Low-frequency noise characteristics of polysilicon emitter SiGe bipolar transistors made by SEG/NSEG process and given F and/or BF2 implants

Noise investigations of Si/SiGe heterojunction bipolar transistors (HBTs) grown by a SEG/NSEG process are carried out in the frequency range f=1 Hz to 100 kHz under different operating conditions. The polysilicon emitter devices given a F implant into the polysilicon emitter and/or a BF₂ implant into the buried oxide as well as the devices without either F or BF₂ implants are studied. It is shown that the 1/f fluctuations of the recombination base current and the 1/f fluctuations of the conductivity of the interfacial oxide layer in the emitter are responsible for the low-frequency noise typical for the devices considered. The analysis of the noise results allow to conclude that two different components contribute into the recombination base current observed. One of these components is accompanied by a high 1/f noise and can be suppressed by a BF₂ implantation. It is shown that this component can be attributed to recombination in the lateral emitter/base depletion region that is partly inside the extrinsic base polysilicon. Therefore, it is revealed that the BF₂ implant can influence not only the collector junction but also the emitter junction and, hence, its role in HBTs studied is more complicated than supposed before. Another recombination component of the base current is accompanied by a much lower noise and is decreased by a F implantation. This component can be due to recombination at the interface between the low doped emitter layer and the oxide deposited on this layer. In addition, a significant decrease of the 1/f noise in the interfacial oxide layer by a F implantation is found. This effect is explained by the decrease of the layer resistance and supports the idea that the F implant promotes the break-up of this layer