alent circuits of the variable capacitor;FIGS. 7A and 7B are a plan view and a cross sectional view, respectively, of an example of a variable capacitor according to a third embodiment;FIGS. 8A and 8B are cross sectional views of the variable capacitor illustrated in FIGS. 7A and 7B;FIGS. 9A and 9B are diagrams illustrating equivalent circuits of the variable capacitor;FIGS. 10A and 10B are diagra