SiCED | Veröffentlichungen

Novel fabrication technology for devices with nearly temperature-independent forward characteristics

S. Heim, A. Albrecht, W. Bartsch, presented at ICSCRM 2009

In this work we discuss a structure of a p-doped Poly-Si layer and a Ni layer deposited onto n-type 4H-SiC in order to form a Schottky-like contact which undergoes a specific temperature budget to establish a temperature independent forward characteristic of the formed rectifying junction. The results of our treatment is discussed in terms of a hetero junction, though temperature treated NiSi-layers normally are expected to show an ohmic behavior.

Fast switching with SiC VJFETs – influence of the device topology

Rudolf Elpelt, Peter Friedrichs, Jürgen Biela, presented at ICSCRM 2009

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.

Optimization of Bipolar SiC-Diodes by Analysis of Avalanche Breakdown Performance

W. Bartsch, R. Schoerner, K. O. Dohnke, presented at ICSCRM 2009

In this work we discuss measurements of the breakdown voltage of diodes with non-punch-through (NPT)- and punch-through (PT)-designs. From the experimental results we deduce the temperature dependent Fulop constants of the effective ionization rate. The data of this work agree very well with ionization rates for electrons and holes determined recently.

High-performance multi-wafer SiC epitaxy – First results of using a 10x100mm reactor

C. Hecht, R. Stein, B. Thomas, L. Wehrhahn-Kilian, J. Rosberg, H. Kitahata, F. Wischmeyer, presented at ICSCRM 2009

In this paper, we present first results of epitaxial layer deposition using a novel warm-wall CVD multi-wafer system AIX 2800G4 WW from AIXTRON with a capability of processing 10x100mm wafers per run. Intra-wafer and wafer-to-wafer homogeneities of doping and thickness for full-loaded 10x100mm runs will be shown and compared to results of the 6x100mm setup of our hot-wall reactor VP2000HW by AIXTRON used for device production since 2001.

Lifetime investigations of 4H-SiC pin power diodes

S. A. Reshanov, W. Bartsch, B. Zippelius, G. Pensl presented at ECSCRM 2008

Lifetime measurements are performed on 4H-SiC pin power diodes (6.5 kV). The lifetime values in the base range from 1.1 s to 2.1 s; these values demonstrate the high quality of the 4H-SiC epilayer and the optimized device processing. The observed lifetimes are correlated with deep defect centers detected by deep level transient spectroscopy. The role of the Z1/2-center as a lifetime killer is discussed.

SiC Power Devices for Industrial Inverters

Dethard Peters, presented at ICSCRM 2007 in Otus (Jp), in press

extended abstract

SiC-Powerdiodes: Design and Performance

Wolfgang Bartsch, Bernd Thomas, Heinz Mitlehner, Bernd Bloecher, Swen Gediga, presented at EPE2007, Aalborg (Dk)

In this work we discuss static measurements on bipolar 6.5 kV-SiC-diodes which were fabricated on 4H-SiC wafers preferentially cut 4° off the (0001) basal plane in order to prove design rules developed for Si-devices. To suppress emitter recombination currents, the p-emitter thickness has to be increased. The switching behaviour of optimized 6.5 kV-Diodes with a 3 µm thick p-emitter at different current levels at DC link voltages of 4 kV and at junction temperatures up to 125°C is shown. For these diodes first results on forward stability are also presented.

Properties of Unipolar SiC Power Devices for Operation Beyond 200°C Junction Temperature

Peter Friedrichs, presented at HiTEC 2006, Santa Fe, NM, USA, May 2006

Up to now generally the available packaging technologies were the root cause for the existing limits regarding the maximum junction temperatures of power semiconductors. Due to the driving force arising from applications in vehicles or in the aircraft industry, more efforts are directed to the development of high temperature packaging technologies with a first target of an allowed maximum junction temperature of approximately 250°C. This temperature range, however, cannot be addressed with silicon devices due to physical limitations, especially for blocking voltages exceeding 600V. Thus, alternatives based on wide band gap materials have to be developed. Among the potential candidates, SiC has gained an extraordinary status due to its maturity regarding base material quality and technology developments. Most advanced are unipolar devices. Especially their potential to enable high switching frequencies is used in applications today. Schottky barrier diodes are already established at the market, especially for high end power supplies. Switching devices still seek for a suited mass application which can provide a sufficient base load for a mass fabrication. High temperature electronics could be one of these target applications. The paper will reflect the present status and running developments with respect the demands arising from high temperature operation. It will be shown that today’s Schottky barrier diodes have certain limitations at higher temperatures due to high static losses and increasing leakage currents. Based on the physical model behind this behavior, technology developments addressing this feature will be sketched. Regarding switching devices, especially the question of a suited device concept will be analyzed. Experimental evidence for the suitability of the favored concepts will be given. In the discussions, challenges as well as potentials and limitations for next generation high temperature electronics based on SiC components will be pointed out.

