Ultra-small MESFET's have characteristic lengths comparable to quantum lengths: wavelength, mean free path, etc. In a first attempt to incorporate these quantum lengths, we develop a model based upon a set of quantum moment equations obtained from the Wigner function equation-of-motion. Interesting time-dependent current oscillation behavior has been observed when a step voltage is applied to the initial steady state. The oscillation frequency is peaked around 500 GHz, which is related to plasma response of the carriers in the channel. Quantum effects, such as barrier repulsion and penetration, have been demonstrated in the simulation. These effects modify the electron density distribution and current density distribution both in the channel and near the source. Modifications of the frequency spectrum of the oscillation current due to the quantum effects are obvious.
