Ulrich Weinert

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

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 m/spl Omega/ cm/sup 2/ and 14.5 m/spl Omega/ cm/sup 2/, 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/sub rr/ 30 nC, t/sub rr/ 20 ns) and fast switching as well as short circuit capability (1 ms) of vertical VJFETs. Finally, a short outlook to future SiC switching devices is given.

Dynamic characteristics of high voltage 4H-SiC vertical JFETs

We have developed a novel structure for a fully implanted, normally-on vertical junction field effect transistor (VJFET) and fabricated prototypes with blocking voltages between 600 and 1000 V. Mounting the SiC VJFET together with a 50 V Si MOSFET on a DCB substrate in a cascode circuit, we obtain a normally-off high voltage switch. The specific on-resistance of the VJFET was sufficiently low, in the range of 18 to 40 m/spl Omega/cm/sup 2/, for various blocking voltages. The dynamic behaviour shows turn-off times between 50 ns and 2 /spl mu/s due to the RC-product of two different p-gate networks.

A simulation system for diffusive oxidation of silicon: a two-dimensional finite element approach

A numerical approach to the simulation of two-dimensional local oxidation of silicon is presented. The key idea is the description of the oxidation as a three-component thermodynamic process involving silicon, silicon dioxide, and oxidant molecules. This results in a reactive layer of finite width, in contrast to the sharp interface between silicon and dioxide in the conventional formulation. The numerical approximation takes advantage of this description in a finite-element approach which models silicon, dioxide, and reactive layer as a whole, removing the necessity of tracking the interface with element edges. It is shown that a suitable parameter identification results in an interface motion equivalent to that of the Deal-Grove model. Numerical examples show the advantages of the approach. >

The potential of fast high voltage SiC diodes

Silicon carbide power devices offer important advantages compared to silicon devices, regarding higher power ratings at clearly reduced static and dynamical losses, better reliability because of possible higher operating temperatures, and significant savings in system cooling equipment. Physically, these benefits are mainly due to the wide band gap of SiC and the by an order of magnitude higher critical electric field. Consequently, the doping concentration will be by 2 orders of magnitude higher and the thickness of the devices about an order of magnitude lower for the same blocking voltage compared to silicon. Therefore, the power rating of unipolar devices such as Schottky diodes, vertical JFETs and MOSFETs is strongly extended to high voltage application. On the other side bipolar devices such as p-n diodes, IGBTs and thyristors will predominantly be used for applications, where blocking voltages considerably higher than 2000 V and/or increased operational temperatures are required. This is a consequence of their higher threshold voltage compared to Si. To clarify differences in the overall electrical behaviour, both types of diodes with blocking voltages below 2000 V should be analysed. In this paper we present the properties and limitations of 4H-SiC diodes and relate the experimental data of Schottky and p-n diodes with results obtained with the device simulator MEDICI.

Switching behaviour of fast high voltage SiC pn-diodes

4H-SiC p-n diodes with an active area of 1 mm/sup 2/ 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 may point to a local lifetime reduction at the p-n junction.

Characterization of fast 4.5 kV SiC p-n diodes

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 todays 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 39 /spl mu/m thick n-type epitaxial layers with a doping concentration of 2/spl times/10/sup 15/ cm/sup -3/. They exhibit a stable avalanche breakdown at 4800 V and a low leakage current (<20 /spl mu/A/cm/sup 2/) prior to breakdown. The on-state is characterized by a voltage drop of 4.0 V at a current density of 100 A/cm/sup 2/, corresponding to 2.2 A. For current densities above 80 A/cm/sup 2/ lower static losses have been achieved compared to equivalent silicon high voltage diodes. The temperature coefficient is slightly positive guaranteeing 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.

Generalization of the Moment Method of Maxwell-Grad for Multi-Temperature Gas Mixtures and Plasmas

Abstract A coupled system of balance equations is derived for the coefficients of orthogonal expansions of the velocity distribution functions. The orthogonal functions are not specified, but the initial func tions must be local Maxwellians with different temperatures for different species of particles. Closed expressions for the matrix elements of the non-linearized Boltzmann operator are given, whose dominant terms are determined and compared.

On the Evaluation of Transport Collision Frequencies I. The Integration of Empirical Data

Abstract The computation of the transport collision frequencies is discussed, provided the transfer cross sections are given at least for a discrete set of relative kinetic energies. Numerical investigations show that the most satisfying results are achieved by the use of rational interpolation of the transfer cross sections.

Matrix Elements of the Linearized Collision Operator for Multi-temperature Gas-mixtures

For a multi-component and multi-temperature gas-mixture the matrix elements of the linearized Boltzmann collision operator are investigated for isotropic interaction potentials. The representation by means of Burnett basis functions simplifies the algebraic structure and enables closed expressions for the general results, which can also be used for an investigation of inelastic collisions. For the elastic case those collision terms are given explicitely which appear in the balance equations for mass, momentum, energy and heat flux-vector.

On the Inversion of the Linearized Collision Operator

Abstract Some features are discussed in connection with the representation of the linearized Boltzmann collision operator and its inversion. It is shown that under certain assumptions the inverse operator can be given explicitly as an integral kernel function.