Michael Treu

Reliability of SiC power devices and its influence on their commercialization - review, status, and remaining issues

The following paper will give an overview about the main reliability aspects of silicon carbide power devices. After a brief review of the key device concepts it covers reliability topics of bipolar devices, Schottky diodes, metal oxide semiconductor field effect devices, and junction field effect devices. Special attention is paid to the influence of the different reliability topics on the commercialization of the different device types. It will be shown that for some device types the reliability is at a very high level being not hampering commercialization (e.g. Schottky diodes or junction field effect devices) whereas other devices concepts still need improvement until commercialization (e.g. bipolar devices).

2nd Generation SiC Schottky diodes: A new benchmark in SiC device ruggedness

A new commercially available SiC diode is reported and key features like widely improved surge current capability and avalanche ruggedness are described and related to the merged pn/Schottky device structure. The clearly positive temperature coefficient of the avalanche allows high avalanche power dissipation in the range of 20W/mm2 even in the frame of long term (1000h) testing. Additionally the device has the same excellent dynamic properties as already known from pure SiC Schottky diodes. The dV/dt ruggedness of the device was also proven with a long term test (3.6E11 cycles with > 90V/ns). No noticeable difference in switching losses occur when the device is switched off from nominal current at room temperature or from 10times nominal current at 150-C. The long term stability of the device under bipolar operation at high current densities (> 2kA/cm2) at the p-areas was proven over a stress time of 1h

A Surge Current Stable and Avalanche Rugged SiC Merged pn Schottky Diode Blocking 600V Especially Suited for PFC Applications

Today silicon carbide (SiC) Schottky diodes are mainly used in the power factor control (PFC) unit of high end switched mode power supplies, due to their outstanding switching performance compared to Si pn diodes. In the case of the PFC it is required that the diodes are capable of handling surge currents up to several times the current of normal operation. The paper shows the surge current capability of a merged pn Schottky diode where the p-areas are optimized as efficient emitters. During normal operation the diode is behaving like a normal Schottky diode whereas during surge current condition the diode is behaving like a pn diode. For a sine half wave of 10 ms we achieved a non repetitive peak forward current capability of about 3700 A/cm2 which is about ten times rated current (for comparison: destructive current density of a standard Schottky diode ~ 1650 A/cm²). Additionally the device shows a stable avalanche and is able to withstand a single shot avalanche of 9.5 3s and 12.5 mJ.

Reliability of SiC Power Devices Against Cosmic Radiation-Induced Failure

Cosmic radiation has been identified as a decisive factor for power device reliability. Energetic neutrons create ionizing recoils within the semiconductor substrate which may lead to device burnout. While this failure mode has gained widespread acceptance for power devices based on silicon the question whether a similar mechanism could also lead to failure of SiC devices was left to be debated. Radiation hardness intrinsic to the SiC material was generally assumed but as experimental data was scarce reliability problems due to radiation-induced device failure could not be ruled out. Recent accelerated testing results now show that cosmic radiation will indeed affect the reliability of SiC power devices, as it is the case for its silicon counterpart, but the problem can be contained very effectively by device design.

2nd generation 600V SiC Schottky diodes use merged pn/Schottky structure for surge overload protection

A new generation of silicon carbide Schottky diodes has been developed which address surge current overload thermal runaway and lack of avalanche clamping by the use of a merged pn/Schottky diode structure. A PN structure with low ohmic contact is used in parallel with Schottky structure to greatly extend peak current handling and also provide avalanche energy capability without compromising dynamic performance.

SiC JFET: Currently the Best Solution for an Unipolar SiC High Power Switch

Today a main focus in high efficiency power electronics based on silicon carbide (SiC) lies on the development of an unipolar SiC switch. This paper comments on the advantages of SiC switching devices in comparison to silicon (Si) switches, the decision for the SiC JFET against the SiC MOSFET, and will show new experimental results on SiC JFETs with focus on the production related topics like process window and parameter homogeneity which can be achieved with the presented device concept. Due to material properties unipolar SiC switches have, other than their Si high voltage counterparts, very low gate charge, good body diode performance, and reduced switching losses because of the potential of lower in- and output capacitances. The most common unipolar switch is the MOSFET. However, the big challenge in the case of a SiC MOSFET is the gate oxide. A gate oxide on SiC that provides adequate performance and reliability is missing until now. An alternative unipolar switching device is a normally-on JFET. The normally-on behavior is a benefit for current driven applications. If a normally-off behavior is necessary the JFET can be used together with a low voltage Si MOSFET in a cascode arrangement. Recently manufactured SiC JFETs show results in very good accordance to device simulation and demonstrate the possibility to fabricate a SiC JFET within a mass production. A growing market opportunity for such a SiC switch becomes visible.

Influence of Overgrown Micropipes in the Active Area of SiC Schottky Diodes on Long Term Reliability

Other than open micropipes (MP), overgrown micropipes do not necessarily lead to a^significantly reduced blocking capability of the affected SiC device. However they can lead to a degradation of the device during operation. In this paper the physical structure of overgrown micropipes will be revealed and their contribution to the leakage current will be shown. The possible impact of the high local power dissipation in the surrounding of the overgrown micropipe will be discussed and long term degradation mechanisms will be described. Failure simulation under laboratory conditions shows a clear correlation between the position of overgrown micropipes and the location of destructive burnt spots.

Si, SiC and GaN power devices: An unbiased view on key performance indicators

This paper discusses key parameters such as capacitances & switching losses for silicon, SiC and GaN power devices with respect to applications in switch mode power supplies. Whereas wide bandgap devices deliver roughly one order of magnitude lower charges stored in the output capacitance, the energy equivalent is nearly on par with latest generation super junction devices. Silicon devices will hence prevail in classic hard switching applications at moderate switching frequencies whereas SiC and GaN based power devices will play to their full benefits in resonant topologies at moderate to high switching frequencies.

Application specific trade-offs for WBG SiC, GaN and high end Si power switch technologies

There is an increasing choice of power switches in the 600V to 1700V range for the application engineers. Besides the well-established Si SJ (Super Junction) MOSFETs and IGBTs now also silicon carbide (SiC) and latest gallium nitride (GaN) power switches are available for new designs. Complete new system optimizations are possible driven by totally different trade off options e.g. between static and dynamic losses and their temperature dependencies. In this paper we explain these trade-offs for the different device types and show the consequences based on some prominent sample applications.

1200V SiC JFET in Cascode Light Configuration: Comparison versus Si and SiC Based Switches

A 1200 V SiC JFET has been demonstrated to achieve ultra-low switching losses ten times lower than for industrial grade 1200V Si IGBT. The low switching losses are also shown to compete with the fastest 600V class MOSFET in the market, yielding 1.1% higher PFC stage efficiency for 340 kHz switching frequency, when same device on-resistances were measured. The proposed normally-on JFET also differentiates over the IGBT by its purely Ohmic output characteristics without any voltage threshold, and by a monolithically integrated body diode with practically zero reverse recovery. In this paper we outline as well how the other pre-requisites for a 1200 V SiC switch in applications such as photovoltaic systems and UPS can be fulfilled by the proposed JFET solution: long-term reliability, product cost optimization by low specific on-resistance combined with reasonable process window expectations. Finally, a normally-off like safe operation behavior is ensured by a dedicated driving scheme utilizing a low-voltage Si MOSFET as protection device at system start-up and for system failure conditions.