  Collective vibrations of proteins, rotations of small molecules, excitations
of high-temperature superconductors, and electronic transitions in
semiconductor nanostructures occur with characteristic frequencies between 1
and 10 THz. Applications to medicine, communications, security and other fields
are emerging. However, mapping the coldest parts of the universe has been the
largest driver for developing THz detectors. The result is a family of
exquisitely-sensitive detectors requiring sub-4K temperatures. For earthbound
THz science and technology, sensitivity remains important but many applications
require high speed and operating temperatures. Room-temperature Schottky diodes
enable some of these applications. Here we demonstrate a new type of detector
in which THz radiation excites a collective oscillation of ~25,000 electrons
between two gates in a microscopic four terminal transistor. The energy
dissipates into other modes of the electron gas, warming it and changing the
source-drain resistance. The detector shows amplifier-limited rise times near 1
ns and has detected THz laser radiation at temperatures up to 120K. The
frequency of the collective oscillation tunes with small gate voltages. The
first-generation tunable antenna-coupled intersubband Terahertz (TACIT)
detectors tune between 1.5 and 2 THz with voltages <2V.
