PENTACENE TFTs

Gundlach DJ, Jia LL, Jackson TN (2001) Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes. IEEE Electron Dev 22 571-573... 

Optimization of pentacene TFTs for mobility has led to some very promising results, but implementation of pentacene circuits in product applications has been limited, because of poor understanding and limited data on operational and shelf life stability in different circuit architectures. A notable exception to this is the implementation of Philips pentacene active-matrix backplanes in Polymer Vision s... 

Fig. 2.6. Some representative methods for effective surface-treatment preparation of both SAMs and polymers to improve the performance of pentacene TFTs.

The Infineon group reported a pure polymer dielectric which has also been shown to improve pentacene performance, both as an unmodified polymer, and with a silane treatment performed on it [7b]. In addition, Samsung SDI has recently reported a proprietary polymer dielectric which enables them to achieve high mobility in pentacene TFTs [12]. Table 2.2 emphasizes some of the reports of increasing pentacene mobility. 

Tab. 2.2. Reports of elevated mobility in pentacene TFTs from the past 10 years. The table shows that although very high mobility is still rather rare, mobility exceeding 1 cm2 V-1 s-1 has been widely reported [53],...

There have been many reports of elevated mobilities in pentacene TFTs. Table 2.2 summarizes some of these results, some of which have been discussed in the text. New reports appear frequently, continuing to push the limits for high-mobility pentacene devices. 

Fig. 6.9. Statistical distribution of (a) mobility, (b) switch-on voltage, and (c) subthreshold swing of 49 pentacene TFTs on the same substrate.

Fig. 6.12. Dependence of hysteresis in pentacene TFTs on exposure to UV light (Ushio model UXM-501XD lamp, 365 nm, 7 mW cm-2).

Figure 6.15 shows the electrical characteristics of a pentacene TFT with top-contact configuration. The transistor has a carrier field-effect mobility of... 

Fig. 8.12. Schematic cross-section of an organic pentacene TFT on metallized PET film.

Fig. 8.14. Electrical characteristics of a pentacene TFT and output signal from a five-stage pentacene ring oscillator, both fabricated on reel-to-reel aluminized PET.

All these calculations show that pentacene TFTs are feasible for active-matrix OLED display backplanes. For the Penn State/Kodak practical realization mentioned above, a 48 x 48 pixel bottom-emission display panel was designed on a 64 mm x 64 mm glass substrate. To obtain good yield, a design rule of 10 pm was used for the minimum feature size (line width or separation) for most structures on the test panel. The coarseness of this design rule is not related to the use of organic compounds, but rather to the simplicity of photolithographic processes. 

Fig. 15.6. Characteristic of a PVA-patterned pentacene TFT (W = 240 pm, L = 10 pm), showing relatively poor subthreshold slope, but relatively good on/off ratio in the...

Passivation is needed to insulate the backplane from the OLED stacks everywhere except the ITO and bonding contact areas. Unlike poly-Si and a-Si H backplanes, on which both organic and inorganic passivation layers can easily work, the device passivation technique needs extra consideration for pentacene TFTs. We explored several different materials for passivation of pentacene TFTs, including poly(vinyl alcohol) (PVA), room temperature plasma-enhanced chemical vapor deposition silicon nitride (RT PECVD SiN), and vapor-deposited parylene. 

RT PECVD SiN has been tried as the passivation layer for pentacene TFTs. The device was measured and compared before and after the deposition. Figure 15.7 shows TFT drain current as a function of gate-source voltage before and after passivation. The TFT performance is greatly degraded from the SiN deposition - the... 

Fig. 15.9. Bottom-contact pentacene TFT characteristics before and after parylene passivation. The device has W/L = 400/80 pm. The passivation degrades the mobility whereas the threshold potential is relatively steady.

Although there are claims that parylene can passivate pentacene TFTs without degradation [18], this has only been shown where initial mobility before the passivation was less than 0.1 cm2 V 1 s 1. On our devices, for which starting mobility is uniformly much higher, the results reveal the degradation effect of parylene on good pentacene TFTs. 

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