Effects of negative differential resistance in high power devices and some relations to DMOS structures

Due to the feedback of free carriers to the electric field, branches with negative differential resistance (NDR) occur in high power devices. NDR usually leads to current filaments. These current filaments might be non-destructive if they move. Most critical effects are found if impact ionization occurs at the n^--n⁺-junction with a positive electrical feedback to avalanche or dynamic avalanche at the pn-junction. Against dangerous double-sided dynamic avalanche, countermeasures have been found in high power devices. These structures suppress or limit the formation of electric fields at the n^--n⁺-junction. They lead to devices with dramatically increased ruggedness. Even if ESD and DMOS devices operate at lower voltage, the very high current density levels lead to a density of free carriers above the specific doping of the layer, and free carriers dominate the shape of the electric field. Similar effects of negative differential resistance and moving filaments have been found in ESD protection devices. Snap-back effects have been found in DMOS devices. Possible relations are discussed.