Heinz Mitlehner

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 .

Bipolar SiC-Diodes – Challenges Arising from Physical and Technological Aspects

In this contribution we summarize measurements on bipolar high voltage SiC-diodes which were fabricated on 4H-SiC wafers preferentially cut 4° off the [0001] basal plane, whereas the p-emitter thickness was varied in predetermined ratios to the n-base thickness. The switching behaviour of optimized 6.5 kV-Diodes at a current level of 25 A is shown at DC link voltages up to 4 kV and at a junction temperature of 125°C. Experimental results are discussed in terms of snappiness.

High-Voltage Modular Switch Based on SiC VJFETs - First Results for a Fast 4.5kV/1.2Ω Configuration

The success of high voltage SiC switching devices is curr ently retarded by not solved questions regarding the technology of bipolar devices and th e available material quality which does not allow the fabrication of large area chips. High cur rents, however, are mandatory for most of the targeted application in energy distribution or traction. T hus, a unipolar high voltage switch based on a serial connection of vertical JFETs is presented. T his concept comprises the advantage of paralleling in order to achieve high currents and the fas t switching behavior of majority carrier devices. It is controlled by a simple silicon low volt age power transistor at low side (source connected to ground). Theoretically a 12kV device with less than 1Ω resistance for fast switching above 10kHz can be realized using existing 3kV SiC VJFETs. F irst results for a 4.5kV configuration with an on-resistance of 1.2 Ω are presented. The transient analysis reveals the ab ility to switch extremely fast. The devices are placed in an appropriate p ckage designed for applications up to 9kV. Switches utilizing the full capability of the package w ill be realized in a next step. Introduction Silicon carbide devices can revolutionize the field of high voltage (>3kV) solid state switching technology. Compared to silicon solutions (IGBT’s, GTO’ s, etc., stacked or in single configuration), considerable improvements for static and dynamic losses can be achieved. Moreover, by using SiC devices the physical limit for the blocking voltage of a single device can be theoretically shifted to values much higher than 10kV. However, in this applications , high power, e.g. high currents are mandatory. Since the quality of commercially available SiC wafers does not allow reasonable active areas above 10mm2 today, suited devices must be able to be conn cted parallel. This holds in general for unipolar devices because of their positive temperature coefficient. This work presents a new approach for a modular high volta ge SiC switching device based on the unipolar SiC VJFET / Si cascode system as presented earl ier [1]. Device concept In order to enlarge blocking voltage capability, the casc ode [3] can be extended by a serial connection of an arbitrary number of VJFETs with the gate connected to the proceeding stage or ground potential, respectively, via a commercial silicon diode capable of avalanche at a certain blocking voltage (see Fig.1). The approach is similar to t he serial connection of MOSFETs [2], however, the realization presented here is simpler and h s much less restrictions than the MOSFET arrangement. The complete device works like a three term inal switch with source, drain and gate. Like the simple cascode it can be controlled easily by a low voltage Si-MOSFET at low side. This is an important advantage compared to conventional solutions for erial connection of high voltage switches, where individual gate control at different vol tage levels is essential for each single stage. The presented system can be theoretically extended to a ny blocking voltage of choice. It should be Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 793-796 doi:10.4028/www.scientific.net/MSF.433-436.793

To Be ''Snappy'' or Not - a Comparison of the Transient Behaviours of Bipolar SiC-Diodes

Introduction Though advanced electronic devices of conventional semiconductor materials are very successfully used in industrial applications like power management, t he availability of such components will touch limits because of increasing demands for high te mperature and high frequency capabilities in advanced circuits. Therefore, power electr oni s engineering has to seek new solutions as developed by the silicon carbide (SiC) technology. S the superior switching behaviour of silicon carbide devices can be the pushing key in e stablishing SiC technology with advanced components even in high power management. Considerable improvements for static and dynamic losses have been achieved in bipolar high voltage SiC diodes [1-6]. The greatest challenge in all these efforts is the mat rial quality of wafers and epitaxial layers, which limits current capability and yield of such devices with respect to the chip area. Paralleling of small area devices is one way to com pensate for the defect density limiting the yield. However, much more attention should be paid to the chara cterise the diodes in switching experiments with an appropriate rate of current decay and DC link voltages in order to optimise the n-base thickness and doping to exploit the full volta ge capability without snappy effects.

