40 kV Silicon Vacuum Transistor: An MIT-led team will describe the first Si vacuum transistor operating at ~40 kV and with the potential to have a semiconductor-like footprint. Such a high voltage level is normally reserved for wide-bandgap materials like SiC and GaN. The proof-of-concept device consists of a gated field emission array or FEA (i.e., an electron source), a vacuum drift region and a metal anode. Electrons are emitted from the gated field emission array into the vacuum through tunneling, travel through it and are collected at the anode. The vacuum determines the transport properties and the high-voltage isolation. Using this technology as a baseline, the researchers will provide intrinsic benchmarks for vacuum transistors. They say the high critical electric field and unbounded carrier velocity of these devices can lead to compact, high-performance vacuum devices able to outperform solid-state devices on all metrics, making them suitable for a range of high-power and high-frequency applications, and also as next-generation X-ray sources. (Paper #5.2, “Demonstration of a ~40 kV Si Vacuum Transistor as a Practical High Frequency and Power Device,” W. Chern et al, MIT/Harvard/Massachusetts General Hospital)

The top image is a schematic of different vacuum device test setups. In both, the FEA electron source is below a metal anode housed in vacuum, yielding a vertical device architecture. In device A, a floating ball anode with a 1mm diameter is moved to a distance, d, away from the FEA chip. In device B, a fixed 45 degree molybdenum anode is ~1.5 cm away.

On the left in the middle set of images are (a) tilted and (b) cross-sectional SEMs for gated FEAs. (a) shows an array of gated sharp tips in the middle of a ~200-300nm aperture, formed through a self-aligned fabrication process; (b) shows the high aspect ratio nature of the nanowires (~6-10μm tall) with a diameter of 200 nm. On top of this nanowire a sharp tip exists to concentrate the E-field of the surrounding polysilicon gate.

On the right in the middle set of images is a schematic of the full FEA structure. The table on the bottom compares the basic material properties of various semiconductor materials and their

Johnson Figures of Merit (JFOM, a measure of a semiconductor material’s suitability for high-frequency power transistor applications). It shows that the JFOM for a vacuum transistor far exceeds any semiconductor, owing to its high saturation velocity and critical electric field.