Stored charge in tantalum capacitors: Latent and manifest mavericks

Abstract

Relatively permanent space-charge fields can be established in Ta-Ta 2 O 5 -MnO 2 capacitor structures by any of the following. 1. Ionizing radiation with forward bias (tantalum electrode positive), 2. Reverse bias without radiation, 3. Forward bias without radiation. All of the capacitors store charge by the first two mechanisms. Only when a permanent space-charge polarization field can be established by condition 3 can the unit be called a “maverick”. The existence of a space-charge field is manifested when the capacitor is exposed to radiation at no bias or low forward bias. An anomalously large release is observed as the space charge is relaxed by the ionization-generated carriers. The polarization is relaxed by successive radiation pulses, and the normal zero-bias signal is finally obtained. The polarization behavior arises due to nonuniform trapping of charge in the oxide. In a normal capacitor, the polarization under irradiation occurs when the ionized electrons and holes are separated by the electric field in the oxide. Since the time and radiation history of the charge release in a maverick is similar to the charge release in a polarized normal capacitor, mavericks are apparently capacitors that can become polarized in the absence of radiation — for example, just due to an applied bias. This model has been simulated with the computer code PN, and the characteristics of the experimental data are reproduced. A maverick capacitor can be either latent or manifest, and a simple test for the zero-bias radiation response will not necessarily reveal a maverick unless a space-charge field has previously been established. These results are used to suggest tests to eliminate mavericks from the population.