing equally suitable dielectrics and electrical contacts. This study combines fundamental materials science based on atomic-scale manipulation with modern semiconductor technology to address that integration challenge."
One of the most important demonstrations involved converting the upper layers of WSe₂ into WOₓ directly above the semiconductor channel. The resulting oxide served as the gate dielectric without the need to deposit a separate high-k material and formed an insulating layer with an effective thickness below one nanometer while maintaining strong insulating performance.
Using this structure, the researchers fabricated WSe₂ transistors in which a change of only 61.3 millivolts in gate voltage increased the current tenfold. This is very close to the theoretical room-temperature limit of about 60 millivolts per decade, indicating highly efficient switching. The devices also achieved an on/off current ratio exceeding 10 million.
The WTe₂ conversion also improved the electrical contacts, reducing the barrier for charge injection to about one-third of that observed with direct platinum contacts. This allows charge carriers to enter the semiconductor more efficiently.
The team further demonstrated n-type MoS₂ transistors using the same WOₓ dielectric and vertically stacked p-type and n-type transistors to construct a complementary van der Waals inverter, a basic building block of digital logic. The successful voltage inversion showed that the method can be extended beyond individual transistors toward more highly integrated circuitry.
Director JO Moon-Ho added, "By directly converting selected regions of a two-dimensional crystal into different electronic functions, this approach could provide a new route toward ultralow-power, ultrascaled, and vertically integrated semiconductor devices."
