Criterion for determining the origin of Ohmic currents in insulators

For thin insulating layers the observation of a linear I-V curves is often cited as prima facie evidence that the carriers originate from thermal bulk generation. In the present paper we point out that a linear I-V curve can also arise from an emission-limited or contact-controlled current. We present an unambiguous experimental criterion to determine whether a linear I-V curve results from contact emission or bulk generation.