http://hdl.handle.net/10366/4531 Mon, 27 Apr 2026 06:35:44 GMT 2026-04-27T06:35:44Z http://hdl.handle.net/10366/170603 [EN]We investigate the frequency-dependent nonlinearities of an AlGaN/GaN high electron mobility transistor (HEMT) involved in terahertz (THz) detection by means of 2-D Monte Carlo (MC) simulations. The analysis shows that, below 1 THz, the responsivity roll-off is mainly governed by the passive microwave behavior of the device rather than by any limitation of the intrinsic detection mechanism. At higher frequencies, an inverse extraction method able to provide the intrinsic nonlinearity coefficients of the device reveals just a marginal broad enhancement around 1 THz, followed by a steep roll-off. Tue, 24 Feb 2026 00:00:00 GMT http://hdl.handle.net/10366/170603 2026-02-24T00:00:00Z http://hdl.handle.net/10366/169272 [EN]The breakdown of GaN-based Schottky barrier diodes associated with impact-ionization events initiated by electrons injected by tunneling is physically analyzed by means of a Monte Carlo simulator self-consistently coupled with a two-dimensional solution of the Poisson equation. Simulations of a realistic topology where different geometrical parameters are modified allow to identify their influence on the breakdown voltage. The correct physical modelling of two-dimensional effects is essential for a proper prediction of the breakdown. Epilayer doping and thickness, dielectric used for the passivation and lateral extension of the epilayer are analyzed. As expected, the lower the doping and the thicker the epilayer, the higher the value found for the breakdown voltage, but, interestingly, the results also indicate that the peak electric field present at the edge of the Schottky contact, which may be reduced by means of high-k dielectric passivation and a short lateral extension of the epilayer, plays a key role in the breakdown. Wed, 01 Jan 2025 00:00:00 GMT http://hdl.handle.net/10366/169272 2025-01-01T00:00:00Z http://hdl.handle.net/10366/168892 [EN]Self-heating significantly impacts the performance and reliability of GaN high electron mobility transistors, but capturing these effects with electrothermal Monte Carlo (MC) simulations is computationally intensive. This paper presents the application of a hybrid AI-thermal model, previously tested on another device, to the electrothermal analysis of GaN HEMTs. This first component consists of an artificial neural network (ANN) trained on isothermal MC data to predict drain current and lattice temperature. To extend the framework, a set of ANN-based microscopic models is introduced, composed of three dedicated networks that reconstruct spatially resolved quantities—electric field, carrier velocity, and sheet electron density. The system is coupled with compact thermal resistance models and iterated until convergence. The proposed approach achieves excellent agreement with electrothermal MC simulations while reducing computation time by approximately an order of magnitude. In addition to global performance metrics, it provides detailed internal profiles under electrothermally consistent conditions, making it a practical tool for fast device evaluation, in-depth analysis, and integration into compact modeling flows. Sun, 07 Sep 2025 00:00:00 GMT http://hdl.handle.net/10366/168892 2025-09-07T00:00:00Z http://hdl.handle.net/10366/167239 [EN]Impact ionization originated by the buffer leakage current, together with high electric fields (>3 MVcm−1) at the anode corner of the isolating trenches, has been identified as the failure mechanism of shaped planar GaN Gunn diodes when biased above 20 V, so that no evidence of Gunn oscillations in fabricated devices has been observed yet. In order to avoid the avalanche, we propose the addition of a Schottky substrate terminal, which, by means of Monte Carlo simulations, has been confirmed to be able to suppress such not-desired leakage current when applying a negative substrate bias. When the substrate bias is positive, impact ionization is also reduced due to the lower electric field at the hotspot, but a vertical cathode-substrate current degrades the device operation. In order to avoid such current, we propose the use a MIS configuration for the substrate contact, which is the optimal solution. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/167239 2024-01-01T00:00:00Z http://hdl.handle.net/10366/167233 [EN]AlGaN/GaN nanodiodes consisting of an array of several nanochannels in parallel are potential candidates for detection in the terahertz (THz) range. The nanochannels are fabricated by etching two isolating trenches and show a current–voltage characteristic strongly influenced by the presence of surface charges at the channel sidewalls. Transient current effects have been characterized at room temperature and found to be associated with electron capture and emission mechanisms by surface traps. The conductance of these devices increases or decreases depending on the history of applied voltage since it changes the occupation of the surface states and thus the depletion region present near the sidewalls. Moreover, the lat eral field effect plays an important role, since, in addition to promoting trap charging or discharging, modifies the depletion region around the trenches, both of these processes determine the conductance of the channel. In addition, the increase of the bias induces an effect analog to the drain-induced barrier lowering (DIBL) of FETs. In this article, the static behavior and transients of current of these nanochannels were characterized from the experimental point of view thanks to very short duration voltage pulses, while Monte Carlo (MC) simulations were able to mimic the observed trends providing as well a physical interpretation that the charges trapped at the sidewalls of the trenches of the channels act as the gate in a FET. