Threshold Voltage (Vt) of MOSFET controls transistor’s on and off state. Vt is usually depends on gate oxide thickness and operating temperature. Systematic failure analysis for a Vt shift issue, should also consider the channel doping which affects the inversion layer formation. In this article, the failure case of a shift in the Vt of a Power MOSFET V is studied. Secondary Ion Mass Spectrometry (SIMS) is found to be the most direct way for detecting any abnormality in the channel doping profiles. A comprehensive simulation is performed showing that the Phosphorus level diffusion from substrate was so high that it affects the doping concentration of channel.