Papers by Author: Michael Treu

Abstract: 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.

Abstract: Starting with the production of Infineon´s first silicon carbide (SiC) Schottky diodes in 2001, a lot of progress was achieved during recent years. Currently, a 3rd generation of MPS (merged pn Schottky) diodes is commercially available combining tremendous improvements with respect to surge current capability and reduced thermal resistance. In this work we present the implementation of SiC switches in power modules and a comparison of these units with the corresponding Si-based power modules. Also the frequency dependence of the total losses of the 1200V configurations using Si-IGBTs or SiC-JFETs as active device is shown, indicating that modules solution with a state of the art SiC JFET outperforms all other options for switching frequencies of 20 kHz and beyond. Additionally a total loss vs. frequency study will be presented. Furthermore, it is show that the switching losses of JFET based modules can be further reduced by reducing the internal distributed gate resistivity.

Abstract: With the help of an improved die attach the Rth,jc of SiC Schottky diodes can be reduced by 40-50% at a given chip size. This enables a significant higher power density for these SiC diode chips, resulting in a chip shrink of ~ 35% for a given nominal current. This has a significant impact not only on the cost position of the device but also on the switching performance of the diodes, as their capacitive charge directly scales with the chip area. Of course these advantages are accompanied by a small penalty in static losses as the Vf of the diodes at nominal current also slightly increases by the chip shrink. However, the reduction of switching losses dominates upon the marginally increased static losses besides full load operation conditions (which are pretty exceptional in today’s SiC Schottky diode applications) combined with frequencies below 130 kHz. This allows a better competitive positioning against fast Si-based diodes and improved system efficiency at the same time.

Abstract: In this paper we describe how a merged pn Schottky diode (MPS diode) is capable to drive surge current levels far beyond the normal current of the diode and how to improve the device in order to achieve even higher surge current levels. For a sine half wave of 10 µs an 8A MPS diode (size: 2.52mm2) with conventional Al pad metallization shows surge current levels of greater than 500A, using Cu it can be increased to ~900A. For 10ms pulse length a different behaviour was observed, here diodes with Al pad metallization show a higher surge current level (80A) compared to Cu pad metallization (~ 40A). The root cause for this negative result at longer pulse time is based in a chemical interaction between Cu and the Schottky metal (Ti). Additionally, an outlook is given how Cu can contribute to improved surge current capability also at longer pulse lengths.

Abstract: After the successful introduction of silicon carbide Schottky-Barrier diodes in 2001, next commercial devices will be switching components. The development focus is targeted to MOSFETs and VJFETs. Regarding VJFETs, a promising device was presented several years ago and tested successfully in several applications. Since the unconventional device structure does not allow the use of classical JFET models, a new electro-thermal model was developed, taking into account the features of the design as well as the targeted enlarged range of operating temperatures.

Abstract: SiC Diodes in the 300 to 1200V range have steadily increased their market penetration in the last 7 years. Especially the 600V SiC diodes are a nearly mandatory device for further increase of power density in modern switch mode power supplies. Those devices entered the market from the high end side due to the still significant higher costs in comparison with conventional fast Si diodes. On the other hand, these high end markets like server or telecom power supplies also require very high reliability of the devices used. In previous papers we showed, that Merged-PN-Schottky (MPS) diodes can be designed for avalanche ruggedness [1,2]. In this paper we will describe, how this feature supports overall reliability improvement. Addditionally, we will show, how a conventional SiC Schottky diode without MPS structure can be modified in order to achieve stable avalanche breakdown in combination with strong reliability improvement.

Abstract: 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.

Abstract: 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.

Abstract: 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.