Short minority carrier response time in HfO2/Ge metal-insulator-semiconductor capacitors

In this work, we elucidate the role of the intrinsic carrier concentration n(i) in the unusual low frequency behavior of the high frequency capacitance voltage curves in inversion which is observed in HfO2/Ge n-type metal-insulator-semiconductor capacitors. We show that the ac inversion conductance is an increasing function of ni showing thermal activation behavior with temperature. This is an indication that, irrespective of the particular mechanism involved, the high n(i) value in Ge, provides a "pool" of carriers which could generate a high density of minority carriers thus inducing the low frequency behavior in inversion. Our analysis shows that the minority carrier response time tau(R) varies inversely proportional to n(i) at room temperature, therefore this time is very short measured in the microsecond range. The short response time results in a fast build-up of inversion charge, so that a low frequency behavior is observed even at high frequencies, in the kHz range.