Transistors are the basic building blocks of electronics, used to build circuits capable of amplifying electrical signals or switching them between binary states. However transistor fabrication is a very complex process, requiring high-temperature, high-vacuum equipment.
In a new study, researchers at the University of Pennsylvania have shown a new approach for making these devices by sequentially depositing their components in the form of liquid nanocrystal “inks.” This study opens the door for electrical components to be built into flexible or wearable applications, as the lower-temperature process is compatible with a wide array of materials and can be applied to larger areas.
The study was led by Cherie Kagan, Stephen J. Angello and Ji-Hyuk Choi in collaboration with Christopher Murray and his other Murray lab members. They started by taking nanocrystals, or roughly spherical nanoscale particles, with the electrical qualities necessary for a transistor and dispersing these particles in a liquid, making nanocrystal inks.
Researchers developed a library of four of these inks: a conductor (silver), an insulator (aluminum oxide), a semiconductor (cadmium selenide) and a conductor combined with a dopant (a mixture of silver and indium).
The electrical properties of several of these nanocrystal inks had been independently verified, but they had never been combined into full devices. So the question was whether Kagan’s group could lay nanocrystal inks down on a surface in such a way that they work together to form functional transistors.
First, the conductive silver nanocrystal ink was deposited from liquid on a flexible plastic surface that was treated with a photolithographic mask, then rapidly spun to draw it out in an even layer. The mask was then removed to leave the silver ink in the shape of the transistor’s gate electrode. The researchers followed that layer by spin-coating a layer of the aluminum oxide nanocrystal-based insulator, then a layer of the cadmium selenide nanocrystal-based semiconductor and finally another masked layer for the indium/silver mixture, which forms the transistor’s source and drain electrodes. Upon heating at relatively low temperatures, the indium dopant diffused from those electrodes into the semiconductor component.
“The trick with working with solution-based materials is making sure that, when you add the second layer, it doesn’t wash off the first, and so on,” Kagan said. “We had to treat the surfaces of the nanocrystals, both when they’re first in solution and after they’re deposited, to make sure they have the right electrical properties and that they stick together in the configuration we want.”
Because this ink-based fabrication process works at lower temperatures than existing vacuum-based methods, the researchers were able to make several transistors on the same flexible plastic backing at the same time.
The inks' specialized surface chemistry allowed them to stay in configuration without losing their electrical properties.
