The performances of a two-dimensional electron gas (2DEG) based planar nanodevice are studied by a two-dimensional-three-dimensional (2D-3D) combined model and an entirely 2D model. In both models, 2DEGs are depicted by 2D ensemble Monte Carlo (EMC) method. However electric field distributions in the devices are obtained by self-consistently solving 2D and 3D Poisson equations for the 2D model and the 2D-3D model, respectively. Simulation results obtained by both models are almost the same at low bias while showing distinguished differences at high bias. The 2D model predicts larger output current and slightly higher threshold voltage of Gunn oscillations. Although the fundamental frequencies of current oscillations obtained by both models are similar, the deviation of wave shape from sinusoidal waveform obtained by the 2D model is more serious than that obtained by 2D-3D model. Moreover, results obtained by the 2D model are more sensitive both to the bias conditions and to the change of device parameters. Interestingly, a look-like second harmonic oscillation has been observed at DC bias. We contribute the origin of divergences in simulation results to the different coupling path of electric field in the two models. And the second-harmonic oscillations at DC bias should be the result of the appearance of concomitant oscillations beside the channel excited by strong electric-field effects.

Gunn oscillations in a GaN based planar nano-device have been studied by ensemble Monte Carlo (EMC) method. Simulation results show that when the channel length of the device reduces to 450 nm, THz oscillations (about 0.3 THz) can be obtained. Also the phase of the oscillations can be controlled by the initial conditions that excite the Gunn domains. Moreover, through adjusting the phase difference between the oscillations in a double-channels device, which attained by parallel connecting two single-channel devices, the frequency of the device shifts from 0.3 THz to 0.6 THz. This phenomenon remains in devices with shorter channel-length, unless the channel-length is too short to support Gunn oscillations. The possible underlying mechanisms are also discussed.