Ionic transistors may enable super-fast switching

Advances in the understanding of ionic materials may allow for much faster switching in transistors. Ann Deml, a doctorate student in the Materials and Metallurgical Engineering department, discussed her progress toward understanding ionic field effect transistors. The applications for Ionic Field Effect Transistors (Nanoionics) are quite broad, with uses in various forms of energy storage such as batteries and fuel cells. Currently, Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) are used, which uses a semiconductor such as silicon with a vapor-deposited layer of a metallic oxide for the switch. This transistor operates by applying a voltage across the semiconductor and the metallic oxide layer, which then allows current to flow across the semiconductor.

Nanoionics would allow for a true solid-state field-effect transistor, and if a few challenges can be overcome, this nanoionic transistor would operate much faster then the equivalent MOSFET. The basic idea behind the operation of a nanoionic transistor is making use of what is known as the “space charge layer.” This layer develops between a metal and a semiconductor when a voltage is applied across the two elements. The layer consists of a distribution of positive and negative charges, arranged in a way that can be hard to define. The difficulty is that this space charge layer is not well understood, as there has been very limited study of it in the past. Deml has been focused primarily on understanding the space charge layer, running various experiments on transistors and testing three different models of the layer itself.

Deml explained that she approached the nanoionic transistor with much the same design intent as a standard MOSFET, modifying it to work with nanoionic materials. The primary difference between a MOSFET and her experimental models is the presence of a very thin film of nanoionic material, which is the “active ingredient,” so to speak. This thin film functions much the same as the semiconductor in the MOSFET; when you apply a voltage across it and the conductive material, it allows current to flow across it. Initial experiments with the model showed similarities to the MOSFET, with some current modulation due to the switching. Only one experimental setup gave the hoped-for data, leading Deml to believe there is another undesirable interaction going on that she does not fully understand yet.

After further study and experimentation with the nanoionic model, Deml discovered that the primary reason for the bad results was a significant current leakage, which at this time is hard to prevent due to limitations in manufacturing the models and measuring the data itself. Deml will no longer be working on this project, and will be moving on to different areas of study in pursuit of her doctorate. She offered a few recommendations to any other students who may be interested in continuing this research. One way the nanoionic transistor could be modified is to find a way to eliminate or at least significantly reduce the current leakage out of the thin film. One way to do this may be to reduce the dimensions of the model, which would require investment in high precision instruments.

This research presents some unique and intriguing challenges, with the possibility of revolutionizing the transistor industry if successful. Deml’s work should serve as a springboard to students interested in solving this unique set of problems, and they can be quite certain that the reward will not be small for doing so.

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