An element which is sometimes an insulator and sometimes a conductor is called a:

A semiconductor is a material whose electrical conductivity falls strictly between that of a good conductor (like copper) and a good insulator (like glass). Depending on applied conditions such as temperature, voltage, or the addition of chemical impurities (doping), it can act as either. This unique dual-property is what allows components like diodes and transistors to function. They can be manipulated to block current flow (acting as an insulator) under certain electrical conditions and allow it to pass (acting as a conductor) under others. Options like intrinsic, N-type, or P-type conductor are incorrect because they refer to specific states or doped versions of materials, whereas "semiconductor" is the broad, fundamental term for this class of elements.