Back to vacuum tubes: at the nanoscale

April 24, 2017 // By Julien Happich
In their search for intrinsically noise- and radiation-immune transistors, researchers from the NASA Ames Research Center have investigated the design of nanoscale vacuum tubes that would beat traditional CMOS field-effect-transistors (FET) transistors in harsh space environments.

Describing their "Nanoscale Vacuum Channel Transistor" or NVCT in the Nano Letters journal, the researchers come back to the fundamentals: vacuum provides superior electron transport compared to all semiconductors (no collisions or scattering in the absence of crystal lattices).

Reviving the defunct vacuum tube, the researchers leveraged modern silicon nanofabrication technology to create a novel type of transistor sporting a vacuum channel instead of a doped semiconductor. Learning from the evolution of planar CMOS FETs to FinFETs and more recently gate-all-around FETs, the team from NASA opted for a surround gate design for optimal local field enhancement.

Their vacuum channel transistor consists of sharp source and drain electrodes separated by a nanoscale vacuum channel (less than 50nm apart), and a surrounding gate delimiting the cylindrical vacuum channel (with a channel radius of 10nm, equivalent to the source-to-gate distance).

Although the device is not "sealed" and the channel is technically an air-channel, because the 50nm channel distance is smaller than the mean-free-path of air molecules under atmospheric pressure, it can be considered as quasi-vacuum, the researchers explain.

Schematic illustrations of (a) a silicon nanowire gate-all-around transistor and (b) a nanoscale vacuum channel transistor. A comparison of the energy band diagrams in the source to channel direction, xz-plane, of the (c) silicon nanowire gate-all-around transistor and (d) the nanoscale vacuum channel transistor.

The paper draws similarities between the NVCT structure and that of a nanowire gate-all-around transistor, except that the silicon channel is replaced by an empty gap and the source is sharpened for local field enhancement. Both share analogous operation mechanisms, the researchers write.