A hybrid voltage-variable capacitor

A new voltage-variable tuning capacitor is described. Using conventional fabrication techniques, metal-insulator-semiconductor and p-n junction structures are combined to obtain the high voltage sensitivity and low capacitance tolerances required to tune the AM broadcast band. Other potential applications are cited. A simple theory of operation based on nonequilibrium conditions is given together with experimental verification. Devices fabricated with a 530-Å-thick SiO
2
insulator on n/n
⁺
epitaxial silicon exhibit capacitance ratios of 15:1 in 14 V with a minimum Q of 170. The reverse current is two orders of magnitude higher than expected. This is attributed to growth defects in the epitaxial layer which give rise to a low minority carrier lifetime τ
0
∼0.1 µs and a high surface recombination velocity S
0
∼500 cm/s.