An integrated circuit is described which includes a test mode circuit that allows a substrate of the integrated circuit to be forced to a voltage level dictated by an external connection during a test operation, and provides an improved substrate isolation from the external connection during non-test operations. Both n-channel transistor and p-channel transistor isolation circuit embodiments are described. An integrated circuit memory device is described which incorporated the test mode and isolation circuits. The external connection can be coupled to a negative voltage during non-test operation which is more negative than a threshold voltage below a substrate voltage without inadvertently coupling the external connection and substrate together.