Bipolar SiC-diodes – Challenges arising from Physical and Technological Aspects

W. Bartsch, H. Mitlehner, S. Gediga, presented at ECSCRM 2006 in Newcastle, published in Mat.science Forum Vols. 556-557 (2007), pp 889 - 894

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.

Epitaxial Growth of 4H-SiC on (000-1) C-Face Substrates by Cold-Wall and Hot-Wall Chemical Vapor Deposition

René A. Stein, Bernd Thomas, Christian Hecht, presented at ECSCRM 2006 in Newcastle, published in Mat.science Forum Vols. 556-557 (2007), pp 89 - 92

Epitaxial layers have been grown on the (000 ) C-face of 2- and 3-inch 4H-SiC wafers. Growth conditions like temperature, pressure, C/Si ratio, and silane partial pressure have been varied. The influence of these parameters on layer properties, e.g. the Nitrogen incorporation, will be presented. The results of epitaxial growth in a cold-wall reactor will be compared to those in a hot-wall system. In both systems smooth surface morphologies could be obtained. The main challenge of epitaxial growth on the C-face of 4H-SiC for electronic device applications seems to be the control of low doping concentrations. The incorporation of nitrogen as donor in epitaxial layers on C-face substrates is proportional to the N₂ partial pressure as it is known from the growth on the Si-face, but the incorporation coefficient is much higher. Our results show that the Nitrogen incorporation has only a weak dependence on the C/Si ratio, but the influence of temperature and pressure is remarkable. The hot-wall CVD system allows the use of higher temperatures and lower pressures than the cold-wall equipment. The lowest doping concentration of 2.8x10¹⁵ cm^-3 has been achieved by hot-wall epitaxy using a temperature of 1625 °C, a system pressure of 72 hPa , a C/Si ratio of 1.4, and a growth rate of 6.5 µmh^-1. Good doping homogeneity on 2–inch and 3-inch wafers could be achieved. For a doping level of N_D-N_A= 3×10¹⁵ cm^-3 sigma is about 15%.

Behavior of high voltage SiC VJFETs under avalanche conditions

Peter Friedrichs and Tobias Reimann, presented at APEC 2006, Dallas, TX, USA

The paper discusses the behavior of SiC VJFETs under avalanche conditions. Safe operation under avalanche is a precondition for the effective use of switching devices in switch mode power supplies. It will be shown how a VJFET device can be designed avalanche stable. Measurements of the avalanche capability of realized devices prove the expected ruggedness of SiC components. Achieved single pulse energies exceed the values possible with silicon components of the same die size.

Bipolar 6.5 kV-SiC-Diodes: On the Road to Industrial Application

Wolfgang Bartsch, Rudolf Elpelt, Reinhold Schoerner, Karl-Otto Dohnke,
Bernd Bloecher, Klaus Koerber, published at 11th European Conference on Power Electronics and Applications 2005, Dresden, Germany, ISBN 90-75815-08-5

This work presents the transient behaviour of paralleled 6.5 kV bipolar SiC diodes with Aluminium implanted emitters. The switching behaviour at a current level of 30 A is shown at DC link voltages up to 4 kV and at a junction temperature of 125°C. Different IGBT gate resistor conditions realise different rates of current decay up to 1000 A/µs. Experimental results are discussed in terms of snappiness.

