Thin-film transistors (TFTs) continue to enjoy a wealth of popularity and intensive research interest—some of which is leading to the next-generation of TFTs.

Chances are that if you turned on your laptop this morning, or your LCD (liquid crystal display) TV, you turned “on” a thin-film transistor. These tiny giants have been a pivotal addition to the consumer electronics space, revolutionizing image stability and display resolution levels, since their creation in the early 1960s. All indications are that their future will continue to be bright.

Austin, Texas-based market research firm DisplaySearch, which specializes in flat panel displays, forecasts that in 2007, the total TFT-LCD market will grow 11% to $77.5 billion, capturing the lion’s share (84%) of the flat panel display market. That said, TFTs will still have work to do to infiltrate burgeoning markets such as RFID tags and flexible, electronic paper. Academia and industry have been on the case.

B A penny, seen through an array of 70 indium oxide thin-film transistors on .7 mm-thick glass, demonstrates the transparency of the new transistors developed at Northwestern.

Performance measures
Among the central issues surrounding TFTs has been their charge carrier or electron mobility and ease of manufacturing. Carrier mobility dictates how fast an electrical current travels through the transistor, essentially outlining how fast the switching speed (“on” or “off”) of the transistor can be.

Typically, TFTs, including variants such as organic thin-film transistors (OTFTs), have had carrier mobilities on average 3-4 orders of magnitude less than other transistor varieties, such as MOSFETs or metal oxide semiconductor field effect transistors. Research conducted at the Univ. of Wisconsin-Madison (UWM) may offer a solution. Led by Jack Ma, the UWM team recently demonstrated a set of flexible, TFTs capable of operating at a record-setting speed of 7.8 GHz.

“Until now, flexible TFTs have been relatively slow, operating in the 0.5 GHz range, says Hao ChihYua, project researcher. This is fine for applications such as LCD, but not for applications such as military surveillance antennas that require high-performance but flexible circuitry for easy storage.

To bring their TFTs to higher speeds, the UWM team harnessed nanoscale-thin membranes of single-crystal silicon, which has greater electron mobility and speed. According to UWM, the membranes can be peeled off the bulk silicon used for fabrication with an inexpensive, patent-pending method. However, “mobility is not enough to bring the TFTs up to speed,” says Ma. Low-resistance electrode contacts are also important.

Achieving this is challenging because of the high temperatures needed to activate the low-resistance electrodes. These high temperatures typically damage the flexible substrates to which the TFTs are affixed. “That is the major obstacle to realizing the high-speed operation of TFTs, regardless of the fact that high mobility has already been demonstrated in single-crystal silicon on flexible substrates,” says Ma.

To solve this, the UWM team fabricated the transistors in a series of “hot” and “cold” steps. First, they made the contact connectors on a bulk silicon substrate to achieve low resistance, and then transferred the single-crystal nanomembranes to the flexible substrate to continue fabrication. A twist in the gate chemistry also helped to boost performance. Rather than using silicon dioxide, the research team fabricated the gates out of silicon monoxide. “Silicon monoxide has a higher electric capacity and can be made thinner than the dioxide. As a result, the device speed becomes even faster,” adds Yuan.

Breakthrough TFT-LCD Among the innovations unveiled at this month’s Consumer Electronics Show (CES) in Las Vegas, Nev., was a unique LCD put forth by display giant, Samsung Electronics, Seoul, South Korea. The new display is touted as the world’s first truly double-sided LCD display. According to the company, the new display can show two entirely different pictures or sets of visual data simultaneously on the front and back of the same screen. Other conventional double-sided LCDs can only show a reverse image of the same video data. This breakthrough was realized through the use of the company’s double-gate, thin-film transistor (TFT) architecture, which uses two gates operating a single pixel, so the screen on the front can display different images than the one on the back. For mobile phone applications, it would alleviate the need to have separate internal and external displays, creating one effective display for thinner and possibly, less-expensive cell phones. Samsung Electronics, www.samsung.com

Invisible electronics
Indeed, subtle changes in TFT chemistry seem to be at the heart of their advances. At Northwestern Univ., Evanston, Ill., researchers have recently demonstrated a new set of transparent, high-performance TFTs transistors capable of being assembled at low temperatures, on both glass and plastics substrates.

This breakthrough was fueled through the combination of organic and inorganic chemistries. To create their transparent, thin-film transistors, the group combined films of the inorganic semiconductor indium oxide with a multilayer of self-assembling, organic dielectric molecules. According to Northwestern, “In addition to being transparent, the transistors outperform the silicon transistors currently used in LCD screens and perform nearly as well as high-end polysilicon transistors.”

“Our development provides new strategies for creating transparent electronics,” adds lead project researcher, Tobin Marks, professor of chemistry, materials science and engineering. “You can imagine a variety of applications for new electronics that haven’t been previously possible—imagine displays of text or images that would seem to be floating in space.”

The team has formed a start-up company, Polyera, to bring their transistors to market in the next 12-18 months.

—Jeannette Mallozzi