 This resistance is a result of the finite conductivity of the p-type doped substrate.When a positive ESD event appears upon the I/O pad (drain), the collector-base junction of the parasitic NPN BJT becomes reverse biased to the point of avalanche breakdown.At this point, the positive current flowing from the base to ground induces a voltage potential across the parasitic resistor, causing a positive voltage to appear across the base-emitter junction.