Page 24 - Power Electronics Handbook

Power rectifier principles 17

Equipotential Cathode with field Equipotential
ring plate extension ring 1

IC)
phac 1.7 Control of dcpktion layer surface junction: (a) separate field plates; (b) cathode
extension plate; (c) addition of equipotential ring

c. This gives improved breakdown voltage performance. Conversely, a
negative voltage on the field plate causes the layer to change to curves d
and e, reducing the breakdown voltage.
Figure 1.7(b) shows a practical arrangement of the field plate, which
does not need a separate field supply. The strength of the field under the
field plate is controlled by the thickness of the oxide, and it distorts the
depletion layer as before. It is also usual to surround the field plate with an
equipotential ring, as shown in Figure 1.7(c). This prevents the slow
drifting breakdown, which field plate structures are prone to exhibit.