It has been more than a year since our paper “Nanoscale vacuum channel transistors fabricated on silicon carbide wafers” appeared in print (Nature Electronics, vol. 2, 405-411, September 2019, https://doi.org/10.1038/s41928-019-0289-z).  This paper has certainly got a decent number of citations in such a short period, especially for something that was not dealing with graphene or 2D material (though graphene is a good candidate material for field emitters in vacuum electronics, given its favorable work function).

The article has given the community hope that wafer scale fabrication of vacuum devices with sub-micron feature scale is feasible, and increasing the current (i.e. current scaling) is possible simply by increasing the number of emitters on the source pad. At NASA Ames, we are further improving the vacuum channel transistor for use in future Europa (Jupiter Icy Moon) lander mission. The current focus is on enhancing reliability and lifetime and I thank Viet Nguyen and Gary Hunter of NASA Science Mission Directorate for their support of our work.

Since this publication, I have been fortunate to be invited to attend and listen to program reviews of two large Multi-University Research Initiatives (MURIs) in the US, both funded to work on nanoscale vacuum electronics.  Prof. Tayo Akinwande of MIT heads the first MURI with several participating faculty from MIT, Boise State University, Southern Methodist University and University of Colorado. The second MURI is led by Prof. Dimitris Pavlidis from Florida International University with participation from Boston University, Penn State University, Ohio State University and Purdue University.  These teams have started doing amazing work on pushing the nanoscale vacuum electronics, exploring various materials systems such Si, GaN, graphene and others along with very interesting geometries, very thorough characterization of emitters for lifetime, robustness, adsorbates and their impact and atmospheric pressure operation, developing reliable models and addressing all other relevant aspects. The range of topics covered by these teams is broad and incredible. It is heartening and exciting to see the interest in bringing back vacuum tubes to the nanoscale era. In addition, the International Vacuum Electronics Conference (IVEC), a legacy vacuum electronics conference - that normally covers primarily traveling waveguide tube (TWT), Klystron, Gyrotron and Magnetron - has decided to include nanoscale vacuum electronics in their program. With that, IVEC 2021 will hold a special session focused on nanoscale vacuum electronics, for which I will honorably serve as the organizing chair.

What have I done since then in vacuum electronics? In my Before the paper blog, the answer to the question “ what is next? “ was: circuits. I have put together some basic circuits using the transistor components described in the paper. That was neither hard nor very exciting, but just routine.  I saw what I expected.  But having been stuck at home since March 2020 due to the COVID-19 pandemic and not having access to the lab (even now at this writing), I had some interesting discussions with my colleagues at NASA Ames about the lack of complementary devices in vacuum electronics; there are no n-type and p-type vacuum-state devices as we have in CMOS solid state transistor. This has not changed since the invention of vacuum tubes over hundred years ago and the first report by Fleming in 1906 and Lee De Forest in 1907. Availability of complementary devices will help with high noise immunity and low static power consumption as we have with CMOS. I was able to come up with a device design that combines electron field emission from a pair of asymmetric electrodes with a NEM (nanoelectromechanical) cantilever actuation of the gate. The latter is like a NEM relay switch. Thanks to the ambipolarity of electrostatic NEM actuation and unipolarity of field emission due to asymmetric cathode and anode geometry, the device was able to be structured to emit current selectively either at positive or negative terminal voltages. The resulting pair of complementary devices shows output and transfer curves similar to CMOS, confirming the success of the approach. Self-consistent modeling and simulation was used to verify the proposed concept, and results for an inverter circuit and the electrical characteristics of the complementary devices were generated. 

You can read about the first-in-a-century complementary vacuum field emission transistor in ACS Applied Nano Materials (https://dx.doi.org/10.1021/acsanm.0c02587).  Alas, we could not publish this work in Nature Electronics (we tried but no luck) because we did not have any experimental demonstration. It was not just the matter of lack of access to our lab and fab due to COVID-19 but fabrication would take easily a year or so, if not longer.  But, staying at home, I could conceive the concept, do the verifying simulations with the help of my colleagues and get it published (of course, after getting rejected by a few journals without even sending out for review) all in about nine months. Anyway, we suggest potential process steps to fabricate the complementary vacuum channel transistor in the above article.  I sincerely hope that the community will fabricate these complementary devices. I hope to do it too down the road. NEM actuation is mechanical and has its limitations but someone may come up with other clever ideas for actuation, hopefully inspired by our concept.