Semiconductor power modules for the control of high currentsand high voltages have important applications in motor drives,traction and power transmission. Dynamic avalanche is of atechnological interest since it may limit the safe operatingarea for bipolar switching devices and for power diodes, whichare important integral parts of a power module. Dynamicavalanche is caused by a current-controlled space charge, whichmay enhance the electric field to the critical field strengthat voltages below the static breakdown voltage. Measurementsand simulations of reverse recovery at high power densitieswere performed for Si and 4H-SiC p+nn+power diodes to investigate the failure limitcaused by dynamic avalanche. A special optical measurementtechnique was used to eliminate influence of the junction edgeand thereby probe the bulk diode area. It was found experimentally that it is possible for Si powerdiodes to sustain dynamic avalanche at very high powerdensities (>1MW/cm2) during reverse recovery. It is proposed thatthese diodes eventually fail as a result of impact ionizationat the diode nn+junction, which according to device simulations,results in current filamentation and an excessive localizedheating. No temperature dependence was found for the failurelimit and similar results were obtained for diodes with andwithout carrier lifetime control. The high failure limitmeasured with the optical technique correlates with a very gooddynamic ruggedness in conventional electrical reverse recoverymeasurements. Device simulations indicate that a deep n+profile may improve the diode failure limit. A low failure limit close to the onset of dynamic avalanchewas found for another Si power diode, which had a structuredp-zone with a lowly doped p profile extending to the metalcontact in a fraction of the active area. A suggested failurecause for this diode is punch-through of a high electric fieldthrough the p region to the metal contact, an effect which maybe enhanced by an inhomogeneous current resulting from dynamicavalanche. Some of the investigated 4H-SiC p+nn+diodes showed no dynamic reduction of thebreakdown voltage in reverse recovery measurements. The highpredicted onset level of dynamic avalanche for 4H-SiC couldhowever not be reached, since breakdown at the junction edgelimited the static breakdown voltage to between 800 and 1200 V.A similarly processed test diode showed a clear avalanche inthe bulk during reverse recovery at 300 V, even though thediodes could block more than 2 kV in static measurements. Basedon device simulation, it is proposed that this largediscrepancy between static and dynamic breakdown voltage iscaused by temporary trapping of holes in deep donor levelswhich enhance the space charge and thus the peak electricfield. Deep hole traps were found in the vicinity of the pnjunction by DLTS measurements. Keywords:dynamic avalanche, power diode, failure limit,dynamic ruggedness, avalanche injection, silicon, siliconcarbide, impact ionization