SiC Power MOSFETs – Status, Trends and Challenges

Dethard Peters, R. Schoerner, Peter Friedrichs, Dietrich Stephani, presented at ICSCRM 2005 in Pittsburgh, published in Mat.science Forum Vols. 527-529 (2006), pp 1255 - 1260

SiC power MOSFETs are attractive electronic power switches for innovative power supply and motor drive solutions. The paper discusses this statement and specifies market segments offering the best chances for a commercialization. Due to well-known difficulties in achieving adequate channel conductivity, a lot of SiC-MOSFET publications focus on the channel mobility. However, for a power MOSFET this is only one important parameter affecting the performance. Other characteristics have to be considered too for an honest evaluation: transfer characteristics and blocking capability over the standard operation temperature range, handling of gate oxide stress and related reliability issues, capability of paralleling, dynamic stability, body diode characteristics, reproducibility of the fabrication process and device size. Various attempts have been made in recent years in order to address these features. Approaches differ in the use of different crystal orientations and polytypes, accumulation or inversion channel, implanted or epitaxially grown channels and novel oxidation techniques, e.g. Worldwide a trend to the planar DIMOS concept can be observed. Our present results are shown for a power SiC MOSFET designed for 10 A / 1200V. Key data are a specific on-resistance of 12 mΩcm2, the desired low but positive increase of the on-resistance with temperature, static avalanche (20 mA DC @1574 V), short-circuit stability at 600 V for 20 µs and robust switching behavior.

Charge Controlled Silicon Carbide Switching Devices

Peter Friedrichs, presented at the MRS Spring Meeing 2004, San Francisco, CA, USA

Charge controlled power switching devices fabricated in 4H-Silicon Carbide are discussed in this paper. After comparing possible structures, results on prototype devices are presented. The presentation will give an overview about the developments of SiC power switches at SiCED, in addition some potential applications serving as an accelerator for the SiC power switch development will be sketched. The performance of vertical JFETs will be analyzed in detail. These can be operated as a single device as well as in combination with a low voltage silicon power MOSFET. The result of the hybrid assembly is a normally off device which behaves for the user more and more like a classical MOSFET with respect to the input as well as the output characteristic. Several improvements where performed which make the device more attractive for the customer. It will be shown which factors drive these optimization and how they can be implemented. Although the primary target for this device is the >1000V blocking voltage range, it will be discussed how the huge 600V power switch market can be made accessible for SiC power devices too. Intensively the high temperature performance of SiC JFETs and Si/SiC cascodes is discussed. Additionally, other developments like silicon power MOSFETs or high voltage switches will be mentioned.

Large Area, Avalanche-Stable 4H-SiC PIN Diodes with V_BR > 4.5 kV

Dethard Peters, Rudolf Elpelt, Reinhold Schörner, Karl Otto Dohnke, Peter Friedrichs and Dietrich Stephani, presented at ECSCRM 2004 in Bologna, published in Mat.science Forum Vols. 483-485 (2005), pp 977 - 980

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

Serial connection of SiC VJFETs – features of a fast high voltage switch

R. Elpelt, P.Friedrichs, R. Schörner, K.-O. Dohnke, H. Mitlehner, and D. Stephani, EPE2003, European Conference on Power Electronics and Applications, 2-4 Sep 2003 in Toulouse

We present a high voltage stacked switch (up to 8 kV / 10 A), based on the serial connection of vertical Silicon Carbide (SiC) junction field effect transistors (VJFETs) together with a standard power MOSFET in a cascode-like circuit. After introduction and discussion of the basic principles a careful analysis of the static characteristics as well as the dynamical switching behavior is conducted by means of experimental measurements together with two-dimensional device simulation. The presented stacked switch can, in principle, be extended to any desired blocking voltage. The current capability depends largely on the cooling efforts and the active SiC area used in each stage of the stack. The dynamic performance of the switch also yields a very fast switching ability. Thus unipolar SiC switches become feasible, exhibiting good on-state performance with blocking voltages exceeding 4 kV and switching frequencies well above 10 kHz.

State of the Art and Technological Challenges of SiC Power MOSFETs designed for high Blocking Voltages

Dethard Peters, Heinz Mitlehner, Rudolf Elpelt, Reinhold Schörner, Dietrich Stephani EPE2003, European Conference on Power Electronics and Applications, 2-4 Sep 2003 in Toulouse

