On the use of a P+N+ mosaic contact for fast switching diode applications

This paper investigates the possibility of using a P+N+ mosaic (ohmic) contact to improve the switching characteristics of a P+N-N+ diode. The mosaic should aid the removal of minority holes from the N-base region. This is the first time such a detailed numerical study has been carried out on the P+N+ mosaic contact and the results are not only applicable to rectifiers, but thyristors and IGBTs as well. The reverse recovery process, both inductive and resistive, of a P+N-N+ diode is reviewed. The mosaic case is then introduced for both a diode fabricated by double diffusion and one fabricated using an epitaxial process. Comparisons are drawn between the mosaic and nonmosaic cases and the effectiveness of the technique at improving switching speed and forward voltage drop for both device types is investigated. It is shown that rather than being a robust technique which can improve switching in any rectifier, the P+N+ mosaic contact is effective only in a limited set of epitaxial rectifiers and it´s effectiveness varies dramatically as a function of the circuit conditions.