OFET Performance Characteristics

Even though the origin of the improvement in OFET device characteristic upon a thermal treatment seems unveiled it is still unclear what causes the enhancement due to electrical cyclic stress. Flowever, additional experiments have shown that a electrical cyclic conditioning of annealed transistors does not yield a significant additional improvement in the device performance. We therefore suggest that the observed OFET improvement for both device treatments, have the same consequence. This implies that the electrical cyclic conditioning also leads to a promoted oxidation of the metallic fraction in the Ca passivation, as discussed for the substrate aimealing. 

Fig. 33. The pattern for drop-cast CP(Ni) displays no measurable peaks. Upon heating, numerous diffraction peaks indicative of the formation of crystal planes appear. OFETs output characteristics are shown in Fig. 34, the mobility of this transistor being of the order of 0.1 and 0.2 cm /Vs, which is the highest value among solution-processed OFETs using porphyrins or Pcs as semiconductors. OFETs based on TBP(Cu) exhibit a similar performance with the mobiUty of 0.1 cm /Vs. A polarized optical micrograph of a spim-cast TBP(Cu) thin film is shown in Fig. 35 and the polycrystalline nature of the TBP(Cu) thin films is displayed. The electrodes are 20 pm wide, indicating...

Due to the great potential applications and the widespread interests of OFETs, several review articles with emphasis on different aspects of OFETs have been published [8-13], However, despite the extensive studies and increasing research interests in phthalocyanine as well as porphyrin semiconductors, there is still no review article that systematically generalizes the research achievements in the field of tetrapyrrole organic semiconductors for OFETs. Thus, the present contribution should interest scientists in both industrial and theoretical fields. The main contents of this chapter is as follows Firstly, some basic introduction of the structure and characteristics of OFET is provided then theoretical factors that influence the performance of OFET devices is discussed finally, the progress in phthalocyanine-based OFETs in the order of p-type, n-type, and ambipolar semiconductors is summarized. 

Analogous to the electrical conditioning, the OFET characteristics can be improved by a thermal treatment prior to the deposition of the source-drain metallisation. The improvement in the device performance can be seen in the insets of Figures 24.8 and 24.9, showing the OFET parameters, measured at RT, for devices exposed to different annealing temperatures for / = 1 h. The examined devices comprised a 8 A Ca passivation layer. It can be noted that an annealing temperature of 160 °C resulted in a device performance, comparable to the one obtained by the electrical cyclic conditioning. 

The excellent performance of the OFETs was also confirmed by results of downscaling in the sub-pm range. Characteristic transistor behaviour was observed for channel lengths down to 180 nm [23]. 

As already mentioned above, the transfer characteristics of an OFET structure can be tested both in the BOC and TOC configuration. However, when performing our measurements in the BOC design using gold contacts resulted in no drain current at all. A probable reason for this is the channel for-... 

Finally, the electrical performance of an OFET-structure made of vacuum deposited DCNDBQT, which form a closed packed and dense molecular layer, was tested. The output characteristics of the OFET clearly show the behaviour of a p-type semiconductor. The resulting field mobilities of 10 cmWs reflect a high current density as interpreted in the framework of the ultra-thin molecular QFET structure. The sensitivity of the ultra thin material towards the environment neeessitates that in the future, the OFET has to be produced and measured under vacuum-like conditions to the exclusion of air, which may be realised by appropriate passivation processes. 

OFET. The/c of the OSIT is approximately 3000 times higher than that of the OFET. These results indicate that device structure, such as the channel length and the effective area, have a significant effect on the device characteristics, and also that the OSlTs have excellent device performance. 

The device structures, operations of OFETs, and estimation of the FET characteristics from current-voltage curves have been summarized previously [2]. The important parameters to evaluate the quality of an OFET are carrier mobility /d), on/off ratio, and threshold voltage 8 performance of the transistor... 

Transient experiments were also performed on pentacene OFETs with tantalum oxide as the dielectric. The channel length used was 50 j,m, so that the effect of the initial displacement current can be eliminated. Fabrication scheme A was used in making the OFETs. The substrates were treated with vapors of HMDS. The HMDS treatment resulted in decreased threshold voltage, and an increase in lon/Ioff ratio by more than an order of magnitude [33]. The steady state characteristics of the transistor are shown in Fig. 15a. The saturation region mobility obtained from the FET... 

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