We propose an unbiased electrical fault isolation methodology for locating gate oxide breakdown failures in MOSFETs. The test vehicle involves a sub-15nm technology DRAM device which failed due to time-dependent dielectric breakdown (TDDB). This methodology introduces an implementation of Optical Beam Induced Resistance Change with no applied external bias (zero input voltage). From OBIRCH analysis, a change in current was achieved near failure site. This principle was explained based on Seebeck effect and equivalent circuit modeling of the MOSFET drain within Seebeck generator. A physical cross section using the Focused Ion Beam (FIB) revealed a gate oxide breakdown along the location of the OBIRCH spot, illustrating the benefit of an unbiased fault isolation to preserve the failure mechanism. This study proves that gate oxide breakdown site can still be located even with no external voltage applied, preserving the device condition of nanoscale DRAM, and eliminating the chances of altering the failure mechanism as a result of the applied external voltage stress.