Paolo Cova

Breakdown walkout in pseudomorphic HEMT's

In this work we show for the first time evidence of gate-drain breakdown walkout due to hot electrons in pseudomorphic AlGaAs-InGaAs-GaAs HEMTs (PHEMTs). Experiments performed on passivated commercial PHEMTs show that hot electron stress cycles induce a large and permanent increase of the gate-drain breakdown voltage. Three-terminal and two-terminal stress conditions are compared, the former producing a much larger walkout due to hot electrons flowing in the channel. Experimental results indicate that a build-up of negative charge in the region between gate and drain is responsible for the breakdown walkout, due to a local widening of the depletion region and a reduction of the peak electric field.

On the effect of power cycling stress on IGBT modules

Abstract IGBT reliability is becoming of great relevance, due to the range of application of these devices. Nevertheless, no standard test methods have been established, in order to evaluate their power cycling reliability. On this paper we report on the effect of ΔT and Tjmaz on the power cycling capability of IGBT dice, by means of a matrix of stress cycles with different values of ΔT and Tjmax. Failure analysis has been performed, in order to understand the failure mechanisms induced by the stress.

Hot electron degradation of the DC and RF characteristics of AlGaAs/InGaAs/GaAs PHEMT's

This paper reports on hot electron (HE) degradation of 0.25-/spl mu/m Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.2/Ga/sub 0.8/As/GaAs PHEMTs by showing the effects of the hot electron stress on both the dc and rf characteristics. The changes of dc and rf behavior after stress turn out to be strongly correlated. Both can be attributed to a decrease of the threshold voltage yielding different effects on the device gain depending on the bias point chosen for device operation and on the bias circuit adopted: a fixed current bias scheme will minimize the changes induced by the stress. The work also presents a study of the dependence of device degradation on the stress bias condition.

Trapped charge modulation: a new cause of instability in AlGaAs/InGaAs pseudomorphic HEMT's

A new degradation mechanism of PM-HEMTs subsequent to hot electron stress tests or high temperature storage tests is presented. A noticeable increase in drain-to-source current, I/sub DS/, is observed after the tests. We show that this I/sub DS/ variation is slowly recoverable and is correlated with the presence of deep levels in the device. Stress tests cause a variation of trapped charge. Trapping of holes created by impact-ionization and/or thermally stimulated electron detrapping induce a variation of the net negative trapped charge, leading to a decrease in the threshold voltage, V/sub T/ and a consequent increase in I/sub DS/. The correlation between g/sub m//spl Delta/V/sub T/ and /spl Delta/I/sub DS/ clearly demonstrates that the variation of trapped charge induced by hot electron tests is localized under the gate.

Enhancement and degradation of drain current in pseudomorphic AlGaAs/InGaAs HEMTs induced by hot-electrons

New failure mechanisms induced by hot-electrons in AlGaAs/InGaAs pseudomorphic HEMTs have been identified by means of accelerated testing of commercial devices from four different suppliers. Different degradation modes have been observed, depending on the device type, namely: (a) recoverable increase of I/sub D/ and |V/sub T/|, which has been attributed to recombination of electrons trapped under the gate with holes generated by impact ionization; (b) enhancement of the kink in the output characteristics, possibly due to the generation of deep levels with subsequent electron trapping/detrapping; (c) permanent increase of the breakdown voltage, due to creation of negatively charged traps in the gate-drain region, yielding a wider space-charge region, hence a reduced maximum electric field. The link between the observed degradation modes and the underlying physical mechanisms is investigated by means of different techniques, and the main functional effects of the degradation modes are addressed.

Lifetime prediction and design of reliability tests for high-power devices in automotive applications

Different procedures are defined and compared to extract the statistical distribution of the thermal cycles experienced by power devices installed in hybrid vehicles operated according to arbitrary mission profiles. This enables both to design efficient accelerated tests tailored on realistic data and to provide the input for lifetime prediction models. Initially, the system lifetime is predicted under the assumption of linear accumulation of the damage produced by low cycling fatigue. In the second part, a novel prediction model based on some fundamental equations is introduced which takes into consideration the creep experienced by compliant materials when they are submitted to thermal cycles.

Thermal characterization and modeling of power hybrid converters for distributed power systems

We have developed and experimentally validated a 3D model for thermal analysis and reliability-conscious design of hybrid power converters, using the commercial finite-element solver COMSOL™. The model was tuned using an accurate packaged MOSFET model and a specially-designed test board for multi-point temperature measurements. Measured and modeled temperatures showed good agreement for various dissipated power levels. A simplified version of the packaged MOSFET was then used in a bridge configuration to build a thermal model of the converter. After thermal studies, we performed preliminary finite-element analysis of the thermally-induced stress distributions.

Thermal modeling of planar transformer for switching power converters

Abstract This paper presents thermal simulations for reliability-oriented design of planar transformers for medium-power, high-frequency DC–DC converters. The modeling approach is based on accurate 3D finite-element thermal simulation of the transformer structure; inputs to the finite-element thermal model are the magnetic and electrical power losses extracted by experimental characterization of a planar transformer test-bench we designed and assembled. The simulation results are compared with those of infra-red thermal measurements. The simulations allowed to evaluate the effect of frequency and output current on the temperature distribution inside the transformer, thus setting limits for reliable operation, and to analyze alternative designs aimed at improving thermal management and, consequently the transformer reliability.

Power converters for future LHC experiments

The paper describes power switching converters suitable for possible power supply distribution networks for the upgraded detectors at the High Luminosity LHC collider. The proposed topologies have been selected by considering their tolerance to the highly hostile environment where the converters will operate as well as their limited electromagnetic noise emission. The analysis focuses on the description of the power supplies for noble liquid calorimeters, such as the Atlas LAr calorimeters, though several outcomes of this research can be applied to other detectors of the future LHC experiments. Experimental results carried on demonstrators are provided.

Thermo-mechanical finite element analysis in press-packed IGBT design

Abstract The reliability of press-packed insulated gate bipolar transistors (IGBTs) depends on satisfactory contact conditions applied at assembly stage and maintained throughout the service life. The objective of this work is the simulation of stresses and strains in press-packed IGBTs due to assembly and thermal cycling. Single-chip as well as multi-chip devices were analyzed with 2D and 3D models including an elastic–plastic material description and the contact between components using the abaqus code. The assembly process was initially modeled and the factors affecting the contact pressure uniformity between contact disks and chip discussed. The thermal cycling associated with accelerated stress test was then introduced to examine contact pressure evolution as well as local stress/strain concentrations and stick/slip conditions. The device sensitivity to potential damage initiation due to thermo-mechanical fatigue and/or fretting is addressed.