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/167233 2024-01-01T00:00:00Z http://hdl.handle.net/10366/167232 [EN]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 475K, we demonstrate the presence of two leakage mecha nisms 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 200K, while trap-assisted tunneling prevails for lower tempera tures, 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 occu pancy 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. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/167232 2024-01-01T00:00:00Z http://hdl.handle.net/10366/167231 [EN]This work shows that for a correct analysis of Schottky barrier diodes operating under strong reverse-bias conditions, it is necessary to account for the self-consistency between the shape of the energy barrier and carrier concentration in the depletion region since the full-depletion approximation fails to estimate the current. This happens for very high applied voltages, at which impact ionization by electrons and holes must also be considered. Two example GaN diodes with different doping concentrations and barrier heights are analyzed. The results are relevant to regions of the diodes where a very high tunnel injection takes place, like the contact edge or surface inhomogeneities. Sun, 01 Jan 2023 00:00:00 GMT http://hdl.handle.net/10366/167231 2023-01-01T00:00:00Z http://hdl.handle.net/10366/167225 [EN]A model to predict the ideal reverse leakage currents in Schottky barrier diodes, namely, thermionic emission and tunneling components, has been developed and tested by means of current–voltage–temperature measurements in GaN-on-SiC devices. The model addresses both current components and both forward and reverse polarities in a unified way and with the same set of parameters. The values of the main parameters (barrier height, series resistance, and ideality factor) are extracted from the fitting of the forward-bias I–V curves and then used to predict the reverse-bias behavior without any further adjustment. An excellent agreement with the I–V curves measured in the forward bias in the GaN diode under analysis has been achieved in a wide range of temperatures (275–475K). In reverse bias, at temperatures higher than 425K, a quasi-ideal behavior is found, but additional mechanisms (most likely trap-assisted tunneling) lead to an excess of leakage current at lower temperatures. We demonstrate the importance of the inclusion of image-charge effects in the model in order to correctly predict the values of the reverse leakage current. Relevant physical information, like the energy range at which most of the tunnel injection takes place or the distance from the interface at which tunneled electrons emerge, is also provided by the model. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/10366/167225 2022-01-01T00:00:00Z http://hdl.handle.net/10366/167222 [EN]By means of Monte Carlo simulations of gallium nitride(GaN) planar Gunn diodes,the epilayer structure on which they are fabricated is optimized in order to achieve ultrahigh-frequency oscillations. Practical considerations, such as the limitations of the technological fabrication process and the mitigation of the huge self-heating effects expected to appear in the devices, have been also contem plated for the choice of the optimal epilayer parameters.The best results are obtained for an active layer thickness of 150 nmwith doping of 5 × 1018 cm−3, which would provide 350GHzGunnoscillationswhenusingacontactseparation of 0.5 μm. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/10366/167222 2022-01-01T00:00:00Z http://hdl.handle.net/10366/163596 [EN]This study investigates the performance of planar Gunn diodes based on highly doped Gallium Nitride using Monte Carlo simulations. The conversion efficiency is evaluated in geometrically V-shaped channels with an active region length of 500 nm, an input channel width of 200 nm, and output widths of 200 nm, 220 nm, and 250 nm. The diodes are subjected to various biasing conditions to assess DC-to-AC conversion efficiency under different AC biases, simulating operating conditions similar to those found in tuned circuits (comprising R, L, and C elements). The efficiency is analyzed for an AC voltage of 2 V superimposed on a 20 V DC bias, considering four distinct doping levels in the active region. These devices demonstrate conversion efficiencies of up to 0.36 % at frequencies of 340 GHz for a channel doping level of N_D=1.0x10^18 cm-3 and an output width of 250 nm. The increase of lattice temperature reduces the efficiency of the diodes, although the obtained values indicate that the devices would still remain operational. Additionally, the frequency range where efficiency is positive (generation band) decreases as temperature increases. Wed, 01 Jan 2025 00:00:00 GMT http://hdl.handle.net/10366/163596 2025-01-01T00:00:00Z http://hdl.handle.net/10366/160316 [EN]GaN-on-sapphire Schottky barrier diodes (SBDs) have been fabricated for frequency multiplier applications. A complete set of characterization has been done, including DC and RF measurements. A model to extract the Schottky parameters from S-parameters measurements in small diodes is proposed, obtaining good capacitance agreement compared with that extracted from capacitance-voltage (C-V) measurements carried out in large-area diodes where the parasitic effects are not significant. Sun, 01 Sep 2024 00:00:00 GMT http://hdl.handle.net/10366/160316 2024-09-01T00:00:00Z http://hdl.handle.net/10366/160314 [EN]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 compatible with the existence of a trap energy band 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 occupation within the trap energy band, originating the hysteresis cycle. