Behaviour of 1.2 kV SiC JBS diodes under repetitive high power stress
Introduction
Wide bandgap (WBG) semiconductors show better electrical and physical properties than Silicon for developing power devices. Silicon carbide (SiC) is the WBG semiconductor with the most mature technology. Since 2001 Infineon commercializes 600 V and 1200 V SiC Schottky diodes. The SiC rectifier overcomes the Si PiN diode, which limits the performances of power systems in application such as power factor correction (PFC). However, SiC Schottky diodes exhibit a limitation in terms of their surge current capability. A new generation of Schottky diodes has appeared in the market in 2006 to solve this problem. The surge current limitation has been addressed by means of an additional bipolar contribution at high current level [1]. This is the concept of the junction barrier Schottky diode (JBS). Very few works on reliability measurements of 4H-SiC JBS diode have appeared in recent literature due to the lack of such devices and to the need of new methods to evaluate the real long term performances of SiC devices. This paper presents a new methodology to evaluate the power cycling capability and reliability of 1.2 kV JBS diodes fabricated in our facilities. A set-up for the surge current measurement has been developed to test high current SiC diodes. The same set-up has been adapted to perform electrothermal stresses. The aim is to stress the SiC JBS diodes at 40% of the surge current capability, which is significantly higher than the nominal current. Infineon has reported a test in which a 3 ms rectangular current pulse, between 4 and 7 times higher than the nominal current, is used to evaluate their 2nd generation SiC Schottky diodes [2].
After a short description of the design and the technological process, the static and switching electrical characteristics in the 25–300 °C temperature range are shown. A new test set-up is explained in detail, which is able to determine the surge current of diodes and to apply a power stress to the diodes. The results of the surge current of the two different JBS diodes and a Schottky diode are analized. The impact of the power stress on the three different diodes is also discussed.
Section snippets
Diodes design and fabrication
4H-SiC JBS diodes have been fabricated on a CREE N+-substrate on which an N-type epilayer, 13 μm thick and 8 × 1015 cm−3 doped, has been growth. Fig. 1 shows the cross-section of the diodes. The edge termination consists in a junction termination extension (JTE) with a 120 μm length and an implantation dose of 1013 cm−3. The JBS design alternates P+-regions (high temperature Al implants) with Schottky contacts areas. The electrical characteristics basically depend on two geometrical parameters (LP
Static characteristics
The diodes have been packaged using a Au/Ge die attach in a TO-247 DBC test package. The wire bondings have been performed with two 320 μm Al wires. The diodes have been forward characterized in the 25–300 °C temperature range. The influence of the design has been studied previously [3]. Fig. 2 reports the forward electrical characteristics at 25 °C and 300 °C of JBS D5 and D6 diodes and that of a 4 mm2 SiC Schottky diode. At 25 °C, the Schottky and the JBS D6 diodes show similar characteristics
Switching characteristics
To characterize the diodes in switching mode, a DC/DC converter in buck configuration has been developed using a 1.2 kV–40 A Si-IGBT. The inductive load was optimized in order to obtain a dI/dt = 200 A/μs. The diodes can be characterized in the 25–300 °C temperature range at 300 V–10 A. Fig. 4 shows the current waveforms at the turn-off of the diodes at 25 °C and 300 °C. At 25 °C, the 4 mm2 Schottky diode exhibits a higher reverse peak current than the 2.6 mm2 JBS diodes. This is due to the larger area of
Surge current and power cycling test bench
A schematic of the test bench is shown in Fig. 5. An autotransformer T1is used to provide the pulse amplitude tuning. A second transformer T2 provides the maximum current level needed for the test. The test bench can operate either as
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Surge current tester (switch is set in position 2)
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Power cycling bench (switch is set in position 1)
For the surge current test, the current pulse is manually enabled by the test button. Every push on of the button provides a 10 ms semi-sinusoidal current pulse to
Surge current test results
The surge current capability test consists in applying a high current pulse with a short duty cycle for allowing the device temperature to return to its starting value after applying each individual pulse, according to the US Military Standard -STD-750E 4000 Series. A 10 ms half sinusoidal forward current pulse is used for this test. The device temperatures are 25 °C and 225 °C. The current amplitude is increased step by step up to the device destruction. The data acquisition from the waveforms
Power cycling results
It is well known that the main stress of a power electronic device comes from a temperature variation, either from an external source or from the device self-heating. In turn, the temperature variation induces mechanical stresses. After a large number of cycles the material could suffer from either mechanical fatigue produced by the material aging, or composition changes due to thermal fatigue. Moreover, local current imbalance can develop non-uniform heating of the device, leading to
Discussion and conclusions
The paper presents the results of surge current test and of a new accelerated reliability test method of 1200 SiC (JBS and Schottky) diodes.
A dedicated test bench was developed both for the surge current test and for power cycling reliability test. The test signal is 10 ms half sinusoidal current.
Two important results have to be mentioned about the surge current:
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JBS diodes show a ×2.66 (×4.16) higher surge current capability at 25 °C (225 °C) than pure Schottky diodes.
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The surge current capability
Acknowledgements
This work has been supported by the European Space Agency (ESA) in the framework of the RTP project CHPCA AO4616 supervised by Dr. Benoit Lambert and by the Spanish Ministerio de Educación y Ciencia under contract TEC2005-087392 (SPACESIC Project).
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