Nanowire transistors have overtaken amorphous silicon devices in research at Purdue University in Indiana.

“The issue that is intriguing is we can get relatively high carrier mobility,”  Professor Tobin Marks told EW. “That translates into higher drive current or higher operating speed.”

According to Marks, most current thin film materials have relatively low mobility, for example: one to 10cm sq./Vs for amorphous silicon and under 1cm sq./Vs for organic materials.

In contrast, in the Purdue ZnO and InO nanowire transistors “looks like we are getting in the order of a few hundred, at least an order of magnitude of current or speed”, said Marks.

Polysilicon is far better than amorphous silicon, achieving around 50cm sq./Vs. “But we essentially do processing at room temperature with just a little bit of substrate heating when doing evaporation. Polysilicon needs higher temperatures that are difficult in plastics”, said Marks. In addition, the nanowires transistors have a very high on-off ratio and they are transparent.

“Our study demonstrates that nanowire electronics can be fully transparent, as well as flexible,” Marks said. “Other researchers have previously created nanowire transistors, but the metal electrodes in the transistors were non-transparent, which made the overall structure opaque.” 

The nanowire transistors are constructed by first depositing a InZnO gate on the substrate, then covering it with an Al2O3 gate insulator.

Next, nanowires are solution deposited over the gates. This is not necessarily an exact science. “There are some groups developing technologies to place nanowires where they want them,” said Marks.

Lastly, source and drain contacts are sputtered over the top with 1.5µm source-drain spacing.

Current capability scales with the number of nanowires in the structure. “We can put in up to 20,” said Marks.

The devices have been demonstrated on both glass and plastic substrates and Purdue is proposing them, amongst other applications, as thin-film transistors for OLED displays.