what is Drain Punch through in vlsi

When the drain is at a high enough voltage with respect to thesouce, the depletion region around thedrain may extend to the source, thus causing current to flow irrespective of the gate voltage( i.e., even if itis zero). This is known as apunchthroughcondition.Currently, this effect is used in I/O protection circuits to limit the voltages across internal circuit nodes, although it will impact design as devices are scaled down by requiring that internal circuit voltages be reduced to a point where the effect does not occur.