  A technically simple and physically clear method is suggested for the direct
measurement of brightness temperature of two-dimensional electron gas (2DEG) in
the channel of a high electron mobility transistor (HEMT). The usage of the
method was demonstrated with the pseudomorphic HEMT as a specimen. The optimal
HEMT dc regime, from the point of view of the "back action" problem, was found
to belong to the unsaturated area of the static characteristics possibly
corresponding to the ballistic electron transport mode. The proposed method is
believed to be a convenient tool to explore the ballistic transport, electron
diffusion, 2DEG properties and other electrophysical processes in the
heterostructures.
