Universal voltage-current characteristics are presented for a planar diode, showing the general transition from the Fowler-Nordheim relation to the Child-Langmuir law. These curves are normalized to the intrinsic scales that are constructed from the Fowler-Nordheim coefficients A, B. They provide an immediate assessment of the importance of the space charge effects, once the gap voltage, gap spacing, and the Fowler-Nordheim coefficients are specified. An example in the parameter regime of vacuum microelectronics is presented.