Air-Bridged Schottky Diodes for THz Applications
Alderman, B.; Sanghera, S; Thomas, B; Matheson, D
Rutherford Appleton Laboratory

Low capacitance GaAs Schottky diode technology is essential for millimetre and sub-millimetre wave heterodyne receivers. Schottky diodes operate at both ambient and cryogenic temperatures and are uniquely able to cover the frequency range from DC to above 1 THz. Schottky diode technology has been evolving for many years and has traditionally been driven by the demands of radio astronomy and remote sensing of the atmosphere. Ground based applications, e.g. security imaging, are now increasing in importance. For these applications, Schottky based technology offers an attractive alternative to detectors and sources that require cryogenic cooling.

Precision fabrication techniques are required to form the diode structures. These typically consist of two parallel 20 µm long gold air-bridges passing over a channel, of depth 4 µm, to make contact to anodes which have a sub-micron radius. This approach is required to reduce parasitic capacitance, which increasingly dominates the response as the frequency rises. The on-wafer diode-to-diode variation in electrical parameters for fabricated devices is low and mechanical yields in excess of 95 % are achieved. Incorporation of the Schottky devices in heterodyne detectors yields near state-of-the-art results at 183 GHz, the characteristic frequency of a molecular line that is important for remote sensing.

A process to further improve the reliability of this Schottky technology will be reported. In this, we integrate the air-bridged diode structures with impedance matching networks. This eliminates the traditional process step of flip-chip soldering discrete diodes onto gold-on-quartz filters.