Oxide semiconductors, alongside low-temperature polysilicon, have been the high mobility alternative to amorphous silicon backplanes for a few years now. Flexible display manufacturers in particular seem to have embraced oxide semiconductors for future generations and new form factors. It’s interesting to note that oxide semiconductors have other primary uses besides displays, in large part due to their extremely low leakage current values.
I talked with Johan Bergquist (see photo below) of Semiconductor Energy Laboratory (SEL) about these uses and how they relate to SEL’s C-axis aligned crystalline indium gallium zinc oxide (CAAC IGZO) technology. Within SID, SEL is best known for its many advances in active backplane technologies. Most recently, SEL has demonstrated high-definition CAAC IGZO displays, both on glass and flexible substrates. The president of SEL, Dr. Shunpei Yamazaki, is recognized in many societies, including SID, for this work. Beyond displays, SEL has shown the capability of CAAC IGZO in representative circuits, such as system-on-glass processing units and multi-level memory circuits. In fact, large-scale integration (LSI) applications are some of the most promising uses for IGZO.
“IGZO doesn’t have the short gate effect that silicon has,” said Bergquist. “This means that reducing design rules does not reduce the mobility, and SEL has in fact demonstrated 30 nm technology in CAAC IGZO.” CAAC IGZO is aligned in the transverse direction, while it is nanocrystalline in the lateral dimensions. Because the mobility is still below that of crystalline silicon, oxide semiconductors are, unlike CPUs and other circuits requiring fast switching, best suited for circuits that can utilize the next-to-nonexistent leakage current, such as nonvolatile memories. These can be used, for instance, as register backup memories and frame buffers. Because the technology is analog in nature, multilevel memories are possible. “SEL’s goal is to make an 8-bit memory, but we are not there quite yet. 4-bit and 5-bit memories have been realized, though,” says Bergquist. “The trick is to use hybrid circuits where silicon and IGZO are used together.”
At the moment, the reliability of oxide semiconductors is not at the same level as that of silicon-based circuits, so memory chips based on oxide logic are not available. There is a lot of promise in IGZO circuits, especially integrating these with oxide semiconductor backplane displays, and future efforts with chip maker partners of SEL will likely see to it that product reliability will reach silicon levels. We may have to wait for a few years before this happens but until then, we can enjoy the benefits of IGZO in fantastic mobile-sized displays, and beyond these, moderate-sized high-pixel-density displays, both on glass and flexible substrates. -- Jyrki Kimmel for Information Display