a circuit for testing for mobile ion contamination of a semiconductor chip has an enabling circuit , a voltage pump , and a regulating circuit . in a normal operating mode , the voltage pump is enabled whenever the substrate voltage drops below a normal operating voltage . in a test - mode , the voltage pump is continuously enabled to continually drive the substrate toward the normal operating voltage . however , the voltage regulating circuit is enabled to shunt current from the substrate to ground whenever the voltage pump drives the substrate beyond a test voltage that is intermediate the normal operating voltage and ground . the regulating circuit thus holds the substrate at the test voltage .