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A Survey of Wide Bandgap Power Semiconductor Devices

Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

SiC Schottky Diodes for Harsh Environment Space Applications

This paper reports on the fabrication technology and packaging strategy for 300-V 5-A silicon carbide Schottky diodes with a wide temperature operation range capability (between -170 °C and 300 °C). These diodes have been designed for harsh environment space applications such as inner Solar System exploration probes. Different endurance tests have been performed to evaluate the diode behavior when working at a high temperature and under severe thermal cycling conditions (ranged from -170 °C to 270 °C). The radiation hardness capability has been also tested. It has been found that the hermeticity of the package in a neutral atmosphere is a key aspect to avoid an electrical parameter drift. Moreover, the use of gold metallization and gold wire bonds on the anode allows reducing the diode surface and bonding degradation when compared to Al-containing technology. On the back-side cathode contact, the Ti/Ni/Au metallization and AuGe combination have shown a very good behavior. As a result, the manufactured diodes demonstrated high stability for a continuous operation at 285 °C.

Thermomechanical Assessment of Die-Attach Materials for Wide Bandgap Semiconductor Devices and Harsh Environment Applications

Currently, the demand by new application scenarios of increasing operating device temperatures in power systems is requiring new die-attach materials with higher melting points and suitable thermomechanical properties. This makes the die-attach material selection, die-attaching process, and thermomechanical evaluation a real challenge in nowadays power packaging technology. This paper presents a comparative analysis of the thermomechanical performance of high-temperature die-attach materials (sintered nano-Ag, AuGe, and PbSnAg) under harsh thermal cycling tests. This study is carried out using a test vehicle formed by four dice (considering Si and SiC semiconductors) and Cu substrates. Thermally cycled test vehicles have been thermomechanically evaluated using die-shear tests and acoustic microscopy inspections. Besides, special attention is paid to set up a nano-Ag sintering process, in which the effects of sintering pressure or substrate surface state (roughness and surface activation) on the die-attach layer are analyzed. As a main result, this study shows that the best die-attach adherence is obtained for nano-Ag when pressure is applied on the dice (using a specifically designed press) during the sintering process (11 MPa provided die-shear forces of 53 kgf). However, this die-attach presents a faster thermomechanical degradation under harsh thermal cycling tests than other considered high-temperature die-attach materials (AuGe and PbSnAg) and PbSnAg shows the best thermomechanical performances.

Analysis of Clamped Inductive Turnoff Failure in Railway Traction IGBT Power Modules Under Overload Conditions

This paper studies the overload turnoff failure in the insulated-gate bipolar transistor (IGBT) devices of power multichip modules for railway traction. After a detailed experimental analysis carried out through a dedicated test circuit, electrothermal simulations at device level are also presented. The simulation strategy has consisted in inducing a current and temperature mismatch in two IGBT cells. Results show that mismatches in the electrothermal properties of the IGBT device during transient operation can lead to uneven power dissipation, significantly enhancing the risk of failure and reducing the lifetime of the power module. Concretely, simulations qualitatively demonstrate that localized hot-spot formation due to a dynamic breakdown could lead to a second breakdown mechanism.

Long-Term Reliability of Railway Power Inverters Cooled by Heat-Pipe-Based Systems

This paper analyzes the impact of a nonuniform temperature distribution inside insulated-gate bipolar transistor (IGBT) power modules on the reliability of railway power inverters. The interaction between the chosen cooling system (a heat-pipe-based one) and the power module is considered in detail. After showing the experimental setup and thermal conditions, thermal mapping inside the power module is carried out. Then, the effects of the thermal cycles on the constitutive elements of the IGBT module are pointed out when considering a real mission profile. Finally, the experimental results from the thermal cycles are linked to problems on the power-inverter reliability. Concretely, the thermal-grease distribution is analyzed on failed IGBT modules coming from the field, and the solder-delamination pattern observed in IGBT modules after endurance cycling tests is also reported.

Irradiance-based emissivity correction in infrared thermography for electronic applications

This work analyzes, discusses, and proposes a solution to the problem of the emissivity correction present in infrared thermography when coatings with known emissivity cannot be deposited on the inspected surface. It is shown that the conventional technique based on two reference thermal images and the linearization of the blackbody radiation dependence on temperature is not a reliable and accurate solution when compared with the coating procedure. In this scenario, a new approach based on the direct processing of the output signal of the infrared camera (which is proportional to the detected irradiance) is proposed to obtain an accurate emissivity and surrounding reflections map, perfectly compensating the thermal maps. The results obtained have been validated using a module as a test vehicle containing two thermal test chips which incorporate embedded temperature sensors.

Internal infrared laser deflection system: a tool for power device characterization

In this paper, a set-up based on the internal IR-laser deflection technique is described. This technique allows measurement of the temperature gradient and free-carrier concentration inside power semiconductor devices with high spatial (35 µm) and time (less than 1 µs) resolution. The internal IR-laser deflection technique consists in the measurement of deflection and absorption of an IR-laser beam passing through a biased power device. After describing the operational principle and the experimental set-up, the postprocessing methodology followed to extract the temperature gradient and free-carrier concentration is detailed. In order to show the set-up functionality, the drift region of a 600 V PT-IGBT device has been studied. These measurements are very helpful for performing reliability or thermal management studies on power semiconductor devices.

Thermal resistance investigations on new leadframe-based LED packages and boards

In Solid State Lighting, thermal management is a key issue. Within the C-SSL consortium, we have developed an advanced leadframe based package to reduce the thermal resistance of the component. Numerical simulations have been implemented using Ansys® software and thermal measurements have been carried out using the forward voltage method to derive the thermal resistance. The T3ster® transient thermal analysis has been used to determine the different thermal resistance contributions in the package and in the board showing good correlation between experimental and simulation results. Low thermal resistances of 5.5 K/W have been obtained on our advanced leadframe based package and have been compared with standard Rebel LEDs on board.

IGBT module failure analysis in railway applications

This work reports two different characteristic patterns detected in IGBT chips failed in real operation (railway application) by failure analysis procedures. The analysed chips have been recovered from the rheostatic chopper leg and from the three legs which supplies the traction motor. It is observed that depending on the location and characteristics of the detected default (burn-out spot), this failure can be attributed to a latch-up process or a secondary breakdown mechanism. These results are corroborated with tests at limit, obtaining the same result. Consequently, each failure can be linked to overcurrent (latch-up) or overtemperature (secondary breakdown) events, which makes possible to distinguish between problems coming from driving strategies or thermal issues (uneven temperature distribution inside the module or packaging wear-out).

Gate Oxide Degradation of SiC MOSFET in Switching Conditions

Under realistic switching conditions, SiC MOSFETs reliability issues remain as a challenge that requires further investigation. In this letter, a specific aging test has been developed to monitor and characterize the electrical parameters of the SiC MOSFET. This allows estimations of the health state and predictions of the remaining lifetime prior to its failure. The gate leakage current seems to be a relevant runaway parameter just before failure. This leakage indicates deterioration of the gate structure. This hypothesis has been validated through analysis of scanning electron microscopy pictures, with a focused ion beam cut showing cracks within the polysilicon.