Niobium SIS junctions provide low-noise mixing at frequencies up to
THz. Above this frequency, their performance falls off
quickly due to their relatively low gap frequency ( 
GHz). However, recently superconducting, niobium microbridge
bolometers have been shown to produce low receiver noise temperatures
at frequencies up to at least 2.5 THz (Karasik et al. 1997).
These devices make use of the large dR/dT occuring at the
resistive transition in a superconducting thin film. In such
materials, the electrons are only loosely coupled to the crystal
lattice and the electron-electron interaction is enhanced. When
bombarded by photons these electrons can equilibriate at a temperature
greater than the lattice. The electrons then can cool either through
phonon coupling to the underlying lattice (Gershenzon et al.
1990) or through diffusion through normal metal contacts (Prober
1993). Which cooling mechanism dominates depends principally on the
length of the bridge; diffusion cooling begins to dominate as the
bridge length becomes smaller. Due to the heating of free electrons by
the absorption of photons, these devices are refered to as Hot
Electron Bolometers (HEB's). To date, the lowest noise temperatures
and widest IF bandwidths have been obtained using diffusion cooled
devices (Skalare, et al. 1994, 1997; Karasik et al.
1997). The theoretical treatment by Prober (1993) suggests these
devices will provide excellent performance up to several 10's of
THz. Furthermore, these devices exhibit an essentially real impedance
(of order 100 
), making them much easier to match over large
bandwidths to antenna and waveguide structures than their SIS
counterparts.
Hot-electron bolometers have been used at submillimeter wavelengths
for over a decade (Phillips 1982). These earlier devices provided low
noise performance, but only over small IF bandwidths ( MHz).
Spectroscopy of astrophysical sources was made possible by sweeping
the local oscillator. A number of ground-breaking observations from
both airborne altitudes and mountain tops were made in this way ( 
Phillips & Huggins 1981). What makes the new generation of
superconducting, hot-electron bolometers so useful is their large
instantaneous bandwidths. Niobium devices have been shown to have
bandwidths of up to 
GHz (Schoelkopf et al. 1996).
Higher 
superconductors may be able to provide much larger IF
bandwidths in the future. For example, YBCO may be capable of
delivering IF bandwidths of up to 
GHz. While such
bandwidths are certainly attractive, they are not essential to meet
the primary scientific goals and instrumental requirements of the
instrument proposed here. Indeed, these needs can be met with the
existing niobium microbridge technology. Therefore, we have chosen
these devices for our baseline design.