Study on Optically Injection-Locked Terahertz Oscillation in Plasma-Wave Resonant Transistors

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We have studied the possibility of optically injection-locked terahertz oscillations of the plasma-wave transistors (PWT's). The PWT is a new type of the electron device that utilizes the plasma resonance effect of highly dense two-dimensional conduction electrons in the field effect transistors (FET) channel. The resonance frequency can be externally controlled by the gate bias potential, which offers the tenability of oscillation. First, the plasma resonant phenomena were experimentally investigated for a sub-100-nm GaAs MESFET. The terahertz excitation was performed by photomixing the two laser sources in a manner of difference-frequency generation. The optically injection-locked plasma resonance modulates the DC drain-source potential. The resonant intensity, thus, can be measured by monitoring the DC modulation component : ΔVds. The gate-bias dependence of ΔVds was measured under several, difference-frequency conditions. The results clearly indicated the occurrence of plasma reson … More ance in the terahertz range. It was verified for the first time that the plasma resonance frequency is controlled externally by the gate bias in a wide terahertz range. Next, we have carried on the first observation of the frequency dependence of the plasma resonant intensity for a 0.15-μm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT (pHEMT). When the gate bias was properly applied to induce a sufficient density of electrons in the channel, the resonance was clearly observed with a double peak. The first peak at 1.9 THz corresponds to the fundamental resonance while the second peak at 5.8 THz corresponds to the third harmonic resonance. Observed resonant frequencies coincided well -to the theoretical calculation. The detected plasma resonances are actually non-radiative-mode oscillation and their energy is estimated to be on the order of nW under mW excitation power. A mean to convert the plasma oscillation to an electromagnetic radiation should be employed onto the device structure. Also, improvement of the quantum efficiency is the key.' These are the future subjects. Less