Intrinsic carrier effects in HfO_2-Ge metal-insulator-semiconductor capacitors

摘要

Germanium metal-insulator-semiconductor capacitors with HfO_2 or other high-k gate dielectrics show unusual low frequency behavior of the high frequency (1 kHz or higher) capacitance-voltage characteristics when biased in inversion. Here, we provide evidence that this effect is partly due to the high intrinsic carrier concentration n_i in Ge. We show in particular that the ac conductance in inversion is thermally activated and it is governed either by generation-recombination processes in depletion, varying proportional to n_i or by diffusion-limited processes varying as n~2_i, depending on whether the temperature is below or above 45 ℃, respectively. From these measurements, we also show that the minority carrier response time in Ge is very short, in the microsecond range (much shorter than in Si), depending inversely proportional to n_i at room temperature. This means that due to high n_i, the inversion charge is built fast in response to high frequency signals at the gate, inducing the observed low frequency behavior.