InGaZnO shows transparent transistor promise
Researchers at the Tokyo Institute of Technology have fabricated transparent flexible thin-film transistors (TFTs) containing an amorphous indium gallium zinc oxide (InGaZnO) active channel (see Nature 25th November).
TFTs are fundamental building blocks for state-of-the-art microelectronics, such as flat panel displays. The devices, when combined with transparent circuit technology, could lead to applications such as integrating display functions onto car windscreens.
InGaZnO is not the first material to be investigated for flexible electronic applications, but its field-effect mobility of 5.6 cm2V-1s-1 is significantly higher than today s well-established alternatives, hydrogenated amorphous silicon (about 1 cm2V-1s-1) and pentacene (1.5-2.7 cm2V-1s-1). InGaZnO is also optically transparent from 390-3200 nm, and so unlike silicon-based devices it is suitable for fabrication of transparent circuits.
The Japanese team formed the active InGaZnO layer by pulsed laser deposition, but they believe that a sputtering or MOCVD method could be used for large-area uniform deposition and mass production.
The top-gate flexible transparent TFTs contained an InGaZnO n-channel active layer deposited on a 200 µm-thick polyethylene terephthalate substrate. Standard photolithography and lift-off techniques defined the source, gate and drain contacts.
The TFT device s behavior agreed with standard transistor theory. The devices were then tested when flexed, by bending the TFT sheet into a curve with a radius of 30 mm. The performance was "almost unaffected by bending, although a slight decrease is observed in the saturation current", according to the researchers.