Abstract: The influence of various geometry sizes of SiGe HBTs on noise performance of the monolithic low noise amplifier(LNA) is investigated in this paper.Four types of LNAs using SiGe HBTs with different geometry parameters are fabricated in a 0.35 μm SiGe BiCMOS process technology.The measurement data indicates that at a given bias condition,a smaller emitter width slightly improves the noise figure(NF) if the ratio of emitter width to emitter length is small,however a proper longer emitter width and less emitter strips can significantly improve the overall NF without significant the gain degradation.Geometry scaling data show that the optimal LNA using SiGe HBT with A_E=4 μm×40 μm×4 has the minimum NF of 2.6 dB,the maximum associated gain of 27.4 dB and the optimum noise source resistance of nearly 50 Ω over 0.2-1.2 GHz compared to other device geometries.The die areas are only 0.2 mm² due to the absence of inductors.