Normally-off n-type inversion channel mode SiC-MOSFETs blocking voltages between 1200 V and 3000 V are presented and discussed. The on-resistance of a 3 kV blocking device with 1.45 mm² active area amounts to 3 W at room temperature, corresponding to a specific on-resistance of 45 mWcm². This value is taken at 20 V gate source voltage corresponding to an electrical field strength in the gate oxide of 2.6 MV/cm. The 4H SiC MOSFET utilizes a 76 nm thick thermal grown gate oxide and a polycrystalline silicon gate electrode. This oxide thickness provides a significantly improved reliability as demonstrated in first reliability tests of 1200 V. The temperature coefficient of the on-resistance is positive and allows easy paralleling of these devices. At a drain current of 1 A in on-state the drain source voltage rises from 3.8 V at 25°C to 5.2 V at a junction temperature of 150°C. The inversion channel mobility could be improved to 10 cm²/Vs at room temperature. For this case the channel resistance still dominates the overall on-resistance but decreases with temperature. At 150°C the channel contributes 20% to the on-resistance. In contrast to high voltage silicon MOSFETs the reverse diode of the SiC MOSFET exhibits excellent switching behavior and might be used as the free wheeling diode.

BIFET – a novel bipolar SiC switch for high voltage power electronics

Heinz Mitlehner, Peter Friedrichs, Rudolf Elpelt, Karl O. Dohnke, Reinhold Schörner, and Dietrich Stephani

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 T_j = 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 voltage considering the used doping concentration of 2*10¹⁵ / cm³. The dynamic performance was investigated in a cascode configuration in a chopper circuit with a clamped inductive load. After a first fast decrease of the load current it passes into a quite long tail phase (>500ns), which is depending on the gate control region.

The Industrial Utilization of SiC Power Devices - Aspects and Prospects

Dietrich Stephani
Lecture given at the Workshop on Future Electron Devices (FED), Yokohama, Japan, on March, 26^th 2003

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Application of SiC Devices in Power Supplies

Tobias Reimann, Juergen Petzoldt, Ilia Zverev, Heinz Mitlehner, Peter Friedrichs
EPE-PEMC 2002, Dubrovnik & Cavtat, September 9^th - 11^th, 2002

The potential of silicon carbide as the basic material for power semiconductor devices is discussed in this paper. Concepts for high-voltage SiC devices (Schottky Barrier Diode, pin-Diode, JFET) are presented. Specific problems of ultra-fast power devices in the parasitic circuit environment are shown. Finally, some realized switched-mode power supply (SMPS) applications for state-of-the-art SiC power semiconductor devices are presented and compared with benchmark silicon solutions.

Enhanced channel mobility of 4H-SiC metal-oxide-semiconductor transistors fabricated with standard polycrystalline silicon technology and gate-oxide nitridation

Reinhold Schörner, Peter Friedrichs, Dethard Peters, and Dietrich Stephani
Simi Dimitrijev and Philippe Jamet
Appl. Phys. Lett., Vol. 80 (2002), No. 22, pp 4253 - 425

This work presents improved channel mobility of n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) on 4H-SiC, achieved by gate-oxide nitridation in nitric oxide. Lateral enhancement mode MOSFETs were fabricated using standard polycrystalline silicon gate process and 900°C annealing for the source and drain contacts. The low field mobility of these MOSFETs was as high as 48 cm²/Vs together with a threshold voltage of 0.6 V, while the interface state density - determined from the subthreshold slope - was about 3x10¹¹ eV^-1cm^-2. The 43 nm thick gate oxide of coprocessed metal-oxide-semiconductor structures exhibited a breakdown field strength of 9 MV/cm.

Comparison of 4H SiC pn, Pinch and Schottky Diodes for the 3 kV Range

Dethard Peters, Peter Friedrichs, Reinhold Schörner, and Dietrich Stephani
Mat.science Forum Vols. 389-393 (2002), pp 1125 - 1128

This paper investigates the static and dynamic performance of 4H SiC pn, pinch and Schottky diodes able to block 3 kV, with identical active area (1.0 mm²) and prepared on the same wafer. In reverse direction the Schottky diodes exhibit the highest leakage current, the pn diodes the lowest. Leakage of the pinch diodes is lower than that of SBDs since the highly doped p regions reduce the field strength at the Schottky contact. Considering reverse and forward characteristics the pinch diode turns out to be the best choice. The active p emitters of the pinch diode improve its surge current stability significantly. The dynamic behavior is Schottky like characterized by a negligable storage charge and a recovery time of 30 ns.

