Electron-beam lithography for the fabrication of air-bridged, submicron Schottky collectors

Abstract

T-gate technology as is commonly used for field-effect transistors and high electron mobility transistors has been adapted for use in Schottky-collector resonant tunneling diodes (SRTDs) devices in which it is necessary for the footprint to be extremely small in both dimensions. By air bridging the contact, GaAs-based RTDs with projected cutoff frequencies of nearly 1 THz have been fabricated. The process is advantageous for the fabrication of terahertz diodes because of the large periphery to area ratio associated with the small footprint (which reduces the parasitic resistance), because small areas provide better impedances, and because the air bridge both reduces parasitic capacitances and provides certain processing advantages. The process is also inherently planar in contrast with other diode implementations for use at submillimeter wave frequencies. In addition to the GaAs-based RTDs, the process is also being used for the fabrication of GaAs Mott diodes, which have cutoff frequencies of 12.5 THz and InGaAs/AlAs RTDs, which appear to have cutoff frequencies of 2.5–3 THz.