Experimental studies on low frequency noise in bipolar junction transistors (BJT) including polysilicon emitter, hetero-junction bipolar transistors (HBTs) and silicon-germanium hetero-junction transistors (SiGe HBTs) are reviewed. The 1/f noise is treated in terms of mobility fluctuations. The validity of a new empirical relation between the 1/f noise corner frequency fc (the frequency where the 1/f noise and shot noise are equal) and the peak cutoff frequency frpeak is investigated. The experimental procedure to investigate the most dominant low-frequency noise source in the equivalent circuit is described. At medium frequencies, the white noise becomes dominant and the noise figure is calculated taking into account the emitter and base series resistance.