Design considerations of hyperabrupt varactor diodes

This paper describes the design calculations and fabrication techniques used to produce hyperabrupt varactor diodes. The computation consists of determining, the doping profile, the capacitance as a function of applied voltage, the breakdown voltage, and the series resistance; allowance is made for the nonlinear variation of mobility with doping density. Fabrication of the retrograded region was accomplished by diffusion from a doped-oxide source. Two groups of diodes were fabricated. The first group had a breakdown voltage VBof 95 V at 1 µA, a capacitance ratio of 12 over a voltage range of 1 to 40 V (CP1/CP40=12), and a quality factorQof 60 at an operating level of 3 V and 100 MHz. The corresponding values for the second group were VB=45 V (CP1/CP40=4) andQ=210. The capacitance tracking capability of these diodes was found to be within 2 percent. These measurements showed excellent correlation with calculated results.