Nanoscale Vacuum Electronic Devices

High-speed electronic devices rely on short carrier transport times, which are usually achieved by decreasing the channel length and/or increasing the carrier velocity. Ideally, the carriers enter into a ballistic transport regime in which they are not scattered. However, it is difficult to achieve ballistic transport in a solid-state medium because the high electric fields used to increase the carrier velocity also increase scattering. Vacuum is an ideal medium for ballistic transport, but vacuum electronic devices commonly suffer from low emission currents and high operating voltages. We have developed a low-voltage field-effect transistor with a vertical vacuum channel (channel length of ∼20 nm) etched into a metal–oxide–semiconductor substrate. We measure a transconductance of 20 nS µm–1, an on/off ratio of 500 and a turn-on gate voltage of 0.5 V under ambient conditions. Coulombic repulsion in the two-dimensional electron system at the interface between the oxide and the metal or the semiconductor reduces the energy barrier to electron emission, leading to a high emission current density (∼1×105 A cm–2) under a bias of only 1 V. The emission of two-dimensional electron systems into vacuum channels could enable a new class of low-power, high-speed transistors.\ud Harboring a two-dimensional electronic system, graphene can be highly conductive in in-plane transport while being transmissive to impinging electrons. Based on these in- and out-of-plane interaction properties, a suspended graphene membrane is promising as an ideal gate (grid) to control electron transport in nanoscale vacuum electronic devices. We have measured capture and transmission efficiencies of very low energy (< 3 eV) electrons impinging upon a suspended graphene anode that is placed on top of a nanoscale void channel formed in a SiO2/Si substrate. Electron capture efficiency of 0.1 % (transmission efficiency of 99.9 %) is observed at 1 V bias. Presence of suspended graphene is also found to significantly enhance electron emission at cathode beyond the level of Child-Langmuir’s space-charge-limited emission.\ud Photocarrier multiplication, the process of generating two or more electron-hole pairs from a single absorbed photon, can occur in semiconductor quantum dots or nanocrystals. Translating this carrier-level performance into a device-level improvement in sensing or converting photon energy, however, remains challenging. We have developed a graphene/SiO2/Si photodetector with a nanoscale void channel that demonstrates internal quantum efficiency of 115-175% measured with photocurrent in UV-Vis range. The self-induced electric field in 2D electron gas of a graphene/oxide/Si structure enables photocarrier multiplication.\u