Rudolf Elpelt

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.

BIFET – a Novel Bipolar SiC Switch for High Voltage Power Electronics

The driving force for the use of SiC in high voltage switches is the potential benefit from considerably reduced static losses and reduced number of devices in serial connection compared with Si-IGBTs. As an appropriate design for high voltage applications we choose a bipolar normally-on JFET structure (so called BIFET, Bipolar Injection FET), which promises additional advantages in a serial connection in a “supercascode” circuit. Simulations of such BIFETs for an aimed blocking voltage of 4.5 kV and minority carrier lifetimes between 0.5 μs and 5 μs demonstrate a reduction of the static loss by more than 25 % compared to the unipolar JFET for the same blocking voltage. Experimentally achieved forward characteristics of BIFETs show a forward voltage of less than 6 V at 70 A/cm2 and Tj = 150°C. The breakdown of these first BIFETs exhibits an avalanche-like behaviour at 2.5 kV , i.e. approximately 70 % of the planar breakdown. The dynamic performance was investigated in a cascode configuration in a chopper circuit with a clamped inductive load. Introduction The demand for devices able to block several kilovolts is still increasing. In power distribution systems, e.g., today many single switches are connected in series in order to obtain high blocking voltages. Another trend is the fact that the users prefer IGBT like switches with powerless voltage control despite their deficiencies of lower blocking voltage compared with thyristors or GTO’s. The requirements typical for these applications are at least several 100 Amps, so that parallel connection of switches becomes mandatory. The driving force for the use of SiC in high voltage switches is the potential benefit from considerably reduced static losses and reduced number of devices in the serial connection. Above 4 kV the power loss should be less in bipolar than in unipolar SiC switches because of the bipolar conductivity modulation. The task of developing bipolar switches, however is not easy to solve since for bipolar SiC switches physical parameters like minority carrier lifetime and its adjustment are a challenge of design and technology. A simple transfer from experience with silicon won’t work. In silicon, state of the art bipolar switches like IGBTs utilise a p-type substrate. However, using the available p-type SiC substrates, all advantages from conductivity modulation gained from bipolar effects are overcompensated by the high resistivity of the substrate (s. Fig.1: VFsubstrate = 5,25 V). Using n-type substrates will overcome this situation, but in consequence the drift zone needs to be p-type, where the carrier transport properties and defect density are today less analysed compared with n type layers. Therefore one has to look for an high emitter efficiency and a suited lifetime management in order to efficiently modulate the drift zone conductivity . Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1245-1248 doi:10.4028/www.scientific.net/MSF.457-460.1245

Optimization of Vertical Silicon Carbide Field Effect Transistors towards a Cost Attractive SiC Power Switch

The concept of a laterally controlled Vertical Junction Field Effect Transistor (VJFET) in 4H-SiC with blocking voltages up to 1500V and a current rating of some Amps is assumed to be a candidate for an attractive first SiC power switch at the market. In order to compete technically and economically with the existing silicon solutions some optimizations are mandatory. Besides a further reduction of the on-resistance other characteristics like the temperature dependence of the on-resistance and the saturation behavior of the drain current are subject of redesign and technology refinements. The reduction of the on-resistance was indirectly achieved by reducing the field crowding at the buried gate. Both, the saturation behavior and the temperature dependence were successfully optimized by a careful design of the controlling head region (lateral part of the JFET). The strategy is to use a remarkably higher doping in the head region in order to take advantage of better homogeneity and incomplete ionization of dopants in SiC, e.g.. Higher doping increases the saturation current and decrease the temperature dependence of the on-resistance. We demonstrate the effect of the optimization by means of simulations first measurements of devices fabricated by implementing the discussed features into the design and the process flow .

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.

Large Area, Avalanche-Stable 4H-SiC PiN Diodes with VBR > 4.5 kV

Large area 4H-SiC PIN diodes have been fabricated which exhibit a stable avalanche ranging between 4.5 and 5.5 kV. The avalanche occurs at an electrical field strength of 2.1 MV/cm at the pn junction. The temperature coefficient of the avalanche is positive (0.3 V/K). The avalanche is tested in DC mode. The device concept as well as the fabrication process is described in detail. Static and dynamic characteristics are shown.

Avalanche Capability of Unipolar SiC Diodes: A Feature for Ruggedness and Reliability Improvement

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.

Layout considerations for improving the on-state performance of vertical SiC switching devices

The presented work shows simulated and experimental results of a method to improve the on-resistance of SiC Power devices by an optimized utilization of the drift zone. Mostly SiC devices have quite thin drift layers compared to silicon devices. Thus, the lateral dimensions of cell-like structures are much larger that the actual cell thickness in the direction of the current flow. Due to design and technology limitations, a certain fraction of the cell is not involved in forward current flow. The presented method - a local doping enhancement - improves the current flow through the cell and reduces the on-resistance of the device. After a short introduction of the investigated device structure, the method will be presented by simulation results and proven by experimental evidence. It will be shown, that for a 1200 V device e.g., the on-resistance can be reduced by approximately 25%.

A new SiC diode with significantly reduced threshold voltage

In this paper we introduce a new generation of silicon carbide (SiC) Schottky diodes with reduced threshold voltage. A detailed comparison with Infineons 5th generation of SiC diodes (G5) is done. With a Mo-based Schottky metal system, the new generation of diodes (G6) was designed in such a way that the increased reverse power loss is more than balanced by the efficiency gained by the low threshold voltage. Therefore, in spite of a higher reverse current, due to a lower Schottky barrier, it is shown that the efficiency of G6 is higher and the ohmic losses are reduced in comparison with G5 over a wide load range. G6 also demonstrates similar surge current capabilities as G5 and high ruggedness of the Schottky barrier.

Fast Switching with SiC VJFETs - Influence of the Device Topology

Since SiC VJFETs are believed to offer extremely fast turn on and turn off processes it is important to understand how these transients are tailored by the layout. Regarding the basic layouts two main topologies are under investigation today – structures with the well known SIT layout with purely vertical current flow and lateral vertical concepts where the current flow through the channel is in lateral direction and the vertical current flow takes place in the drift region only. In this paper we will focus on differences in the electric characteristics of both structures and the relation of the dynamic behavior to the topology and the layout of the switches. For the analysis, 1200V VJFETs based on the two basic topologies were manufactured having approximately the same total and active device area. It turns out that the SIT switches under investigation suffer from a high internal gate resistance in the p-doped layers and a relatively high gate drain capacitance.

Comparative Simulation Study of Dynamic Behavior of the Body-Diode for 4H-SiC JFET and MOSFET

In switching applications with half-bridge like configurations the load current is commutated to the so-called reverse or body-diode of a switching device once each switching cycle. The bipolar charge generated in the switch in principle leads to a reverse recovery current and to additional losses. Though it is well known, that in silicon carbide these reverse recovery losses are very low compared to e.g. silicon devices, it turns out that depending on device structure and switching conditions the reverse recovery charge for the JFET may become larger than can be explainable by the stored bipolar charge. In this paper therefore we focus on a simulation study comparing the body-diode operation of common lateral channel silicon carbide JFET and MOSFET devices in a so-called double pulse measurement. It is shown, that the MOSFET body-diode operation still remains uncritical under very fast switching conditions, while the JFET body-diode exhibits a pronounced recovery current peak originating from a partial channel turn-on, and thus higher losses.