5 - Reliability Considerations

Abstract

Concerns have arisen about die temperature and its effect on reliability. Solid-state RF power designs in the past were made with little concern for thermal properties. Both conventional silicon bipolar transistors and silicon field effect transistors (FETs) normally have bottom-side collector or drain connections to the die. This dictates that the transistor die must be isolated electrically, and that the requirements to isolate the collector or drain from the heat sink normally dictate the use of an insulating material with good thermal conductivity as the interface between the transistor die and the mounting flange or stud. From the viewpoint of reliability, the junction temperature of a solid-state device should not exceed 200°C. Lower temperatures will only tend to enhance reliability The one predictable failure mechanism in RF transistors is metal migration, which is a result of high current densities at high temperatures. In addition to excessive die temperature, there are other failure mechanisms in transistors. Some of these failure mechanisms are inherent in the transistor construction. One of the most common failures in bipolar transistors occurs when the base-emitter reverse voltage (BVEBo) is exceeded. Transistor failure caused by a mismatched load is without question the most frequent source of device failure in using high power RF transistors. A push‑pull amplifier tends to be more vulnerable to transistor failures from over dissipation caused by load mismatches than a single ended one. The MOSFETs have another failure mode, which is unique to them and is fairly common. If gas discharge devices, such as surge arrestors, were available with low enough breakdown voltages, they would make ideal MOSFET gate protectors.