BV on the parasitic diode is different to that of its corresponding iso ring

[HCRain]

Recently I'm searching for the parasitic diode breakdown voltage (BV) for LV MOS of BCD process (many of the values are found in spectre file of PDK).
However, the BV voltage mentioned in the name of iso ring is different fron that of the corresponding parasitic diode.
Take TSMC 180nm BCD as the example: for iso ring "shpdpwnblhvnw8_iso", its two parasitic diodes “shpdpwnblhvnw_dio_iso” and "hvnwnblpsubshp_dio_8_iso" are both with bv_max>20V, which is pretty higher than 8V.
Could anyone provide the possible reason? Thanks a lot.

 

[dick_freebird]

Diode (GR) construction affects low current breakdown, applications have different "care-abouts".

A diode drawn with ortho corners will break down / leak earlier than one drawn with high radius of curvature.
One that sits on device drain node would be more of a leakage interest, than a plain ESD diode if application has a leakage concern in the pA / nA range).

Low BV numbers are sometimes used to flag reliability / stability potential issues for the designer to review. So you should ask the circuit what's up, not just commentators.