Application oriented unipolar switching SiC devices

Peter Friedrichs, Heinz Mitlehner, Reinhold Schoerner, Karl O. Dohnke, Rudolf Elpelt, and Dietrich Stephani
Mat.science Forum Vols. 389-393 (2002), pp 1185 - 1190

The paper is focused on the discussion of unipolar switching devices suited for mass applications. The step to push SiC switches into commercial applications seems to be mandatory in order to get serious impact on material development and market acceptance or silicon carbide. It will be shown that currently, a MOSFET like switch for a blocking voltage above 1200V and a rated current of some amps is a potential candidate to provide the recommended driving force for a successful market entry. In detail, SiCED favors a combination of a silicon switch and a vertical, normally-on SiC junction field effect transistor. Several types of such devices will be presented and discussed. Additionally, an analysis will be given regarding the feasibility of bipolar switching devices with respect to their unipolar counterpart for a given blocking voltages.

Prospects of SiC Power Devices: From the State of the Art to Future Trends

Dietrich Stephani
PCIM 2002, Nuremberg

The prospects of SiC power devices are described and related to the outstanding physical properties of silicon carbide. The state of the art in the development of Schottky diodes, Junction Field Effect Transistors, MOSFETs and pn diodes is presented to the authors knowledge. Future trends are discussed regarding the development of SiC power devices as well as trends regarding the impact of SiC power devices in power electronics.

Ultra low loss and fast switching unipolar SiC-devices

Heinz Mitlehner, Peter Friedrichs, Reinhold Schörner, Karl O. Dohnke, Rudolf Elpelt, and Dietrich Stephani
PCIM 2002, Nuremberg

The development of silicon carbide power devices is encouraged by their superior static and especially dynamic properties compared to equivalent silicon devices. Low specific on-resistance for high breakdown voltages and dynamics properties comparable to low voltage silicon devices are suggested to be the most outstanding features of SiC power switching devices. In order to illustrate these expectations on real devices, vertical normally-on JFETs with blocking voltages from 600V up to 4kV and a specific on-resistance of 8mWcm² to 38mWcm², are discussed. A normally-off device can be achieved by combining such a SiC device with a low voltage silicon power MOSFET. Above 4 kV blocking voltage the simulation of bipolar SiC switching devices demonstrates lower static losses, higher surge current capability in comparison with their unipolar counterparts.

SiC Leistungsbauelemente - an der Schwelle einer neuen Ära in der Leistungselektronik?

Heinz Mitlehner, Dietrich Stephani
ETG Fachtagung 2002, Bauelemente der Leistungselektronik und ihre Anwendungen, S. 19 - 26

Unipolare Siliziumkarbid (SiC) - Dioden und - Schalter zeigen im Sperrspannungsbereich zwischen 600 und 3500 V sehr niedrige Gesamtverluste verglichen mit den entsprechenden Si-Bauelementen. Insbesondere ermöglichen SiC Bauelemente schnelles Schalten mit vernachlässig-baren Schaltverlusten bei gleichzeitig niedrigen statischen Verlusten. So zeigen beispielsweise vertikale JFETs sehr kleine spezifische Durchlasswiderstände zwischen 8 und 25 mWcm² in dem Sperrspannungsbereich zwischen 600 und 3500 V respektive. Durch intensive Nutzung wesentlich höherer Schaltfrequenzen z.B. in Konvertern müssen die dadurch erzielten Systemvorteile jedoch die höheren SiC-Bauelemente Kosten deutlich kompensieren. Bei Sperrspannungen von 3,5 kV sehen wir in etwa die Grenze für den Einsatz unipolarer SiC-Schaltelemente (hier beträgt ein die Materialgrenze charakterisierender Gütefaktor: VB²/Ronsp >> 400 MW/cm² (!)). 4.5 kV Bipolardioden erlauben hohe Schaltgeschwindigkeiten auf Grund geringer Rückströme selbst bei großen Stromsteilheiten. Die statischen Verluste sind mit denen der Si- Dioden vergleichbar. Dieser Fortschritt in der Performance schneller Halbleiterschaltelemente sollte neue Horizonte in der Leistungselektronik eröffnen.

