Electroluminescence in divergent field

Charge carriers with high mobility can only be studied in dielectrics within microscopic dimensions, since the power dissipation would cause thermal runaway for a macroscopic geometry. As a complement to the direct "charge injection" measurement with a guarded needle electrode system, electroluminescence from a semicon needle was monitored to study the space charge formation and dynamics in dielectrics, especially at the interface. Under bipolar excitation, the threshold for luminescence is the same within experimental error as the charge injection threshold determined using a guarded needle because of the alternative bipolar injection in each half cycle. Under half rectified ac condition, almost no light is emitted before twice the threshold voltage for ac condition is reached. The experimental results can be explained through the polarity reversal during the unipolar pulse shot, which is in accordance with the Finite Element Method simulation. This study is also useful for electroluminescence model differentiation.