Conduction Loss Reduction for Bipolar Injection Field-Effect-Transistors (BIFET)

In this study, the potential of forward conduction loss reduction of Bipolar-Injection Field-Effect-Transistors (SiC-p-BIFET) with an intended blocking voltage of 10kV by adjusting the doping concentration in the channel-region is analyzed. For the optimization of the SiC-p-BIFET, numerical simulations were carried out. Regarding a desired turn-off voltage of approximately 25V, the optimum doping concentration in the channel-region was found to be 1.4x1017cm-3. Based on these results, SiC-p-BIFETs were fabricated and electrically characterized in the temperature range from 25°C up to 175°C. In this study, the differential on-resistance was found to be 110mΩcm2 for a temperature of 25°C and 55mΩcm2 for a temperature of 175°C. In comparison to our former results, a reduction of the differential on-resistance of about 310mΩcm2 at room temperature is demonstrated.

Alloying of Ohmic Contacts to n-Type 4H-SiC via Laser Irradiation

In this study, we present the results of alloying nickel as ohmic contact material to n-type 4H-SiC via a continuous wave fiber laser with different laser beam powers and processing times. The laser system exhibits an emitting wavelength of 1070 nm and a beam propagation factor M2 smaller than 1.1. Contact resistance was determined by current-voltage measurement using the two-terminal contact resistance method. The results indicate that a laser beam power of at least 100 W is mandatory to initialize contact silicidation. Although the contact resistance is improvable by longer processing times, our experiments outline the much higher impact of laser beam power to contact silicidation compared with processing time. For laser beam powers of 300 W and processing times of 0.5 s a contact resistance of 6.5 , comparable to contacts alloyed in a lamp heated furnace at 910 °C for 2 min with a contact resistance of 10.3 , was achieved.

Robust Double-Ring Junction Termination Extension Design for High Voltage Power Semiconductor Devices Based on 4H-SiC

In this study, a new robust double-ring junction-termination-extension (DR-JTE) for high-voltage pn-diodes is presented and analyzed using numerical simulations. As figured out, the DR-JTE reduces the electrical field at both, the edge of the single-JTE region and the MESA-transition, respectively. Thereby, due to the reduction of the electrical field, the maximum breakdown voltage is increased to 91.5% of the theoretical, parallel-plane breakdown voltage of 6.5kVand the maximum acceptable deviation of the optimum implantations dose is twice than that of the single-JTE structure. Furthermore, due to the internal ring, the MESA-transition is shielded from the electrical field and therefore the breakdown voltage is much less affected by the angle of the MESA.

Temperature Dependent Characterization of Bipolar Injection Field-Effect-Transistors (BiFET) for Determining the Short-Circuit-Capability

In this study, the electrical performance of Bipolar-Injection Field-Effect-Transistors (BiFET) in dependence on the junction temperature is presented for the first time. Based on these results, the short circuit capability of the BiFET is discussed. Thereby, the saturation current is estimated to be approximately 150mA at 300K and it increases by a factor of 5 by rising the temperature up to 450K as analyzed in this study. Furthermore, the reduction of the gate-voltage window of the BiFET at elevated temperatures is comparable to unipolar JFETs, and indicates a very good controllability over a wide temperature range. Finally, numerical simulations demonstrate the potential to improve the electrical performance of the BiFET drastically by adjusting the doping concentration in the control region and increasing the ambipolar lifetime in the p-doped drift layer without influencing the dependency on the junction temperature.

Temperature and Electrical Field Dependence of the Ambipolar Mobility in N-Doped 4H-SiC

In this study, we present results on electrical characterization of bipolar pn-diodes to investigate the temperature and electrical field dependent behavior of ambipolar mobility in n-doped 4H-SiC. Therefore, static current-voltage measurements to calculate the specific differential resistance and dynamical reverse recovery measurements to determine the mean carrier concentration were carried out for different temperatures and forward current densities. The specific differential resistance of the drift layer decreased from 10 mΩcm2 at 80 Acm-2 to 6.6 mΩcm2 at 180 Acm-2, whereas the mean carrier concentration only increased from 4.1015 cm-3 to 8.1015 cm-3, indicating a decreasing ambipolar mobility. The calculated reduction of the ambipolar mobility from 800 cm2V-1s-1 to 650 cm2V-1s-1 in dependence on the current density has to be attributed to an increasing electric field from 150 Vcm-1 to 250 Vcm-1 and increasing carrier scattering due to higher carrier concentrations. For example, at a constant conduction current density of 160 Acm-2, the ambipolar mobility decreases from 710 cm2V-1s-1 at 300 K to 650 cm2V-1s-1 at 450 K.