Non-technical Abstract. This proposal describes a collaborative, interdisciplinary effort at the University of Minnesota to understand, design, fabricate and test a new class of switching devices for flexible electronics, namely electrolyte gated transistors (EGTs). The two Principal Investigators, Frisbie (chemical engineering and materials science) and Ruden (electrical and computer engineering), have a strong track record of scientific collaboration in transistor development extending over the past ten years. In this proposal, they again join their respective experimental and computational expertise to understand and develop EGTs as a novel class of devices for applications such as flexible displays and wearable sensors. This project will produce experimental and theoretical results that advance the field of flexible electronics; it will support the training of PhD students and undergraduates, and it will also produce educational aids for teaching about electronic materials and devices at the high school level.
In prior work by the principal investigators (PIs) it has been established that electrolyte gated transistors (EGTs) have many advantages for building circuits on impact resistant, bendable plastic substrates. These advantages include: sub-2 V operation compatible with thin film batteries, excellent ON/OFF current ratios of order 106, complementary circuit designs (i.e., p-type and n-type channels), high ON-state transconductances >1 ÃS/Ãm, fast switching speeds approaching 1 MHz, excellent bias stress stability, good air stability without encapsulation, and easy, near-room-temperature fabrication procedures compatible with plastic. These very promising characteristics motivate the PIs to continue to improve EGTs and to develop accurate computational models of their operation that both reflect a deep understanding of the device physics, and facilitate device design for integration into complete circuits. There are a number of challenges associated with understanding EGTs because the gate insulator layer contains mobile ions. For example, the mechanism of EGT switching depends on whether the semiconductor channel is permeable or impermeable to ions, and new transistor models must be constructed to take account of these differences. In addition, the operation of EGTs, and in particular the channel carrier mobility, depends on the type of mobile ions employed in the gate/electrolyte/ semiconductor stack. These effects, and many others, must be understood in order to optimize EGT performance. EGTs also open up substantial opportunities for changing circuit fabrication paradigms in flexible electronics. In particular, the ability to make EGTs entirely from electronically functional liquid inks has inspired the PIs to propose a new, self-aligning fabrication strategy based on nanoimprint lithography and capillary flow. This process, termed SCALE (for Self-Aligned Capillary Flow Lithography for Electronics), will be extensively investigated and developed in connection with fabrication of p-type and n-type EGTs. The PIs will also expand the materials sets compatible with EGT architectures, e.g., new gate electrolytes based on ion gels and ceramic ion conductors, and the incorporation of both organic semiconductors and amorphous oxides as the channel materials.
The broader impacts of this proposal will be in human resource development and the creation of educational tools for teaching semiconductor electronics and materials science. Specifically, this grant will support the training of PhD students and undergraduates in the fabrication and characterization of electronic devices, new microelectronics processing methods, novel device architectures, and sensors, i.e., areas that are important to national workforce development. The PIs will also develop prototype ¡§Flexible Circuit Fabrication Kits¡¨ that will allow high school science students to explore the interplay between microelectronics and materials. These kits will include embossed plastic substrates and non-toxic, environmentally friendly electronic inks that can be easily delivered to the substrates to create circuits that can then be tested using simple supplies such as a battery and a resistance meter.
