Organic field-effect transistor

The organic field effect transistor ( OFET) is a field effect transistor (FET) that uses at least an organic material as a semiconductor.

History

Hideki Shirakawa discovered in 1976, Alan MacDiarmid and Alan Heeger ( Nobel Prize in Chemistry 2000) with chlorine or bromine in oxidized polyacetylene increased by a factor of 109 conductivity (ie to 103 S / cm). The organics were thus in regions of the conductivity, which would otherwise be present only in semiconductors or metals (copper: 106 S / cm). This effect was completely unknown until then, because all polymers hitherto considered insulators.

The discovery of organic semiconductors allows for new applications of eg LEDs ( OLEDs), displays, solar cells, integrated circuits and electronic price signs hope.

The primary advantages are its low weight, mechanical flexibility and the hoped-for lower price compared with conventional semiconductor devices based on inorganic materials such as silicon. The lower price, so it hoping for the researchers is to be achieved by a simpler manufacturing process (eg, spin coating or printing). Problems are far short lifetime ( sensitivity to oxygen and water) and a low operating frequency (resulting from the low charge carrier mobility ) of the components.

Construction

As inorganic field-effect transistors also have the three terminals OFETs source, gate and drain. Most of them are similar to MOSFETs fabricated as a thin film transistor in which the semiconducting layer is only a few nanometers thick. As with the MOSFET, the electric potential of the substrate (bulk) is also important in organic thin film transistors, and is analogous to these can be seen as a fourth connection.

As a semi-conductive layer, various organic materials are especially suitable. Thus, both polymers and oligomers are (for example, poly (3- hexylthiophene ) ), as well as small molecules (English small molecules ), such as pentacene, tetracene used. Recent research has shown that natural substances such as indigoids or anthraquinones can be used in field-effect transistors.

In the current state of research of organic field- effect transistors, an oxidized silicon wafer is used as substrate usually. This structure is already well investigated, and the layer properties can also be very well controlled. Thus, many foreign influences that might occur with new coating systems can be prevented. As the drain and source electrodes of gold is frequently used, since the work function of gold is in the range of the work functions of the organic materials. As potential barriers can be minimized at the interfaces.

In addition to the structure described above, there are also so-called All- Organic - OFETs consisting entirely of organic materials. These are located on a substrate made ​​of PET, PEN, or PVC film. Are for the conductive members, as mentioned above, the polymers polyaniline, polythiophene or polyparaphenylene considered. For the dielectric (insulator ), the polymers PMMA, PHS / PVP and other polymers, such as melamine resins, or phenols are suitable.

Since the organic semiconductors are sensitive to water and oxygen, they must be protected against them. As so-called encapsulation or barrier layers are inorganic as well as organic materials in the conversation. In the long term is attempted organic encapsulation layers, which are still flexible to develop. However, there are currently no polymers that are sufficiently impervious to oxygen and water vapor, in order to ensure required for production lives. Therefore, layer systems composed of different materials are also possible.

Physical Description

Organic semiconductors are due to their oxidation behavior in large part p-type; n- conductors are usually unstable.

Organic field-effect transistors are usually operated in accumulation, i.e., the majority charge carriers are pulled by an electric field to the semiconductor - insulator interface layer (field effect). This field is generated by the gate voltage. The thus enriched charge carriers can be moved through the drain-source voltage across the semiconductor -insulator interface. The organic field effect transistor therefore behaves similarly to normal inorganic field effect transistors. Modeling the field effect transistor known from classical MOSFETs formulas can be used as a good approximation.

In the linear range applies:

Wherein the drain current, the area-normalized capacity of the insulator, the charge carrier mobility and the threshold voltage is.

In the saturation region, a quadratic dependence between drain current and gate voltage is:

However, this applies only to well-crystallized semiconductor, which have usually 2-3 orders of magnitude lower charge carrier mobilities than silicon. In poor crystallized organic semiconductors gives way to the charge transport from strong and can no longer be explained by the energy band diagram. The resulting lower vagina, such as a voltage-dependent transconductance stage, must be observed in the transistor model.