On the use of n-well resistors for uniform triggering of ESD protection elements

N-well resistors are sometimes used to add series resistance to a grounded-gate NMOST protection device in order to ensure simultaneous triggering of multiple fingers. It turns out that such protections may fail far below their nominal ESD threshold depending on the particular layout. It is shown that n-well snapback plays a major role in the failure mechanism. Maximum ESD performance can be obtained by applying a simple design rule for the n-well geometry.