The transistor is the most fundamental building block of electronics, used to build circuits capable of amplifying electrical signals or switching them between the 0s and 1s at the heart of digital computation. Transistor fabrication is a highly complex process, however, requiring high-temperature, high-vacuum equipment. Now, University of Pennsylvania engineers have shown a new approach for making these devices: sequentially depositing their components in the form of liquid nanocrystal "inks."
Their new study, published in Science, 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 researchers' nanocrystal-based field effect transistors were patterned onto flexible plastic backings using spin coating but could eventually be constructed by additive manufacturing systems, like 3D printers.
The researchers began 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.
Kagan's group 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). "Doping" the semiconductor layer of the transistor with impurities controls whether the device transmits a positive or negative charge.
The electrical properties of several of these nanocrystal inks had been independently verified, but they had never been combined into full devices. Such a process entails layering or mixing them in precise patterns.
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.
Because this entirely 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.