Martin Ruff

SiC devices: physics and numerical simulation

The important material parameters for 6H silicon carbide (6H-SiC) are extracted from the literature and implemented into the 2-D device simulation programs PISCES and BREAKDOWN and into the 1-D program OSSI Simulations of 6H-SiC p-n junctions show the possibility to operate corresponding devices at temperatures up to 1000 K thanks to their low reverse current densities. Comparison of a 6H-SiC 1200 V p-n/sup -/-n/sup +/ diode with a corresponding silicon (Si) diode shows the higher switching performance of the 6H-SiC diode, while the forward power loss is somewhat higher than in Si due to the higher built-in voltage of the 6H-SiC p-n junction. This disadvantage can be avoided by a 6H-SiC Schottky diode. The on-resistances of Si, 3C-SiC, and 6H-SiC vertical power MOSFETs are compared by analytical calculations. At room temperature, such SiC MOSFETs can operate up to blocking capabilities of 5000 V with an on-resistance below 0.1 /spl Omega/cm/sup 2/, while Si MOSFETs are limited to below 500 V. This is checked by calculating the characteristics of a 6H-SiC 1200 V MOSFET with PISCES. In the voltage region below 200 V, Si is superior due to its higher mobility and lower threshold voltage. Electric fields in the order of 4/spl times/10/sup 6/ V/cm occur in the gate oxide of the mentioned 6H-SiC MOSFET as well as in a field plate oxide used to passivate its planar junction. To investigate the high frequency performance of SiC devices, a heterobipolartransistor with a 6H-SiC emitter is considered. Base and collector are assumed to be out of 3C-SiC. Frequencies up to 10 GHz with a very high output power are obtained on the basis of analytical considerations. >

Progress in the development of an 8-kV light-triggered thyristor with integrated protection functions

Light-triggered 8-kV thyristors with an integrated breakover diode were fabricated. The adjustment of the breakdown voltage of the breakover diode (BOD) is realized by a well-defined curvature of the junction between the p-base and the n-base. A multiple amplifying gate structure, together with an integrated turn-on current limiting resistor, guarantees a safe turn-on behavior in the case of overvoltage triggering as well as during light triggering. These thyristors have successfully been put into service at the Celilo Converter Station of the Pacific Northwest-Southwest HVDC Intertie by Bonneville Power Administration in Portland/Oregon/USA. There, a mercury arc valve was replaced by a valve containing these light triggered thyristors with integrated overvoltage protection. In a further development, the amplifying gate structure was optimized in order to simplify the fabrication process and to maintain the high light sensitivity while obtaining a higher dV/dt and a higher dI/dt capability. This was realized by shrinking the optical gate, by carefully adjusting the triggering sensitivity of the amplifying gates and by widening of the turn-on current limiting thyristor. With these measures, initial investigations regarding the integration of a forward recovery protection were also performed.

Note on charge conservation in the transient semiconductor equations

A modification of the continuity equations for electrons and holes in materials with deep donors or acceptors is presented. The modification is necessary in order to prevent violation of charge conservation in transient simulations. The influence of incomplete ionization of dopants on the transient characteristics of a SiC power diode is demonstrated. >