Gate dielectrics in printed electronics can make the transistors exhibit higher current and lower voltage. They are therefore a hot area of research. Organic materials are fundamentally limited in their basic permittivity by the covalent bonding. That is why so-called high permittivity dielectrics for printed transistor gates only have a permittivity of five or so - a poor shadow of a typical inorganic dielectric such as barium titanate.
With the latter starting to be printed, that has put those seeking an all organic solution to printed transistor circuits at a disadvantage in getting voltages down and performance up. However, an alternative now emerging is the use of polymers deliberately designed to encourage ionic movement to enhance permittivity. These are called ion modulated transistors. They can have poor response times and there may be challenges of life but Abo Akademi of Finland lectured about such work in the recent IDTechEx Printed Electronics Europe conference and so did Professor C. Daniel Frisbie of Chemical Engineering & Materials Science at the University of Minnesota, in this case with an ion gel electrolyte put down as a paste.
Both paper and plastic film substrates have been used. Abo Akademi put it this way:
- How to achieve low-voltage operation in OFETs with large dimensions?
1. Increase dielectric constant
Problem: Dipolar broadening -> lower mobilities
1. Reduce dielectric thickness
- Our choice: Organic transistors accumulated with ionic drift within the insulator:
1. High capacitive coupling -> low voltage operation
2. Robust device operation -> High yield
This is the Hygroscopic Insulator FET (HIFET).
So far they see the following emerging from this approach:
- Manufacturing in dirty room atmosphere
- Roll-to-roll processable
- Low voltage operation - one volt.
- Thick hygroscopic insulator (1 - 2 µm)
- Slow switching behavior
- Ionic gate-current
- High off-current
They are seen as suitable for roll to roll production and they are even tolerant of rough substrates.
If you missed Printed Electronics Europe 2008 you can purchase the proceedings on line or attend Printed Electronics USA 2008.