Organic Field Effect Transistors

Other Unique Engineering Ideas

Organic semiconducting polymers are promising electronic materials, but for full versatility they need to conduct negative as well as positive charge. A step towards that goal has now been taken.

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

Description

In general organic field effect transistors (FETs) are thin film transistors of a MISFET (metal-insulator-semiconductor) geometry.(see Fig. 1) The transistor may include an inorganic substrate (which may also serve as Gate electrode), insulator or electrodes.Transistors are sensitive devices where the characteristics of the device reveal information about the intrisic properties of the active materials involved, mainly the semiconductor material. Devices are measured as a function of  field, time, pressure, temperature and atmosphere etc.From the characterisics either in the low-field or the saturation regime material parameters such as conductivity, charge carrier mobility, turning on voltage and on/off ratio are estimated. FET results are compared to those obtained using other methods to give an accurate picture of the charge transport in the material.

Why

The advantages of building computer chips out of things other than silicon are numerous, especially within specific fields like medicine. High temperatures during manufacturing and the fact that silicon chips have to be rigid are factors that make standard processors difficult to integrate into certain medical devices.The essential structure consists of two electrical contacts with a channel of semiconductor between them. The researchers found that by applying a specially tailored pretreatment compound to the contacts before applying the organic semiconductor solution, they could induce the molecules in solution to self-assemble into well-ordered crystals at the contact sites.These structures grow outwards to join across the FET channel in a way that provides good electrical properties at the FET site, but further away from the treated contacts the molecules dry in a more random, helter-skelter arrangement that has dramatically poorer properties--effectively providing the needed electrical isolation for each device without any additional processing steps.The work is an example of the merging of device structure and function that may enable low cost manufacturing, and an area where organic materials have important advantages.In addition to its potential as a commercially important manufacturing process, the authors note, this chemically engineered self-ordering of organic semiconductor molecules can be used to create test structures for fundamental studies of charge transport and other important properties of a range of organic electronic systems.

How

The design of the OFET’s is not common in electronics. The layer sequence chosen from technological reasons is as follows: n+-substrate (as gate) / oxide/ active p-layer (organic). In some cases instead of a gate-substrate one has used a substrate covered by a metallic or graphitic gate (followed again by the insulator and the active layer). The organic layer has a floating potential as e.g. the SOIFET or the new vertical MOSFET.Further, the OFET’s operate in depletion or accumulation (till now it seems to be difficult to achieve inversion) and therefore the layer should be sufficiently thin. Such transistors show several peculiarities. This has been shown by 2D simulations of analogous Si-devices for which, of course, the material parameters are known. The following properties are especially of interest.

(a) Since the OFET operates in depletion and accumulation depending on the gate voltage the surface potential can be shifted by more than twice the bulk potential (e.g. more than 0.8V in Si with 1017 doping, in the wide-gap organics more than 1.5..2V) and therefore the use of a constant Uth is a serious error.

(b) The operation of the device depends sensitively on the ratio of layer thickness and depletion length, which is smaller in the organics than in Si due to the smaller dielectric constant.

(c) In both depletion and accumulation the transverse field reduces the mobility but there is no transverse field at the transition from depletion to accumulation (flat band voltage). This peculiarity results in a hump in the transfer characteristics

(d) If one has accumulation at source and due to a high drain voltage depletion at drain than at some position between source and drain there is no transverse field.

Thus, there is no monotonous decrease of the mobility due to the increasing lateral field from source to drain. Since for the organics little is known till now on the transport properties (compared to Si and other inorganic semiconductors) it is important to have an analytical model which allows to determine these properties by fitting the model to the experimental characteristics.But the development of organic FETs has been hampered by a poor understanding of electronic processes in these materials. The ability of a FET to amplify an electromagnetic signal depends upon the density of the charge carriers (electrons or holes) that can be introduced into its conducting channel.In an organic FET, the conducting channel resides in a nanometer-thick layer at the interface of the device's semiconducting and insulating layers; here, high-density charges can result in the formation of polarons, distortions in the crystal lattice that create energy wells.An energy well can trap an electron much like a divot on a fairway can trap a golf ball, and this can impede the flow of charge-carriers, which in turn can impede the performance of the FET.

Future Trends

Li adds that "organic FETs are already employed in the displays of mobile electronic appliances such as cell phones, but they have a potentially much bigger future in large-area displays, which demand a lightweight and structurally flexible material — as well as in chemical and biological sensors, molecular electronics, organic spintronics, and radio-frequency identification tags. Organic FETs are also offer the advantage of being easy to mass-produce at a very low cost."  

Keywords

Organic FETs, semiconducting, organic spintronics, OFET, semiconducting polymers

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