Off-currents

FET is ihe on-off current ratio, which indicates its ability to shut down the current, and is particularly relevant in applications such as active matrix displays and logical circuits. Because of the presence of p-n junctions at both the source and drain electrode, the on-off ratio of MOSFETs is in the I0y range [12J, while that of a-Si H TFT is limited to 106 [13], High mobility ensures high on-current and, hence, also contributes to a high on-off ratio. 

The inkjet-printed (or inkjetted ) TFT operated with a mobility of 6.5cm2/ Vs and an on/off ratio of three digits as shown in Fig. 5.6b. This low mobility is attributed to the poor crystallinity and rough surface, whereas the large off current, which was confirmed to be the current between source and drain rather than a leakage current through the gate insulator, is also attributed to... 

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. 

The first successful application of this method was reported by Mullen and coworkers [18]. The removal of their solubilizing group occurs thermally at temperatures as low as 180 °C, yielding pentacene and a volatile tetrahalobenzene. The tetrachloro compound 4 (Fig. 3.3) formed good-quality films from spin-cast solution, and heating the film of the soluble precursor at 200 °C for 5 s yielded the desired acene film. FET properties for devices made from solution-cast films were quite good - measured mobilities were as high as 0.2 cm2 V-1 s-1 with on/off current ratios of 106. 

Fig. 5.9. On and off currents of TFT as a function of time in the dark in dry, purified air for (a) P3HT and (b) PQT-12. These devices were fabricated in a coplanar geometry and the semiconducting polymer was unencapsulated.

Figure 12.9 shows the output and transfer characteristics of a state-of-the-art, polymer FET fabricated using the Plastic Logic direct-write manufacturing process (L = 10 pm). No encapsulation of the TFT is present other than what is naturally provided by the presence of the PET substrate on the bottom and an inkjet printed silver gate electrode on the top. The device exhibits a field-effect mobility of 0.04 cm2 V 1 s 1 and ON-OFF current ratio of 5 x 10s. The threshold potential is Vt = 5-6 V. These basic TFT performance values can be achieved consistently in a manufacturing environment, and are sufficient to drive a 100 dpi electronic paper display with A5 size. 

Variations of the ON and OFF current of an active matrix array of 100 OTFTs on a PET substrate. (Reprinted with permission from Ref. [72]). 

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