Stacking fault formation sites and growth mechanisms in PiN diodes have been investigated. The diodes were fabricated on a 4H SiC wafer with a 150 mum thick n epitaxial layer and a grown p(+) anode. Stacking faults and their associated dislocations were examined by light emission imaging. Many of the stacking faults originate from extended string-like clusters that are present before electrical stressing and are observed at depths ranging from 10 to 100 mum below the SiC surface. Two possible mechanisms for creating these clusters are discussed: (1) nucleation of dislocation loops due to step bunching during epitaxial growth and (2) faulting of basal plane dislocations. Two alternate driving forces for stacking fault growth are also considered: mechanical stress relief and electronic energy lowering. Based on the growing behavior of the stacking faults, it is concluded that mechanical stress is responsible for the stacking fault growth.