Design of a high frequency transistor with star geometry

It is well known that at high injection levels the current capability of a transistor emitter is proportional to emitter periphery rather than area. The star geometry of the device reported in this paper was designed for maximum edge length to area ratio consistent with fabrication techniques. This NPN silicon transistor was designed to operate in the 150 ma range, with optimum geometry to give maximum high-frequency performance. Coupling of planar-epitaxial construction with the optimum geometry resulted in a structure capable of replacing a number of other types designed for special functions. Nano-ampere leakage currents and long term stability were obtained by planar-passivated construction; Useful current gains have been obtained from 1 microampere to 1 ampere. Current-gain bandwidth, ft, is greater than 300 megacycles Emitter and collector transition capacitance and base spreading resistance are minimized in the geometry used. Use of epitaxial material resulted in less than 3 ohms saturation resistance. As an amplifier, more than 2 watts RF. power output with greater than 10 db gain has been obtained at 70 megacycles.