"The sharp tip and high aspect ratio of a carbon nanotube produce extremely high field enhancement on the tip," researcher David Hsu told nanotechweb.org. "The higher the local electric field on the emitter tip, the higher the field emission current will be. By having the gate electrode in very close proximity - within a micron or so - to the nanotube, such as by having microfabricated integral gates, only relatively low gate voltages are required to produce the high fields."

Hsu and colleague Jonathan Shaw deposited the nanotubes onto arrays of 1 µm diameter silicon posts, each centered in a 2.5 µm diameter gate aperture. An array of 7670 cells occupied 0.1 sq. mm and resulted in an emission current of more than 1 mA.

"The microgating enables high current densities at low gate voltages, which is necessary to produce the high transconductance values required for many electronic device applications," said Hsu. "Transconductance - defined as dI/dV - values up to 170 mS/sq. cm were obtained."

According to the researchers, the current density of 1.2 A/sq. cm and transconductance of 168 mS/sq. cm for a gate voltage of 60 V is a record for any carbon nanotube field emitter.

"Other studies have achieved greater current densities from carbon nanotube emitters in either a diode configuration or a triode configuration, but at much higher voltages - hundreds to more than a thousand volts - due to the macroscopic separations between the carbon nanotube and the biased electrode," said Hsu. "High voltages make it difficult to obtain high transconductance values or achieve very high frequencies."

The team says the current densities are more than adequate for applications such as microsatellite thrusters, portable x-ray sources, miniature gas analyzers, and field emission displays. The carbon nanotube field emitter arrays are also robust, can be regenerated and do not suffer from destructive arcing.

"Future efforts will be focused on reducing the gate current and improving the uniformity of emission over the array," said Hsu. "We are also working on a different configuration of carbon nanotube field emitter arrays - one with an open aperture - that has preliminarily yielded very low gate currents."

The researchers reported their work in J. Vac. Sci. Technol. B.