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/160314 2024-01-01T00:00:00Z http://hdl.handle.net/10366/159401 [EN]—An investigation into self-switching diodes based on highly doped GaN is conducted under direct current (DC) bias conditions. Different device geometries are explored under various lattice temperatures and polarization scenarios. Also, the impact of adopting an intervalley energy ε1−2=0.9eV for this material is examined and compared with results obtained with the traditionally accepted value of 2.2eV. For a rectangular channel configuration, simulations predict oscillation frequencies in excess of 200GHz, much above the expected transit-time value, due to the fact that the Gunn domains are formed near the anode side of the channel. Conversely, structures with a V-shape geometry are able to start the formation of the Gunn domain inside the channel, thus generating oscillations at much lower frequencies (tens of GHz). The key result is that the lower ε1−2 leads to smaller threshold voltage values (and also slightly smaller oscillation frequencies), particularly in diodes with short channels. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/159401 2024-01-01T00:00:00Z http://hdl.handle.net/10366/159400 [EN]This article presents a hybrid artificial intelligence (AI)-thermal model for the determination of the current and lattice temperature of a device under a given bias voltage. The model is based on a neural network trained with isothermal Monte Carlo simulations and the coupling of any thermal model where the lattice temperature depends on the dissipated power. The proposed procedure has been validated on a gallium nitride (GaN)-based self-switching diode, although its application to other electronic devices, such as transistors, is also straightforward. The proposed method allows for a significant reduction in computational cost, in addition to enabling the investigation of various thermal models in an efficient manner. It is capable of reproducing the results that would be obtained through electrothermal Monte Carlo simulations, which are particularly computationally expensive. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/159400 2024-01-01T00:00:00Z http://hdl.handle.net/10366/158649 [EN]In this work, by means of Monte Carlo (MC) simulations, the current responsivity of AlGaN/GaN HEMTs operating as zero-bias detectors is analyzed, reaching the THz frequency range. Two approaches are used for the calculation of the responsivity, trying to get physical information on the detection mechanisms from the comparison of their respective results. First, we determine the responsivity directly from the MC values of dc drain current shift originated by an input ac drain voltage excitation. As second approach, the responsivity is estimated from a closed-form expression involving the MC calculation of both the dc I – V curves of the transistor (to determine Taylor expansion coefficients) and the ac response in terms of the Y parameters (to be converted into S parameters). Both approaches coincide at low frequency, but the closed-form expression starts to deviate from the correct result at frequencies around 1 THz. Moreover, a modest plasma resonance is visible in the 2–5-THz range. Fri, 01 Mar 2024 00:00:00 GMT http://hdl.handle.net/10366/158649 2024-03-01T00:00:00Z http://hdl.handle.net/10366/155349 [EN]This paper presents an analysis of detection in the microwave region with AlGaN/GaN high-electron mobility transistors in terms of the key figures of merit: responsivity (both with voltage- and current-mode detection schemes) and noise equivalent power, in the temperature range between 20 and 400 K. Transistors with three different gates lengths (75, 150 and 250 nm) have been measured and favorably compared with a simple quasi-static model extracted from the DC curves, which is able to reproduce the zero-bias experiments at 1 GHz in the entire gate-bias sweep and operating temperature range. Such model allows to explain the detection experiments performed both with the devices operating in zero-current or zero-voltage conditions, and demonstrates that the bowing coefficient of the Id-Vds curves is the parameter which determines the performances of the devices as RF detectors. The voltage responsivity (in V/W) increases when the gate bias approaches the threshold voltage, but shows different behaviors in subthreshold conditions. Depending on whether the zero-current bias point is near zero drain voltage (for leaky devices, when the channel is not well pinched-off due to the short gate or to the conduction through the buffer at low temperature) or shifted to the point where third quadrant conduction starts (for well-behaved devices), the responsivity presents a decrease or an approximately constant value, respectively. On the other hand, when the detector is biased, the curves of current responsivity (in A/W) show a characteristic bell-shape with a maximum which is approximately the same for all tested devices and hardly depends on the gate bias (it is only shifted to different drain bias points). Sun, 01 Jan 2023 00:00:00 GMT http://hdl.handle.net/10366/155349 2023-01-01T00:00:00Z