Abstract :
The series resistance and capacitance of a silicon-mesa parametric diode are calculated in terms of parent resistivity, geometry and diffusion. These properties have been measured both at 2Gc/s, using slotted-line techniques, and at low frequencies. The capacitance of those diodes tested is in agreement with p-n-junction theory. The diode series resistance, however, has consistently shown an excess resistance of 1.5 to 2¿ when measured at 2Gc/s, this being about twice as great as the series ohmic resistance. A series of blank mesas without p-n junctions had the same resistance using d.c. as they had at 2Gc/s. This result localises the source of excess resistance to the p-n junction. This excess resistance is assumed to be the result of dielectric losses within the p-n junction, due to the strong microwave-frequency fields at the junction, which force ionised donor and acceptor atoms to oscillate about their equilibrium position. The ions transfer energy to the silicon lattice, and hence the energy absorption in the p-n junction appears as a lossy element; i.e. a resistance in series with the junction. A series of diodes made from 0.100¿cm silicon were tested to determine the effect of reducing the density of ions at the junction. These diodes had a series ohmic resistance in substantial agreement with their resistance at 2Gc/s. The losses within the p-n junction suggest a new equivalent circuit for parametric diodes. This circuit includes an absorption resistance in series with the diode capacitance. The absorption resistance may be reduced by outdiffusion prior to formation of the p-n junction, in order to form the junction in a region of low ion density. If successful, this approach would yield silicon-mesa diodes with cutoff frequencies in excess of 100Gc/s at zero bias, using present fabrication techniques.
