New Frontiers of Varactor Harmonic Power Generation in the C-Band

We have designed and developed a new class of varactors that have been noteworthy beneficiaries of the techniques evolved for GaAs IMPATTs. In this paper we report the experimental results obtained from double-stacked diodes for frequency doubling (2 to 4 GHz), and double- and triple-stacked diodes for frequency tripling (2 to 6 GHz). The power and efficiency sought in the present application (S- to 10-W output at 4 and 6 GHz with a minimum efficiency of 70 percent) dictate a low-loss, high-voltage varactor. These conflicting goals are best achieved by a series connection of two or more chips in a single package. 1 Using a Schottky-barrier GaAs chip, the power dissipation per chip is so small that a simple series stacking of one chip upon another is adequate, without heat sinks for any chip except the bottom one. This scheme is facilitated by using dice of typical C-band I M P ATT design, 2 i.e., squat cylinders or truncated cones (0.2 mm in diameter by 0.08 mm tall, for example) in which the active area is that of the cylinder itself. The dice are thermocompression-bounded in "flip-chip" position, one on another, in a package with a diamond heat sink. The series combination not only increases power-handling capacity considerably, but it also alters the input and output impedances favorably. The chips employed here were originally tested as I M P A T T s , where they gave about 3 W at 10 to 12 percent efficiency in C-band. 1839