We report on reflective electro-optic sampling measurements of TeraHertz
emission from nanometer-gate-length InGaAs-based high electron mobility
transistors. The room temperature coherent gate-voltage tunable emission is
demonstrated. We establish that the physical mechanism of the coherent
TeraHertz emission is related to the plasma waves driven by simultaneous
current and optical excitation. A significant shift of the plasma frequency and
the narrowing of the emission with increasing channel's current are observed
and explained as due to the increase of the carriers density and drift
velocity.Comment: 3 figure