Design of a VHF power transistor with second-breakdown protection

An extension of the "overlay" concept has led to the design of a VHF power transistor with integrated second-breakdown protection. This device is capable of delivering 50 watts at 80 megacycles per second in Class C or linear amplifiers for single-sideband applications. The paper shows how the final design was derived from investigation of the factors affecting second breakdown in traditional silicon interdigitated transistors. Temperature-sensitive phosphors were used to observe the formation of "hot spots" prior to second-breakdown. The "hot spots" which result from localized high current densities were correlated with biasing conditions and device geometry. Data are presented showing that localized "hot spots" are an inherent, random condition in a transistor. This conclusion led to the design of a device that protects against second breakdown by limiting the formation of high-current-density regions by resistively ballasting a large number of small emitter sites.