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The 1/f noise of double InP/InGaAs heterojunction bipolar transistors is measured and analyzed. Standard mesa transistors, transistors with an air-bridge-connected base and hexagonal shaped transistors conceived for digital circuits are studied. These differences in the technology will have an influence on the origin of the noise sources. Regarding noise analysis, the base and collector internal current noise sources ib and ic are assumed to be correlated for all devices. This is highlighted by the voltage noise correlation function between the input and the output of the devices presenting an unusual behavior versus bias and geometry. The collector current noise source is divided into a correlated and uncorrelated part with base current. These parts give rise to spectral densities Sc and Snc. They are shown to have distinct origins for the different types of transistor geometry.

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