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Model for explaining the terahertz radiation mechanism for the case of depletion-type FETs. (a) The sample geometry and (b) the circuit diagram of the optical excitation used in this experiment. Expected time evolution of (c) gate electric field, (d) drain-source current , and (e) .
Schematic configuration of the terahertz setup used in this work.
Typical terahertz waveforms radiated from the HEMT samples: (a) sample A and (b) sample B when and . was modulated from at .
Measured terahertz waveforms from sample B under various bias conditions; both and were biased (solid line), and (dashed line), and and (dotted line). Curves were shifted for clarity.
The designed gate length and the cutoff frequency estimated by a network analyzer of the HEMT samples. The transit time and the cutoff frequency determined by the present terahertz technique are also tabulated.
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