Leakage Currents in Multilayer Ceramic Capacitors

Leakage currents in new and degraded (typically at twice rated voltage, 125°C) ZSU and X7R types Of multilayer ceramic (MLC) capacitors show both ohmic and space charge limited current behavior. The near 3/2 power voltage characteristic (la V^3/2) of new devices can be attributed to electron emission from electrode points. The quadratic behavior (I V²) seen for moderately degraded devices represents space charge limited emission from near planar electrodes. This emission may evolve from the point emission due to resistivity decreases that occur in the emission region as a result of ion movement. For these currents, electrons are believed to be the dominant charge carriers. Neither Schottky or Poole-Frenkel currents were identified. Thermal activation energies decrease from ~1.3 eV for new X7R devices, to zero for degraded ones, corresponding to resistivity decreases from ~ 10¹³ -cm to 10⁵ -cm or less (at 125°C). Carrier concentrations and mobilities have been estimated from thermoelectric measurements on new and reduced nonelectroded X7R chips, which exhibit n-type conduction. These parameters both increase substantially with reduction, with mobility dominating the thermal activation energy. These measurements are consistent with a small polaron hopping mode for electron transport in the ceramic.