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Flexible
electronics and in particular flexible thin-film transistors are key
candidates for integration into textiles. Amorphous Indium-
Gallium-Zinc-Oxide (IGZO) was presented for the first time in 2004

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as a
semiconductor material for thin-film transistors (TFTs) on flexible
plastic substrates. IGZO TFTs are beneficial for fabrication on
flexible plastic substrates, because IGZO can be sputtered at room
temperature while the process temperatures to fabricate drain,
source, gate contact and gate insulators are below 150 C. Process
temperatures not exceeding 300 C are imperative due to temperature
stability of for example polyimide foils. Furthermore, the electron
mobility of room temperature sputtered IGZO is about 10 times larger
than the electron mobility of amorphous silicon . IGZO TFTs can be
designed to operate at 5 V with a threshold voltage around 0.5 V.
Since IGZO TFTs can be fabricated on flexible plastic foils, the
impact of bending and hence applying strain in the TFT layers is
investigated . Bending IGZO TFTs is mainly motivated to enable
flexible displays. Bending radii as small as 3 mm corresponding to
0.7 % strain in the TFT layers are reported. Due to the higher
electron mobility of IGZO TFTs compared to TFTs made of amorphous
silicon, faster switching speeds of the tran- sistors are possible
and the fabrication of circuits becomes attractive. To demonstrate
the switching speed of transistors, ring oscillators are

commonly
applied. In, five-stage and seven-stage ring oscillators were
fabricated and propagation delays per stage of 240 ns and 48 ns are
reported, while operating with supply voltages of 18 V and 25 V,
respectively. Basic circuits such as shift registers, inverters and
NAND gates are presented in 44, 45, 46. 44 demonstrates a shift
register operated at 20 V, while in 45 inverters operating at 10 V
are shown. NAND gates and inverters representing the basic building
blocks for digital circuits are demonstrated in 46. The circuits
operate at 5 V and can be bent to a radius of 3.5 mm corresponding to
a strain of 0.6 %. The system on panel concept, described in 7, to
integrate sensors, actuators and the required control electronics on
a single flexible foil, requires analog circuits and in particular
amplifier circuits. Since mechanical strain changes the
characteristics of single IGZO TFTs, it is important to characterize
the behavior of analog circuits subjected to mechanical strain.

A
thin-film transistor (TFT) is
a special kind of field-effect transistor made by depositing of an
active semiconductor layer as well as the dielectric layer and
metallic contacts over a supporting (but non-conducting) substrate. A
common substrate is glass, because the primary application of TFTs is
in liquid-crystal displays. This differs from the conventional
transistor, where the semiconductor material typically is
the substrate, such as a silicon wafer.

TFTs
can be made using a wide variety of semiconductor materials. A common
material is silicon. The characteristics of a silicon-based TFT
depend on the silicon’s crystalline state; that is, the semiconductor
layer can be either amorphous silicon, microcrystalline silicon,or it
can be annealed into polysilicon.

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