Effect of photogenerated electrons on the terahertz plasma-wave resonance in HEMT's under interband photoexcitation

Two-dimensional (2-D) electron plasma in a submicron channel of a high-electron mobility transistor (HEMT) can make resonant oscillation in the terahertz range. The gate bias potential V_gs can control the resonant frequency f_r, which offers the possibility of tunable coherent terahertz oscillators. The terahertz plasma-wave excitation can be performed by means of interband photoexcitation in a manner of laser-photomixed difference-frequency (Δf) generation. The 2-D electron plasma in the electron channel is excited by the terahertz Δf component of the photoexcited carriers. Since the photoelectrons perturb the surface density of 2-D electrons, strong photoexcitation dynamically modulates the f_r, resulting in considerable resonant-spectral broadening. This effect was modeled analytically in the 2-D plasma hydrodynamic equation. The modulation depth of the density of 2-D electrons by the photoelectrons deeply relates to the resonant intensity and f_r. In order to validate the analytical calculation, the plasma-wave resonance was experimentally observed for a 0.15-μm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT in the terahertz range. At the modulation depth of 30%, the resonance was clearly observed with a double peak (the peak at 1.9/5.8 THz corresponding to the fundamental /third harmonic resonance). On the contrary, under a low modulation depth condition, the plasma resonant intensity decreased. Observed resonant frequencies support the analytical calculation.