Reliability and reliability investigation of wide-bandgap power devices
Introduction
Reliability evaluation of Si devices has established test procedures. For intended replacing of Si applications, wide bandgap devices have to meet the reliability requirements. With Si power modules, they are basically described in [1]. Based on [1], the focus of the following chapter is on particularities with SiC. For GaN, reliability test procedures have to be developed and established.
Due to the low intrinsic carrier density, SiC has an advantage in terms of overload capability. This is demonstrated for surge current capability of MPS diodes [2] and avalanche tests for Schottky diodes and MOSFETs [3]. Regarding short-circuit capability, one has to consider: The short circuit current of a SiC MOSFET is above 10 times rated current, while for a Si IGBT is in the range of 5 times rated current. The allowed time in short circuit must be reduced.
In this paper, we focus on the gate oxide reliability and on the power cycling reliability. Here are differences SiC to Si, as described in Detail in [4].
Section snippets
Gate oxide reliability investigation
The channel mobility is a decisive parameter for MOS structures, due to surface scattering it is always lower than the bulk mobility. The quality of the surface is a challenge for SiC, as well as the density of interface defects at the SiC/SiO2 interface. First SiC MOSFETs suffered from a low channel mobility of <10 cm2/Vs, meanwhile 70 cm2/Vs are achieved.
the RON is typically a series of three major componentswhere Rch is the channel resistance of the device, RJFET is the
Power cycling method and junction temperature determination
In a power cycling test, the power chips are actively heated by the losses generated in the power devices themselves. Decisive is the control of the junction temperature. Basically, a pn-junction is used as temperature sensor, its junction voltage is the temperature-sensitive electrical parameter (TSEP). The method to determine Tvj is established since the beginning of power device development [9]. The so determined temperature is named as virtual junction temperature Tvj. A detailed
Power cycling results
When we compare the material parameters of Si and SiC, a significant difference in the mechanical characteristics emerges. The Young's modulus, which represents the stiffness of the material under mechanical stress, is about three times higher in SiC (501 GPa in SiC compared to 162 GPa in Si). The temperature swings and thermal mismatch induce mechanical stress into the package which leads to fatigue. For evaluation of the expected effect in a solder layer, the plastic strain energy density ΔW
Further reliability tests
In the high temperature reverse bias test, Si and SiC are both sensitive, however there are no higher challenges for SiC. The design is in the way that the breakdown occurs in the cell field [22]. The base doping in SiC is about 100 times higher than in Si for the same blocking voltage, therefore 100 times more surface charges are required to create an inversion channel.
The high humidity high temperature reverse bias test has to be executed at 80% of rated voltage in future devices. This is a
Summary
Challenges going from Si to SiC are especially the gate oxide stability and the power cycling capability. For the gate oxide, a new test method is suggested. Different results for different manufacturers were found, one supplier is already showing a reliability which is close to Si IGBTs. Regarding power cycling, the challenge is to find a suited TSEP and to execute the test application-close. SiC needs an improved packaging technology to achieve the lifetime of Si.
For GaN, the first results
Acknowledgment
A part of this work is supported by the German Federal Ministry of Education and Research (BMBF) in the frame of the project GaNMOBIL.
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