Experimental configuration scheme. A tunable optical beating in the range 0.3-0.5 THz is produced by the spatial superposition of two cw-lasers at 1.55 μm (DFB). Using optical elements (lens, prism, mirror), the beating is focused onto the HEMT’s channel (sample) while its stability is controlled by a spectrum analyser. The transistor is biased, using HF probes and 50 loads, with constant voltage between gate and source electrodes and constant current at the drain electrode. The emitted radiation is collected by an off axis parabolic mirror and its intensity is measured by a 4 K-Si bolometer.
Output characteristics at 300 K (solid lines) and at 200 K (dotted lines) for a 50 nm gate-length transistor with a width W = 50 µm. The applied gate voltages are reported on the right side of the figure.
Beam intensity emitted by the HEMTs and measured by a Si-bolometer at 200 K as a function of the power of the optical beating photoexcitation. Dashed lines: and dotted lines (coinciding practically with the horizontal axis): 10 GHz. Solid lines are quadratic fits of the experimental results corresponding to the dashed lines.
(a) Spectral response of the THz beam measured by a Si-bolometer as a function of the optical beating frequency at 200 K. The solid line is a guide for the eyes. (b) Measured relative photoresponse versus the optical beating frequency ( V, mA at T = 300 K (dotted line) and at 200 K (solid line). Error-bars are experimental data joined by eye guidelines.
Calculated source-drain voltages as functions of time for the reported electron velocity relaxation rates = 1.9 (a), 1.75 (b) and 1.7 (c) ps−1. Dotted lines represent the envelopes of the time dependent voltages.
Calculated drain current as a function of source-drain voltage at different gate voltages . Solid line separates stable and unstable regions for the regime of fixed current flowing in the channel.
Calculated relaxation time of 2D plasma excitations as a function of drain current at different gate voltages .
Calculated amplitudes of the (a) harmonic and (b) average photoresponses of voltage as functions of the beating frequency for the reported values of the velocity relaxation rates.
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