The vertical silicon carbide JFET - a fast and low loss solid state power switching device

Peter Friedrichs, Heinz Mitlehner, Reinhold Schörner, Karl O. Dohnke, and Dietrich Stephani
Proceedings of the EPE 2001, in Graz, Austria, August 2001

Silicon carbide power switching devices exhibit superior properties compared to silicon devices. Low specific on-resistance for high breakdown voltages and the capability of operation at higher junction temperatures are believed to be the most outstanding features of SiC power switching devices. In this paper, vertical JFETs capable to block 600V to 3,5kV with a specific on-resistance of 8mWcm² to 26mWcm², are presented. Combining such a device with a low voltage (55V, e.g.) silicon power MOSFET, a rugged and robust normally-off device can be fabricated. However, the commercial use of SiC is currently yet hindere d due to the high material and therefor also device price. Nevertheless, there are additional benefits resulting from the electrical performance which make SiC devices attractive for the system designer. Among others, the authors present fast recovery of the reverse diode, and fast switching as well as short circuit capability in the range of milliseconds for vertical SiC VJFETs.

Turn-off and short circuit behavior of 4H SiC JFETs

B. Weis, M. Braun, P. Friedrichs
Proceedings of the EPE 2001, in Graz, Austria, August 2001

In this paper, the dynamic characteristics if a SiC switching power device are described. The switch is realized as a cascode configuration, consisting of the series connection of a low voltage Si MOSFET and a high voltage SiC JFET. This switch is able to operate both as switch and as freewheeling diode. Turn-off behavior of this switch is reported, whereas turn-off means both turning off in "switch" operation as well as turning off in "diode" operation. Finally, short circuit operation of the switch is demonstrated.

SiC Power devices with low on-resistance for fast switching applications

Peter Friedrichs, Heinz Mitlehner, Karl O. Dohnke, Dethard Peters,
Reinhold Schörner, Ulrich Weinert, Eric Baudelot, and Dietrich Stephani
International Symposium on Power Semiconductor Devices ISPSD Toulouse 2000, pp 213 - 216; ISBN 0-7803-6269-1

Silicon carbide switching devices exhibit superior properties compared to silicon devices. Low specific on-resistance for high breakdown voltages is believed to be the most outstanding feature of SiC power switching devices. In this paper, MOSFETs and JFETs capable to block 1800 V with a specific on-resistance of 47 mWcm² and 14.5 mWcm², resp., are discussed. However, there are additional advantages making SiC devices attractive for the system designer. The authors present fast recovery of the 6H-SiC MOSFET reverse diode (Q_rr 30 nC, t_rr 20 ns) and fast switching as well as short circuitc capability (1ms) of vertical VJFETs. Finally, a short outlook to future SiC switching devices is given.

Characterization of fast 4.5 kV SiC P-N diodes

Dethard Peters, Peter Friedrichs, Heinz Mitlehner, Reinhold Schoerner,
Ulrich Weinert, Benno Weis, and Dietrich Stephani
International Symposium on Power Semiconductor Devices ISPSD Toulouse 2000, pp 241 - 244; ISBN 0-7803-6269-1

New results of silicon carbide P-N diodes show a promising performance for high voltage applications. The diodes are characterized by high power ratings, temperature stability, rugged avalanche and fast switching behavior. Significant savings in system cooling equipment seem possible. However, with today available material the device areas and thereby current ratings which can be fabricated with reasonable yield are restricted to a few square mm resp. a few Amps.
The SiC P-N diodes are fabricated with implanted p-regions on 39µm thick n-type epitaxial layers with a doping concentration of 2x10¹⁵ cm^-3. They exhibit a stable avalanche breakdown at 4800 V and a low leakage current (< 20 µA/cm²) prior to breakdown. The on-state is characterized by a voltage drop of 4.0 V at a current density of 100 A/cm², corresponding to 2.2 A. For current densities above 80 A/cm² lower static losses have been achieved compared to equivalent silicon high voltage diodes. The temperature coefficient is slightly positive guarantying a homogeneous current sharing for operation in parallel. The switching performance is characterized by very low dynamic losses. The reverse recovery current peak is considerably lower than the forward current, with a reverse recovery time as short as 30 ns.

