|Organic Thin-Film Transistors|
A transistor is an electronic valve or switch, where the current flow between the source and drain electrodes is controlled by the magnitude of the or electric field applied at the gate, known as the gate bias. The charge flow in the transistor channel can be dominated by holes (positive charges) or electrons (negative charges), which define whether the semiconductor is p-type or n-type, respectively.
The most common type of transistor is the field-effect transistor (FET). An FET relies on an electric field to control the conductivity of a "channel" of one type of charge carrier in a semiconductor material. A thin-film transistor (TFT) is a special kind of FET made by depositing thin films of materials in a layered configuration known as the stack.
A TFT is a three-terminal device, composed of a source, drain and gate electrodes, a dielectric (insulating) layer, and a semiconducting layer:
Currently a common substrate on which to build OTFTs is glass, since the primary application of OTFTs is in liquid crystal displays. With the increasing demand for flexible electronics, plastic substrates such as PEN are becoming more common. This differs from the conventional FET, where the semiconductor material such as silicon is the substrate.
In OTFTs, there are four different configurations or architectures as determined by the locations of the gate as well as the source and drain within the material stack:
Top Gate, Top-Contact
Top Gate, Bottom Contact
Bottom Gate, Bottom Contact
Bottom Gate, Top Contact
With each architecture having its pros and cons, the choice of device architecture often has real consequences. For example, TGBC devices offer a large injection-face (i.e. the surface from which the charge-carriers leave the source and enter the semiconductor). However, processing-wise, BGBC may be preferred, since the devices with the semiconductor layer as the last one has historically been the easiest to produce. This is due to that the semiconductor layer does not have to be exposed to the potentially damaging chemicals needed to process subsequent layers.
For transistors, the two most important performance parameters are the charge carrier mobility (u - how fast holes or electrons moves) and the current on-off ratio (Ion:Ioff - how efficient the current can be modulated by the source-gate bias). Furthermore, to maximize the transistor speed, the carrier mobility should be as high as possible and the distance between the source and drain electrodes (channel length) should be as short as possible.
When evaluating the performance of an OTFT device, there are three main parameters to consider:
OTFTs can be used for a wide variety of applications, including display backplanes and integrated circuits for lighting, sensors, RFID tags, and any application where logic circuitry is used. Learn more.
Presently, most OTFTs are made with p-type semiconductor materials, as p-types have far superior performance. However, with the recent advances in n-type semiconductors, it is now possible to create circuits using both p- and n-type transistors using a technology known as CMOS. CMOS results in circuitry that is easier to design, cheaper to produce, and more energy-efficient. Learn more.
Despite much progress made in OTFTs - especially the discovery of high-performing n-type materials to enable CMOS circuits - significant challenges still remain: