Role of wide contacts in the ac response of a double-barrier structure

The role of wide contacts is investigated in the ac response of a double-barrier structure. Several physical quantities, such as the distributions of internal potential and charge density, capacitance, and low-frequency ac conductance, are calculated. The results show that when the system is far away from resonance, the charge distribution exists only around the barrier regions as a response to the applied voltage, and hence the contacts have almost no effect on the results. In the resonant case or near resonance, the charge distribution displays large peaks in the contact regions, but almost no charge distribution within the conductor. In the case of small transmission probability, we find a considerable amount of charge distribution both in the contact regions and surrounding the double-barrier conductor. Moreover, we find that qualitatively the presence of contacts does not change the main features of the emittance obtained from a calculation that neglects the widening of contacts. But the contact effect on the capacitance is significant when the chemical potential is very close to the resonant energy: There is a sharp capacitance peak at the resonance, which corresponds to the charge peaks in the contact regions.