Permeable gate Transistor for digital Printing Applications

A novel permeable gate transistor was designed for use in high-resolution, high-voltage transistor backplanes for digital printing applications. While standard designs for digital xerography require thinfilm transistors (TFTs) capable of switching over 5001500V, with this improvement, TFTs would only be required to switch over 5V, improving resolution and ease of manufacturing. Conducting polymers used in the device are incompatible with polar solvents and thus most conventional photoresists, and were consequently patterned using a fluorinated resist system designed for patterning organic films. The finished devices were electrically tested and show behavior consistent with expectations. Summary of Research: Current laser printers use xerography for rapid and inexpensive printing. Xerography is essentially a photographic process by which a charged surface is selectively discharged using light from a laser, creating a pattern that can be transferred onto paper via charged toner particles [1]. This process can be fully digitized through the use of a backplane of addressable thin film transistors (TFTs), which replace the laser in selectively compensating the surface charge [2]. However, this requires TFTs that are capable of switching over 500-1500V, which would have large footprints and be difficult to fabricate [3]. Our solution to this problem is illustrated in Figure 1. Holes are injected from a hole-injection layer into a charge transport layer (CTL), which is hole-transport only. A grounded metal mesh in the CTL collects some of the injected holes, but allows others to pass, and get collected by the top electrode. In this configuration, the TFTs are only required to actuate over 5V to turn the device on and off. This allows the TFT design to be borrowed from current display technology [4]. Devices were fabricated on oxidized 4-inch Si wafers. Figure 2 shows the film stack, in the order of deposition and patterning. Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which is known to form a diode with the CTL material, was chosen as the hole-injector, while aluminum was chosen to form the gate and collector. The CTL material is a proprietary conducting polymer from Xerox. Metals were deposited by thermal evaporation and patterned using lift-off. The PEDOT:PSS and CTL material were spun on to the wafer in solution and patterned using an O2-plasma etch. The key challenge to the fabrication of these devices was the incompatibility of the PEDOT:PSS and CTL material with conventional photoresists. The electronic properties of the Figure 2: Cross-sectional schematic of device, showing materials and film thicknesses.