Diffused GaAs varactors: Epitaxial and nonepitaxial

Diffused epitaxial gallium arsenide varactors have been fabricated with measured (6 Gc/sec, degenerate) noise temperatures of 42°K when operated at room temperature and 14°K operating at liquid nitrogen. The zero bias capacitance of such units ranges from 1.0 to 0.5 picofarad, corresponding to zero-bias cut-off frequencies of 120 to 230 Gc/sec. Calculations suggest that cut-off frequencies well in excess of 500 Gc/sec (with step junction capacitance variations) should be realizable in an epitaxial GaAs structure. The attainment of this performance depends primarily upon continued improvement in the quality of epitaxial films. Nonepitaxial, diffused, GaAs varactors have also been fabricated. In an ultrathin wafer design with graded junctions and zero-bias capacitances of 0.7 to 1.0 picofarad, these have exhibited cut-off frequencies over 200 Gc/sec. The lowest measured noise temperature for the nonepitaxial varactor operating at room temperature is 56° K. Observed cut-off frequencies approach theoretical expectations for this structure. DC reverse bias power tests on nonepitaxial diffused GaAs varactors indicate maximum tolerable power dissipations of over 1 watt per picofarad.