The (r)evolution of thin film transistors
By: Elvira Fortunato
From: Univ. Nova Lisboa
At: Complexo Interdisciplinar, Anfiteatro
[2010-05-05]
We are currently experiencing a fascinating scientific period in the area of thin film transistors using non conventional materials,like oxide conductors and semiconductors. Conductive and semiconductive transparent oxides are a special class of materials, because transparency and conductivity are some what contradictory with each other.
There are two major conditions to be met for a material to be transparent to visible light: the band gap should be above 3 eV and the carrier concentration (free electrons or holes) should be below 2021 cm-3. The band gap is related to how easily the material absorbs electromagnetic waves at what frequencies. Carrier concentration, on the other hand, is related to the plasma frequency, which determines the boundary between reflected electromagnetic frequencies and those which are passed (transparency) or absorbed.
In general a wide band gap material means a low carrier concentration, which makes the material an insulator, like glass. ZnO is an exception because while it is highly transparent it is also a good conductor. Usually dopants are used to supply the carriers needed to make many oxides electrical conductors. ITO, for example is doped with Sn to achieve carrier concentration of the order of 1021 cm-3.
Amorphous InGaZnO4 (a-GIZ O) on the other hand, was created with the objective to increase the carrier mobility(the ease with electrons move) and decrease the carrier concentration, making possible the production of high performance thin film transistors (TFTs).
In this seminar, we start with the beautiful history about thin film transistors along with the major developments achieved for the almost last 80 years, specially focused on the different type of semiconductor material is used. After that we will present some results obtained in our laboratory either on passive and active applications especially for amorphous IZO and GIZO films,respectively. We will finish with some emerging devices based on cellulose, namely the paper transistor . We hope with this seminar to motivate new researchers to initiate their work in this so exciting field as well as to contribute with one more step to the progress of Transparent Electronics in the world.