1700 V SiC Schottky Diodes scaled to 25 A

D. Peters, K. O. Dohnke, C. Hecht, D. Stephani
European Conference on Silicon Carbide and Related Materials ECSCRM, Kloster Banz 2000, pp 675 - 678; ISBN 0-87849-873-7

This paper reports on a study of SiC Schottky diodes focused on high current rating and high blocking voltage: 25 A / 1200 V and 1700 V, resp. With an active area of 10 mm² we successfully explored new ground for SiC devices. The device concept, fabrication process, yield aspects and measured results of static and dynamic characteristics as well as the temperature behavior are described. The reverse currents are very low (< 500 µA) even at 125°C and their temperature dependence is lower than expected by thermionic emission since tunneling mechanisms through the Schottky barrier rule the current transport at high blocking voltages.

Temperature Dependence of Forward and Reverse Characteristics of Ti, W, Ta and Ni Schottky Diodes on 4H-Silicon Carbide

M. Treu, R. Rupp, H. Kapels, W. Bartsch
European Conference on Silicon Carbide and Related Materials ECSCRM, Kloster Banz 2000, pp 679 - 682; ISBN 0-87849-873-7

In this study Ta, W, Ti, and Ni Schottky diodes are characterized under forward and reverse bias for temperatures between +21°C and +200°C. Additionally the Ti Diodes were characterized for temperatures between -168°C and +21°C. It will be shown that the reverse current is dominated by thermionic field emission for all metals, if the junction temperature is in the typical device operating temperature range. Furthermore we will show that non-ideal forward characteristics not necessarily have a negative influence on the reverse characteristics of the diodes. This will be explained by a model proposed by Tung, which considers the pinch off of the defects by the defect free areas around the defect.

Influence of the buried p-layer on the blocking behavior of vertical JFETs in 4H-SiC

Peter Friedrichs, Heinz Mitlehner, Reinhold Schörner, Rainer Kaltschmidt, Karl O. Dohnke, and Dietrich Stephani
European Conference on Silicon Carbide and Related Materials ECSCRM, Kloster Banz 2000, pp 695 - 698; ISBN 0-87849-873-7

For vertical JFETs in silicon carbide, the use of an buried gate with a lateral channel concept in order to achieve a high blocking gain was shown to result in promising device performance. However, due to field crowding at the edges of the buried p-layer, the breakdown voltage is reduced compared to fully planar devices which are able to use the bulk breakdown field of 4H-SiC. The following work presents a possibility how to further enhance the breakdown voltage of SiC JFETs with buried layers by implementing a buried layer with optimized shape and doping. Consequently, for a given blocking voltage, the specific on-resistance can be reduced, resulting in lower losses for the device.

A comparison of modern power device concepts for high voltage applications: Field stop-IGBT, compensation devices and SiC devices

G. Deboy, H. Hüsken, H. Mitlehner and R. Rupp
Bipolar/BiCMOS Circuits and Technology Meeting BCTM, Minneapolis 2000, pp 134 - 141; ISBN 0-7803-6384-1

This article presents a comparison of recently introduced device concepts like the CoolMOSTM to new promising approaches as the field-stop IGBT and the actual trends in SiC devices. All these devices are capable of blocking voltages in the range of 1000 V. They are optimized for switching frequencies up to 100 kHz and beyond and meet with different focus the requirements of applications like switch mode power supplies (SMPS), lighting or industry tasks.

Dynamic characteristics of high voltage 4H-SiC vertical JFETs

Heinz Mitlehner, Wolfgang Bartsch, Karl O. Dohnke, Peter Friedrichs, Rainer Kaltschmidt, Ulrich Weinert, Benno Weis and Dietrich Stephani
International Symposium on Power Semiconductor Devices ISPSD, Toronto 1999, pp 339 - 342; ISBN 0-7803-5290-4

We have developed a novel structure of a fully implanted, normally-on Vertical Junction Field Effect Transistors (VJFET) and fabricated prototypes with blocking voltages between 600 and 1000 V. Mounting the VJFET together with a 50 V MOSFET on a DCB substrate we have built up a cascode circuit, to obtain a normally-off high voltage switch. The specific on-resistance was sufficiently low, in the range of 18 to 40 mWcm² for various blocking voltages. The dynamic behaviour shows turn-off times between 50 nsec and 2 µsec due to the RC-values of two different p-gate networks.

