Organic field effect transistor

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

history

In 1976 Hideki Shirakawa , Alan MacDiarmid and Alan Heeger ( Nobel Prize in Chemistry 2000) discovered that polyacetylene oxidized with chlorine or bromine had a conductivity increased by a factor of 10 ⁹ (i.e. to 10 ³  S / cm). The organic substances came into areas of conductivity that are otherwise only found in semiconductors or metals (copper: 10 ⁶  S / cm). This effect was completely unknown until then, because all polymers were previously considered to be insulators .

The discovery of organic semiconductors suggests new areas of application, e.g. B. LEDs ( OLEDs ), displays, solar cells , integrated circuits and electronic price tags hope.

The main advantages are the low weight, the mechanical flexibility and the hoped-for lower price compared to traditional semiconductor components based on inorganic materials such as silicon. The researchers hope that the lower price will be achieved through a simpler manufacturing process (e.g. spin coating or printing). So far, problems have been caused by the short service life (sensitivity to oxygen and water) and a low operating frequency (resulting from the low charge carrier mobility ) of the components.

construction

Schematic structure of an OFET in cross section ( thin-film transistor ). In this variant, the entire (highly doped) substrate is used as a gate electrode, "bottom gate" OFET. This arrangement can only be used for discrete components or for test structures

Like inorganic field effect transistors , OFETs also have the three connections source, gate and drain. Similar to MOSFETs , they are usually manufactured as thin-film transistors in which the semiconducting layer is only a few nanometers thick. As with MOSFETs, the electrical potential of the substrate ( bulk ) is also important for organic thin-film transistors and is to be seen as the fourth connection analogous to this.

Various organic materials can be used as the semiconducting layer. Thus, both coming polymers and oligomers (eg. B. poly (3-hexylthiophene) ) as well as small molecules (engl. Small molecules ) such. B. pentacene , tetracene are used. Recent research has shown that natural substances such as indigoids or anthraquinones can also be used in field effect transistors.

In the current state of research on organic field effect transistors, an oxidized silicon wafer is usually used as the substrate . This structure has already been very well investigated and the layer properties can also be controlled very well. In this way, many external influences that could occur with new shift systems can be prevented. Gold is often used as drain and source electrodes, since the work function of gold is in the range of the work functions of organic materials. This minimizes potential barriers at the interfaces.

In addition to the structure described above, there are also so-called all-organic OFETs, which consist entirely of organic materials. These lie on a substrate made of PET , PEN or PVC film. As mentioned above, the polymers polyaniline , polythiophene or polyparaphenylene are suitable for the conductive components . The polymers PMMA , PHS / PVP and other polymers such as melamine resins or phenols are suitable for the dielectric ( insulator ) .

Since organic semiconductors are sensitive to water and oxygen, they must be protected from them. Inorganic and organic materials are being discussed as so-called encapsulation or barrier layers. In the long term, attempts are being made to develop organic encapsulation layers that are also flexible. At the moment, however, there are no polymers that are sufficiently impervious to oxygen and water vapor to guarantee the lifetimes required for production. Therefore, layer systems made of different materials are also conceivable.

Physical description

Organic semiconductors are largely p-conductive due to their oxidation behavior; n-conductors are usually unstable.

Organic field effect transistors are usually operated in accumulation , i.e. That is, majority charge carriers are drawn to the semiconductor-insulator boundary layer by an electric field (field effect). This field is generated by the gate voltage . The charge carriers enriched in this way can be moved along the semiconductor-insulator boundary layer by the drain-source voltage . The organic field effect transistor therefore behaves in a similar way to normal inorganic field effect transistors. To model the field effect transistor, the formulas known from classic MOSFETs can be used as a good approximation. ${\displaystyle U_{\mathrm {GS} }}$ ${\displaystyle U_{\mathrm {DS} }}$

In the linear range:

${\displaystyle I_{\mathrm {D} }={\frac {W}{L}}C_{\mathrm {i} }\mu \left(U_{\mathrm {GS} }-U_{\mathrm {T} }-{\frac {U_{\mathrm {DS} }}{2}}\right)U_{\mathrm {DS} }}$,

being the drain current, the area-normalized capacitance of the insulator, the charge carrier mobility and the threshold voltage. ${\displaystyle I_{\mathrm {D} }}$ ${\displaystyle C_{\mathrm {i} }}$ ${\displaystyle \mu }$ ${\displaystyle U_{\mathrm {T} }}$

In the saturation area there is a quadratic dependence between drain current and gate voltage:

${\displaystyle I_{\mathrm {D} }={\frac {W}{2L}}C_{\mathrm {i} }\mu (U_{GS}-U_{\mathrm {T} })^{2}}$

However, this only applies to well-crystallized semiconductors, which usually have charge carrier mobilities 2–3 orders of magnitude lower than silicon. In the case of poorly crystallized organic semiconductors, the charge transport deviates significantly and can no longer be explained by the band model. The resulting differences, such as a voltage-dependent forward slope, must be taken into account in the transistor model.

See also

• Organic electronics

Individual evidence

1. ↑ Hideki Shirakawa, Edwin J. Louis, Alan G. MacDiarmid, Chwan K. Chiang, Alan J. Heeger: Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH) x . In: J. Chem. Soc. Chem. Commun. No. 16 , 1977, pp. 578-580 , doi : 10.1039 / C39770000578 .
2. ↑ ED Głowacki L. Leonat, G. Voss, M. Bodea, Z. Bozkurt, M. Irimia-Vladu, S. Bauer, NS Sariciftci: Natural and nature-inspired semiconductors for organic electronics . In: Proceedings of SPIE . No. 8118 , 2011, p. 81180M-1 , doi : 10.1117 / 12.892467 .