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Inorganic n-Type Thin-Film Transistors

We’ve seen before that organic approaches to transistors have focused on p-type transistors. While n-type materials have become more available, organic CMOS still isn’t widespread.

In an ISSCC paper, imec made reference to an inorganic thin-film transistor (TFT) that makes use of metal oxide (I’m tempted to call these MOxFETs). We’ll have more on that story in another posting; before getting into that, however, I wanted to learn more about exactly what these MOxFETs are.

It turns out that metal-oxide TFTs have been worked on for some time, but with a couple processing variants that limited their use. Vacuum sputtering, which yields the best mobility (> 10 cm2/Vs) costs money and limits the size of the circuit; solution-based materials are more promising because they can be deposited under “ambient” conditions over a larger substrate.

But there has still been one issue: these devices had to be annealed at temperatures of 350 °C and higher, limiting the materials that could be used in the processing. So imec’s contribution was an indium-based solution that could be annealed at 250 °C. This let them deposit this stuff on flexible substrates. While not performing as well as sputtered films, they still achieved mobility over 2 cm2/Vs.

Just as n-type organic devices have left something to be desired, so metal-oxide p-type devices aren’t so great – better materials and a wider processing window are needed. But it’s possible to use the inorganic n-type with the organic p-type to build complementary circuits.

The process starts with the gate contacts and Al2O3 high-K gate at the bottom of the stack, and then lays down the metal-oxide layer, with metal over that for the source/drain contacts. The pentacene organic film is then laid down, contacting that same metal layer from above for its source and drain.

More details can be found in their paper, “Low-temperature and scalable complementary thin-film technology based on solution-processed metal oxide n-TFTs and pentacene p-TFTs” published in Organic Electronics on 8/26 of last year.

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