Pentacene Organic Field Effect Transistors on Flexible substrates with polymer dielectrics S.P. Tiwaril, V. Ramgopal Raolt, Huei Shaun Tan2, E. B. Namdas2, Subodh G Mhaisalkar2 'Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai - 400076, India 2School of Materials Engineering, Nanyang Technological University, Nanyang Avenue-639798, Singapore tEmail: rrao eejitb.acin, Phone: 91-22-25767456 ABSTRACT EXPERIMENTATION Pentacene Organic Field Effect Transistors are fabricated using Figure 1 shows the device structure of pentacene OFET on polymethyl methacrylate (PMMA) as a gate dielectric on flexible flexible Substrates. The ITO coated PET substrates (commercially polymeric substrates like the Indium tin oxide coated polyethylene available from Aldrich Chemicals) are cut into 2.5 cm square pieces, terephthalate (ITO coated PET), and polyethylene napthalate (PEN) cleaned by ultrasonic bath, DI water and IPA followed by a blow dry in top contact configuration. The ITO and aluminium (evaporated on process. The substrates are glued to glass pieces of the same size PEN) act as the gate materials for the OFETs. The devices on ITO with the help of small water drops before dielectric spin coating. coated PET show electron mobility up to 0.07 cm2/V-s and the on/off PMMA (450K) 4wt% in anisole is spin coated at 800 rpm for 60 ratios in the order of 103. seconds to get a thickness of about 500 nm. Pentacene (45 nm) is evaporated as semiconductor. The gold S/D electrodes are deposited INTRODUCTION by shadow masks for top contact structure. For the pentacene OFETs on PEN substrate, Al is evaporated to act as Bottom Gate. The Organic Field Effect Transistors (OFETs) are being investigated substrates are glued on glass pieces with the help of thermo tapes on for a number of low-cost, large-area applications, particularly those the corners. The rest of the process details of PMMA spin coating, that are compatible with flexible plastic substrates [1, 2]. The organic pentacene and gold evaporation are kept identical. materials that have been used as active semiconductor materials include both vacuum evaporated (such as pentacene) and solution Pentacene Au S/D processed semiconductors. However, the vacuum evaporated (45nm) (50nm) pentacene is widely preferred because of its multi crystalline nature which results in higher mobilities and current on/off ratios [3,4]. Development of low temperature processes can make way for PMMA (-500nm) organic FETs to be integrated on low cost plastic substrates. Considerable progress has been made in this area in the recent years, ITO coated PET or though achieving high carrier mobilities and On/Off ratios with Al deposited PEN Substrate organic (polymeric) dielectric materials still remains a challenge. Though many polymeric dielectrics have been employed as gate FIGURE 1 Pentacene OFETs on flexible substrates (ITO coated dielectric, most of the studies are still made with highly doped silicon PET) with polymer (PMMA) dielectric and ITO as gate or a as the gate material. In Polymeric dielectrics, Poly Vinyl Alcohol flexible substrate PEN with Aluminum as gate. (PVA) and cross-linked PVA are studied as gate dielectric materials for pentacene OFETs on PET substrate. However, these dielectrics show poor breakdown characteristics as well as very low immunity The freshly prepared devices are transferred into a nitrogen glove against moisture. A PMMA buffer layer insertion noticeably box containing a micro-probe station, and connected to a HP 4155C improves the performance of these OFETs [5]. Pentacene thin-film system to perform the electrical measurements. The gate was transistors with PMMA as a gate dielectric on a silicon substrate are probed through the dielectric for measurements. For each reported to show good electrical performance, with mobilities around device, the output characteristics (IDS VS. VDS at multiple VGS values 0.01 cm2 /V-s with threshold voltages around 15 volt [6]. It has also from OV to VDD) and transfer characteristics (IDS VS. VGS at multiple been shown that PMMA seems to influence the pentacene thin-film VDS from OV to VDD) were measured. For most of the devices VDD microstructure, increasing the crystallinity which results in a higher was kept constant at -20V. Field-effect mobility and threshold field effect mobility of OFETs [6]. Pentacene OFETs with a voltages were calculated in the saturation regime from the saturation PMMA/Ta2O5 bilayer gate insulator are reported to achieve both region current equation of standard MOSFET, using highest slope of stable and low operating voltage devices. Ta2O5 operates at very low IDS1/2 VS. VGS plot. The current equation of a FET in saturation is: voltages but shows a lack of stability, whereas PMMA, although 1 W _V working at high gate voltages, exhibits a remarkable stability [7]. IDS =1iC0X (VGS oxVT ( 2 L However, in order to realize low cost and large area electronics where pt is the field-effect mobility, Cox is the capacitance per with organic and polymeric materials, the silicon (Gate/Substrate) unit area of the gate dielectric, VT is the threshold voltage, and W and 5i02 (Gate dielectric) need to be substituted with a polymeric (width) and L (length) are the dimensions of the semiconductor substrate and organic dielectric material respectively. Here, we channel defined by the source and drain electrodes of the transistor. present the study OFETs using PMMA as a gate dielectric material, The dielectric constant of PMMA is 3.6. and thermally evaporated pentacene as an active layer. The flexible substrates used for this study are substrates with ITO coated PET and RESULTS & DISCUSSION Al evaporated PEN. Figure 2 shows the output and transfer characteristics of a device with ITO coated PET (ITO as gate) at room temperature. The device 1 -4244-0585-8/07/$20.OO ©C2007 I EEE Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 3, 2008 at 04:18 from IEEE Xplore. Restrictions apply.