The present invention relates to a semiconductor variable capacitance element whose capacitance is controlled by an electric charge accumulated on a floating electrode which is covered with an insulator film provided in the surface of a semiconductor substrate so that the floating electrode is insulated from the outside.
Semiconductor variable capacitance elements have heretofore been described in the literature (see Proceedings vol. 2 in 11th International Congress of Chronometry, '84, edited by The French Society of Microtechnology and Chronometry, p.9).
FIG. 2 is a sectional view showing the structure of a conventional semiconductor variable capacitance element. On the surface of a semiconductor substrate 21 are provided a capacitance or terminal electrode 12 connected to an n-type diffused region 22, a floating electrode 23 having opposed major surface covered with an insulator film 25 so as to be insulated from the outside, and a variable or control electrode 13 for adjusting the amount of electric charge accumulated in the floating electrode 23.
In this semiconductor variable capacitance element, a depletion layer or capacitive region 24 formed in a surface region of the semiconductor substrate 21 which is located under the floating electrode 23 is controlled by a potential generated at the floating electrode 23 by the electric charge accumulated in the floating electrode 23. Since the floating and capacitance electrodes 23 and 12 are strongly capacitance-coupled to each other, the potential of the floating electrode 23 is varied in accordance with the voltage applied to the capacitance electrode 12.
Since the capacitance electrode 12 of the conventional semiconductor variable capacitance element is connected directly to an external circuit, a bias voltage which is generated in the external circuit is applied directly to the capacitance electrode 12. For this reason, any variation in the voltage applied from the external circuit causes a change in the bias voltage, and this leads to variations in capacitance of the variable capacitance element. When the voltage applied from the external circuit is relatively high, a relatively high voltage is applied between the capacitance and floating electrodes 12 and 23, thus causing a minute tunnel current to flow into the floating electrode 23 through the insulator film 25. In consequence, the amount of electric charge accumulated on the floating electrode 23 is changed, and the capacitance value of the variable capacitance element is also gradually changed with time, disadvantageously.