Frequency and temperature tests for lateral nonuniformities in MIS capacitors

Interface states and lateral nonuniformities produce very similar abnormalities in the
C-V
curves of MIS capacitors. Two
C-V
techniques are presented here to aid in distinguishing between them. The first technique is based on the frequency dependence of the interface-state capacitance and utilizes the resulting frequency dispersion of the "high-frequency" capacitance in the depletion regime, which occurs in a frequency range typically between a few hundred Hz and 1 MHz. The second method utilizes a freeze-in of carriers in the interface states at liquid nitrogen temperature. A sweep of bias from accumulation into deep depletion at low temperature produces a
C-V
characteristic which, when compared with the corresponding ideal characteristic for the same semiconductor doping profile, reveals the presence of lateral nonuniformities. A complementary test is provided by temporary illumination of the deep-depleted structure followed by a sweep of bias from inversion into accumulation. A ledge in the
C-V
characteristic reveals the presence of interface states in the central half of the bandgap.