Electrical performance of triple implanted vertical silicon carbide MOSFETs with low on-resistance

Dethard Peters, Peter Friedrichs, Reinhold Schörner, Heinz Mitlehner, Benno Weis, and Dietrich Stephani
International Symposium on Power Semiconductor Devices ISPSD, Toronto 1999, pp 103 - 106; ISBN 0-7803-5290-4

This paper describes results of 6H silicon carbide vertical power MOSFETs designed for different blocking capabilities: 600 V and 1600 V. The fabrication is based on a triple implantation technique with a lateral inversion channel. The MOSFETs are normally off and exhibit specific on-resistances of 22 and 40 mWcm² , resp. A chip area of 1 mm² has emerged as a suitable value in order to achieve an acceptable yield with respect to the blocking capability. A SiC MOSFET of this size can be driven up to 1 A in continuous operation. As expected for a unipolar device short turn-on and turn-off delay times have been measured. In particular, due to a very small accumulation zone the Miller capacitance is small in comparison to Si MOSFETs. The switching speed can be influenced by the gate driving circuit in a wide range. The SiC MOSFET is controllable in all switching states and stable up to 125°C case temperature. The switching behavior tested under conditions typical for motor drives is robust against short cuts and short time overloading.

Rugged Power MOSFETs in 6H-SiC with Blocking Capability up to 1800V

Reinhold Schörner, Peter Friedrichs, Dethard Peters, Heinz Mitlehner, Benno Weis, and Dietrich Stephani
International Conference on Silicon Carbide and Related Materials ICSCRM Research Triangle Park, NC, USA 1999, pp 1298 - 1298; ISBN 087849-854-0

This paper presents the static and dynamic characteristics of 1800V 6H-SiC vertical power MOSFETs. These devices exhibit an on-resistance of 46 mWcm² at room temperature and a steady state current rating of 0.4 A at a power dissipation of 160 W/cm². The forward blocking mode ischaracterized by a stable avalanche breakdown with a current rating of 20 mA/cm². This robust and stable avalanche breakdown could be further improved to a current rating of 2 A/cm² by optimizing the doping profiles. The dynamic behavior of the 1800 V MOSFETs proves to be controllable in all switching states and is also stable at higher temperatures. Particularly the devices withstand a short circuit for 85 µs with a maximum overload current of 2.7 A at 300 V DC supply voltage.

Static and Dynamic Characteristics of 4H-SiC JFETs designed for different blocking categories

Peter Friedrichs, Heinz Mitlehner, Rainer Kaltschmidt, Ulrich Weinert, Wolfgang Bartsch, Christian Hecht, Karl O. Dohnke, Benno Weis and Dietrich Stephani
International Conference on Silicon Carbide and Related Materials ICSCRM Research Triangle Park, NC, USA 1999, pp 1243 - 1246; ISBN 087849-854-0

In this work vertical 4H-SiC power JFETs are presented. The structure consists of two parts, a first, thick epitaxial layer supporting the desired breakdown voltage and a thin head region implemented in a second epitaxial layer on top of the first one, responsible for controlling the device. Sample sets have been fabricated with blocking voltages from 600 V, 1200 V to 1800 V and an active area of 2.3 mm². The devices exhibit stable avalanche breakdown and are rugged under short circuit conditions in the order of several milliseconds. Due to the unipolar conduction mechanism, high switching speeds can be achieved. The temperature dependence of the on-resistance can be modeled on the basis of the temperature dependence of the electron mobility and follows a R_on ~ T^2.58 relation. The experimentally obtained results fit well to previous simulations.

Switching behaviour of fast high voltage SiC pn - diodes

Heinz Mitlehner, Peter Friedrichs, Dethard Peters, Reinhold Schörner, Ulrich Weinert, Benno Weis, and Dietrich Stephani
International Symposium on Power Semiconductor Devices ISPSD, Kyoto 1998, pp 127 - 130; ISBN 0-7803-4752-8

4H - silicon carbide pn-diodes with an active area of 1mm² and up to 3 kV blocking voltage have been fabricated, characterized and compared to simulations. The static forward characteristics demonstrate the expected forward power loss with a negative temperature coefficient. The diodes exhibit a stable avalanche breakdown showing a small positive temperature coefficient (0.3 V/K). The turn-on switching behaviour shows a relatively small voltage overshoot as compared to silicon diodes. The turn-off resembles that of a Schottky diode. In both cases the dynamics can be attributed to a rapid recombination of the storage charge even under high forward injection conditions. Numerical simulations might point to a local lifetime reduction at the pn-junction.

Device Design and Simulation

Device Characterization