Conduction current in the phosphor layer of ZnS:Mn ac thin film electroluminescent (ACTFEL) display device was measured as a function of the amplitude and the rise time of the bipolar voltage pulses: electric field in the phosphor was also determined. It was found that the clamping field of an ac thin film electroluminescent device is not a single‐valued device characteristic. The clamping field depended upon the rise time of the applied voltage pulse increasing in value as the rise time became smaller. These data were interpreted in terms of a nonideal breakdown of the insulator‐phosphor interface and a time delay involved in the emission of electrons from this interface. To account for the nonideality in the interface breakdown the ac equivalent circuit of the ACTFEL device was modified by including a voltage‐dependent resistor in parallel with the phosphor capacitance. The phosphor current measurements, transferred charge measurements and tunneling current calculations indicate that in the nonhysteretic ZnS:Mn devices under investigation, impact ionization in the phosphor layer, if present at all, leads to relatively small carrier multiplication factors.