Dual wavelength continuous wave laser using a birefringent filter

In this paper, we report an analysis of reverse current mechanisms observed in GaN Schottky barrier diodes leading to hysteretic behavior of the I–V curves at low temperature. By means of DC measurements from 33 to 475 K, we demonstrate the presence of two leakage mechanisms when comparing the experiments with the results obtained using a unified model to predict the ideal reverse current of the diode. Poole–Frenkel emission is the dominant mechanism for temperatures above 200 K, while trap-assisted tunneling prevails for lower temperatures, where also, hysteresis cycles are revealed by means of DC dual-sweep voltage measurements. The energy of the corresponding traps has also been determined, being around 0.2 and 0.45 eV, respectively. The hysteresis phenomenon is attributed to the bias-induced occupancy of the energy states originating the leakage-current processes, which leads to the reduction of the reverse current after a high negative voltage is applied to the diode.

The hysteresis cycles observed in the reverse leakage current measured at low temperatures in GaN-on-sapphire Schottky barrier diodes (SBDs) have been deeply studied and interpreted in terms of trap-assisted tunneling (TAT) compatible with the existence of a trap energy band (TEB) near the metal-semiconductor interface. The analysis of the energies for which the maximum tunneling current occurs, both direct and through this trap band, allows us to explain the behaviors found at different temperatures. Starting from empty trap states by previous illumination, transient current measurements performed under different preconditioning voltages evidence a progressive partial filling/release of the trap energy levels, confirmed as well by pulsed measurements. Captured/released electrons modify the number of trap states available for tunneling and thus the current level. As a consequence, tunneling processes in the go and return paths take place through levels with different occupations within the TEB, originating the hysteresis cycle.

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 $W$ .

A generic high-frequency model of two-port RF detectors, based on $I$ – $V$ curves and S-parameters measurements, is proposed and applied to the case of AlGaN/GaN field-effect transistors (FETs). The expression of the current responsivity (A/W) of any kind of transistor detector is derived for RF power injection through both the gate and the drain. The main novelty of the proposed model is the adequate consideration of the often neglected gate–drain coupling. The developed formalism also clarifies the voltage–current dc bias and the progressive and regressive power waves used to describe the RF excitation of the two-port nonlinear device. The obtained frequency-dependent closed-form expressions replicate very satisfactorily the current responsivity measurements made in AlGaN/GaN high-electron-mobility transistors (HEMTs) up to 67 GHz. To improve the physical understanding of the frequency dependence of the current responsivity of transistors, it is expressed in terms of the magnitude and phase of drain–gate voltage ratio. The analysis of different contributions to the RF responsivity reveals that the gate–drain capacitive coupling plays a key role in its frequency dependence.

The responsivity of AlGaN/GaN high-electron mobility transistors (HEMTs) when operating as zero-bias RF detectors in the subthreshold regime exhibits different behaviors depending on the operating temperature and gate length of the transistors. We have characterized in temperature (8–400 K) the detection performance of HEMTs with different gate lengths (75–250 nm). The detection results at 1 GHz can be reproduced by a quasi-static model, which allows us to interpret them by inspection of the output $I_D$−$V_{DS}$ curves of the transistors. We explain the different behaviors observed in terms of the presence or absence of a shift in the zero-current operating point originating from the existence of the gate-leakage current jointly with temperature effects related to the ionization of bulk traps.