Microwave resistance of GaAs varactors

Microwave measurements on epitaxial gallium arsenide varactors have shown that a wide variation in varactor quality is obtained for different epitaxial wafers even though other crystal and diode parameters are equivalent. Using Houlding´s technique for measuring cut-off frequency, the relation between microwave resistance and junction capacity was determined. The linear relation obtained between resistance and reciprocal junction capacity showed a fairly constant loss component of one ohm in addition to the area dependent component. Calculations of diode resistance based on actual geometry show wide discrepancy between calculated value and area dependent component. The extra resistance may be accounted for by the presence of a thin, high resistivity layer contributing extra loss at the measurement frequency of 10 GHz. Measurements were also made on varactors made from homogeneous gallium arsenide andfrom epitaxial silicon. Comparative data will be shown.