  Taking into account both the scattering and the velocity saturation of
carriers, we examine the "shallow-water" instability of the two-dimensional
electron gas in a field effect transistor. It is shown that both the scattering
(which is analogous to friction in a shallow-water channel) and the carrier
velocity saturation lead to damping of the plasma wave instability. Threshold
diagram of instability is calculated. The actual device parameters required for
observation of plasma wave generation are compared with those reported in
recent sub-terahertz emission experiments.
