Fabiana Rampazzo

Surface-related drain current dispersion effects in AlGaN-GaN HEMTs

Drain current dispersion effects are investigated in AlGaN-GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects characterized by time constants in the order of 10/sup -5/-10/sup -4/ s cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

Current Collapse and High-Electric-Field Reliability of Unpassivated GaN/AlGaN/GaN HEMTs

Long-term ON-state and OFF-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN high-electron-mobility transistors on SiC substrates. Because of the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite the fact that they are not passivated. This comes at the price of a relatively high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both ON-state and OFF-state stresses produce simultaneous current-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drain surface degradation and reduced gate electron injection. This study shows that although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation, and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, prepassivation surface treatments, and/or field-plate gate

Evidence of traps creation in GaN/AlGaN/GaN HEMTs after a 3000 hour on-state and off-state hot-electron stress

A long-term 3000-hour test under on-state conditions (VDS =25V, 6W/mm constant dissipated power) and off-state conditions (V DS=46V, VGS=-6V) on GaN/AlGaN/GaN HEMTs is presented. Trapping presence and hot-electrons effect are characterized by means of low-frequency techniques (low-frequency noise measurements, transconductance frequency dispersion, gate-lag). The on-state stress shows the most important degradation. Since our measurements point out to the creation of traps in the gate-to-drain surface region during the stress, this degradation is ascribed to the effect of hot-electrons

A review of failure modes and mechanisms of GaN-based HEMTs

Failure modes and mechanisms of AlGaN/GaN HEMTs, observed during accelerated tests at various bias conditions are reviewed.

Degradation mechanisms of GaN-based LEDs after accelerated DC current aging

This work presents the results of an extensive DC current aging and failure analysis carried out on blue InGaN/GaN LEDs which identify failure mechanisms related to package degradation, changes in effective doping profile, and generation of deep levels. DLTS and photocurrent spectra indicate the creation of extended defects in devices aged at very high current density.

Influence of gate-leakage current on drain current collapse of unpassivated GaN∕AlGaN∕GaN high electron mobility transistors

We report on a correlation between the gate leakage currents and the drain current collapse of GaN∕AlGaN∕GaN high electron mobility transistors. Unpassivated devices on intentionally undoped and doped (Si, 5×1018cm−3) heterostructures were investigated. We observed in the devices that the larger the gate leakage current, the smaller the drain current collapse measured at 50ns gate-voltage pulse turn on, and this correlation is independent of the doping of the structure. The correlation holds for two orders of magnitude in the gate-leakage current and up to 15% in drain current collapse. We believe that the leakage current can modulate trapped surface charge so that the time constant of the current collapse becomes much faster and dependent on the amount of leakage current itself.

Characterization and analysis of trap-related effects in AlGaN-GaN HEMTs

Abstract Traps are characterized in AlGaN–GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce “false” surface-trap signals, i.e. the same type of current-mode DLTS (I-DLTS) or ICTS signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process.

Experimental/numerical investigation on current collapse in AlGaN/GaN HEMT's

Rf current collapse is investigated in AlGaN/GaN HEMTs by means of pulsed, transient, and small-signal measurements. Numerical device simulations are presented, showing that the concomitant presence, at the ungated device surface, of polarization-induced charges and hole traps can explain, without invoking any other hypothesis, all dispersion effects observed experimentally.

Hot-electron-stress degradation in unpassivated GaN/AlGaN/GaN HEMTs on SiC

The paper discusses long term on-state and off-state stress on GaN/AlGaN/GaN HEMTs on SiC substrates. Hot carrier effects and their dependence on bias conditions are evaluated with electroluminescence measurements. Both hot-electron stress conditions produce drain current gate-lag dispersion and gate current decrease. However on- and off- state stresses induce degradation in different gate-to drain surface device regions, i.e. close to the drain contact for the on-state stress and close to the gate contact in the off-state stress. Furthermore, a correlation between gate-leakage current and gate-lag dispersion is also observed.

Developments on DC/DC converters for the LHC experiment upgrades

Prototypes of DC/DC power and Point of Load (PoL) converters were designed and built with the aim of satisfying the foreseen working parameters of the High Luminosity (HL) LHC experiments, using both Silicon (Si) MOSFETs and/or more recent devices substantiated of better power performance, like Silicon Carbide (SiC) and Gallium Nitride (GaN) transistors. Optimization of their design, based on the comparison between the simulated and measured thermal, electrical and mechanical performance, is in progress, and many improvements with respect to the previous versions are under implementation. We discuss in this paper the results of the last modifications. In addition, many tens of discrete component samples, chosen among the devices commercially available in the three different technologies (Si, SiC and GaN), were electrically characterized and tested under γ-rays, neutron, proton and heavy ion radiation, also using a combined run method. We have also planned to test some commercial DC/DCs under the extreme conditions of radiation and magnetic field expected in the upgrades of the LHC experiments. Here we show the first results on few samples.