Current OFETs

The development of microelectronics cannot be envisaged without a comprehensive modeling of the devices. The modeling of OFETs is currently hampered by several features. First, charge transport in organic semiconductors is still not completely understood. The situation is clear at both ends of the scale. In high mobility materials (//>IOcnr V-1 s l), transport occurs within delocalized levels when temperature... 

Figure 14-22. Variation of the square root of the saturation current as a function of the gate voltage for a typical lightly doped DH6T OFET.

Figure 14-27. Drain current-voltage charaeieristics of a doped DH6T OFET sliowiug both accumulation (V cO) and depletion (Fx>0) regimes.

For an OFET, the dependence of the drain current / on the source-drain voltage Vd and the source-gate voltage Vb is described hy the classical equations derived from inorganic-hased thin him transistors (Horowitz, 1998) ... 

An important parameter, from the application point of view, in OFETs is the on-off current ratio. For pentacene this ratio can be as high as 10 near Vx = 0 (Lin et al, 1997), comparable to values typically obtained for hydrogenated amorphous silicon, making pentacene suitable for display and other low-voltage applications. 

When a third electrode is added to the structure shown in Fig. 8.12a, below the substrate, it creates an electric field in the insulating substrate, which is perpendicular to the path of the current between the two contacts to the selective layer. Such a structure became known as the Organic Field-Effect Transistor (OFET). It is discussed here and not in Chapter 6 because it belongs to the family of organic chemiresistors. 

It has been known that operation of OFET is affected by the chemical ambient, namely by the presence of oxygen and humidity. This problem has been conveniently turned into an advantage by promoting OFET as new type of chemical sensor for gases and vapors (Torsi et al., 2000). They lack an explicit quantitative relationship between the concentration of the detected species and the device output. The results are invariably given as change of current or relative change of current. ... 

In Fig. 13.7 the experimental drain currents are compared with modeled characteristics. Below, the simple nine-term OFET model that is used here will be outlined. Three regimes of operation are distinguished ... 

Fig. 17.1. OFET and current-drain voltage plot, current in amps, potential (negative) in volts, gate voltages 0-100 V in 20-V increments, for a diphenylbithiophene oligomer on Si/Si02.

Using naphthalenetetracarboxylic dianhydride as the semiconductor, changes in bulk conductivity, field-effect mobility, and threshold voltage were separately observed in response to exposure to water and oxygen [35, 36], Another more elaborate kind of pattern was produced by a virtual array of eleven different semiconductor OFET monitoring on-current in response to polar and nonpolar organic vapors [37]. Responses (0.8-0.3-fold reductions and 1.5-2-fold increases) were dis-... 

As the structures and current-voltage characteristic of OFETs have been well described in many other articles [8-13], herein we only give a very simple introduction on the basics of phthalocyanine-based OFET devices. 

Fig. 3 Schematic diagram of current-voltage characteristics of (a) p-type, (b) n-type, and (c) ambipolar OFET device...

CuPc thin films, and the enhanced physical connection between source-drain electrodes and semiconductor channel associated with the PMMA polymer layer, the OFET performance of this bottom-contact device was significantly improved with leakage current being reduced by roughly one order of magnitude and on-state current enhanced by almost one order of magnitude. The hole mobility of this bottom-contact OFET device reached 0.01 cm2 V-1 s 1, which is comparable with that of top-contact device but much higher than that of normal bottom-contact device without polymer layer [45],... 

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