The effects of internal R. F. attenuation on the power output performance of bipolar transistors

In the design of microwave bipolar transistors, there has been extensive utilization of three basic geometries: interdigitated, overlay and mesh emitter. In view of the obvious cost-performance-reliability trade-offs of the geometrical designs, an understanding of attenuation along un-metallized stripes is imperative. This paper summarizes an experimental and theoretical investigation of the variation of output power by the r.f. attenuation along a diffused stripe where current was fed from one end only. The experiments have employed the overlay geometry with 2.5 µm linewidths and 4 µm spaces. Diffused stripe length, Le, was varied while the emitter periphery, Pe, the base area, Ab, and the emitter area, Aewere kept constant. At a frequency of 1 GHz, it was observed that for the same input power, the output power decreased by in Class C operation and in Class A as Lewas increased from 19 µm to 36 µm. Interestingly, in both modes of operation, no change in output power was noted for Le\´s between 12 µm and 19 µm. The obvious implication is that in these structures r.f. attenuation does not play a dominant role until the stripes reach a length of ∼ 20 µm, allowing the use of quite wide emitter metallization. A simple analysis has been carried out to better understand the r.f. attenuation mechanism and relate it to experimental data. The attenuation constant, , was calculated using ordinary transmission line theory. Then, the collector current, Ie, was found employing this value of . Variation of the output power was, then, inferred from the variation of Ie². Correlation between theory and experiment is satisfactory considering the simplifications introduced into the analysis.