ROCKWELL INTERNATIONAL THOUSAND OAKS CALIF SCIENCE CENTER
Tailoring of the doping profile has been shown to be a powerful tool in reducing the intermodulation distortion in GaAs power FETs. Reproducible and uniform preparation of the required sophisticated profiles exceeds the present capability of epitaxial techniques, which has motivated the present investigation of fabricating highly linear power FETs by ion implantation. An analytical device model was developed for exploring the relationship between the active layer profile and the intermodulation distortion. These calculations revealed a complex behavior in the variation of the distortion levels due to partial correlation in the contributions due to nonlinear transconductance and output conductance. The device model was used to identify implant doses and energies for approaching an optimum active layer profile. Based on the results, a deep Se implant followed by a shallow Be implant to reduce the doping level close to the surface was used in the device fabrication. The intermodulation distortion of the transistors were measured by the two-tone method. Conventional epitaxial FETs with a flat doping profile were evaluated for comparison purposes. This comparison demonstrated that a 4dB increase in the intercept point for the third order intermodulation product can be realized by using tailored implant profile. The tuning conditions for maximum output power and minimum intermodulation distortion turned out to be virtually identical for the implanted transistors, in contrast to the behavior of conventional devices with flat doping profiles.