Measurement of low-frequency base and collector current noise and coherence in SiGe heterojunction bipolar transistors using transimpedance amplifiers

Transimpedance amplifiers have been used for direct study of current noise in silicon germanium (SiGe) heterojunction bipolar transistors (HBT´s) at different biasing conditions. This has facilitated a wider range of resistances in the measurement circuit around the transistor than is possible when using a voltage amplifier for the same kind of measurements. The ac current amplification factor h _fe and the sum of the base and emitter series resistances (r _b+r_e) have been extracted from the noise. It has been established that the dominant noise source is situated in the base emitter junction at the emitter side and is not related to contact resistance noise. The simultaneous measurement of both the base-lead noise and the collector-lead noise and the calculation of the coherence between the signals has facilitated the pinpointing of the dominant noise source in the device and the extraction of (r_b+r_e )