Planar Gunn diodes on In 0.53 Ga 0.47 As with lengths between 2 and 5 µm have been fabricated and characterized in a temperature range of 10 to 300 K. Two different oscillation regimes are observed depending on temperature. At the higher values, the frequency of the oscillations decreases as the bias increases, as expected for a well-established transit-time domain mode. But below approximately 75 K, the behavior is the opposite, the frequency of the Gunn oscillations increases with the bias. This fact, together with a much lower amplitude of the oscillations, indicate the possible switch to a different oscillation mode in which the domains are not able to attain their complete maturation before reaching the anode.

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Robust coupling between a pulsed laser beam and a photoelectric circuit is an important issue in the development of miniaturized, integrated, and embedded terahertz instrumentation. Here, we present a study of the effect of varying the focus and alignment parameters of an excitation laser pulse on the emission characteristics of a standard Hertziandipole type terahertz photoelectric source. The objective is to quantify the tolerance of a terahertz time-domain spectroscopy system, and we study the variation of peak amplitude, waveform, phase, and energy distribution as a function of excitation position and defocus. We find that a terahertz source can be made relatively tolerant to variations in focus, alignment, and details of the geometry of the photoelectric system, providing a window for a more robust field operation.

In typical terahertz time-domain spectroscopy systems, the use of the lock-in technique is necessary because of the low current induced at the receiver so that the laser pump beam must be modulated (chopped) at a frequency much lower than the laser repetition rate. This work shows that, in the case of semi-insulating GaAs (SI-GaAs) antennas, this modulation has an important effect on the antenna current and consequently, on the radiated electromagnetic pulse. There exists a threshold bias (whose value depends on the chopping frequency) where an abrupt increase in the current and consequently, in the terahertz emission takes place. The calculated energy of the pulse below and above the threshold shows that the energy doubles. The exact bias voltage at which this occurs changes with the laser modulation frequency when this is below 350 Hz, but at higher frequencies, the threshold remains almost constant. The experiments show that the responsibility for this behavior is the S-shape negative differential conductance exhibited by SI-GaAs originated by a slow field-enhanced charge trapping mechanism, which is also an important source of noise at the receiver of the system. Index Terms-Negative differential conductance (NDC), photoconductive switch, terahertz time-domain spectroscopy (THz-TDS). I. INTRODUCTIONO VER the past years, there has been an increase of new applications at terahertz (THz) frequencies for which Manuscript

We report on the measurements of the two main figures of merit for microwave detection, namely, responsivity and noise equivalent power (NEP), in HEMTs based on two of the most broadly used material systems AlGaN/GaN and AlInAs/InGaAs. A comparison between their performances as RF detectors in a wide temperature range is provided by means of experiments made under probes with direct connection to the drain contact. InGaAs HEMTs exhibit much better responsivity and NEP, which are further improved when lowering the working temperature. Moreover, we analyze the possibility of optimizing the current-mode detection of the transistors by improving the impedance matching conditions through an adequate choice of the device width [Formula: see text].