Collapse of MOSFET Drain Current After Soft Breakdown and its Dependance on the Transistor Aspect Ratio W/L

Gate oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The soft breakdown effect on the MOSFET characteristics strongly depends on the aspect ratio W/L: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W/L after soft breakdown. As W/L increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse are due to the formation of an oxide defective region around the SB spot, the area of which is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing, any increase of gate leakage current.