Semiconductor surface varactor

The semiconductor varactor using surface space charge is analyzed and measurements made on several experimental units are described. The chief characteristics of this device are its capacity-voltage dependence and its negligible dc conduction. The particular system used in this work is a thermally grown oxide on silicon. A theory developed from the surface charge relation is shown to agree with the experimental data over a wide range of silicon resistivity. The theory for optimum operation for both dc and ac biasing is derived and used to compare the performance of this device with that of the p-n junction varactor. The result of this comparison shows that with careful design the semiconductor surface varactor will be able to compete favorably with the junction varactor for many possible applications, including those of ultra high frequency.