RF breakdown effects in microwave power amplifiers

Abstract

Electrical stresses in the transistors of high-efficiency switching power amplifiers can lead to hot-electron-induced "breakdown" in these devices. This thesis explores issues related to breakdown in the Transcom TC2571 PHEMT, and the effects this has on the Draper Laboratory 2.3 GHz microwave power amplifier in which the transistor is used. Characterization of breakdown was performed under DC and RF drive conditions, and shows the surprising role of impact ionization at low temperatures in DC off-state breakdown, as well as the apparent prevalence of on-state breakdown under RF drive. DC characterization shows that breakdown walkout and recovery both proceed more quickly at higher temperatures, and also shows that breakdown stress might lead to the permanent creation of traps that degrade breakdown voltage. Walkout under RF drive decreases amplifier gain at lower levels of RF input drive, but appears to have no negative effect on amplifier saturated output power. The use of temperature-compensated input drive and a diode to clamp negative gate voltage swing are also explored as circuit design techniques that can mitigate device degradation due to breakdown stress.