Thermal instability in two-finger bipolar transistors

A novel analytical formulation for the biasing condition leading to thermal instability in a two-finger bipolar transistor is derived. The temperature dependence of the current gain as well as the mutual thermal coupling is accounted for. Different technologies are compared by varying the model parameters. It is demonstrated that SiGe devices are more thermally stable than comparable Si devices. Finally, the influence of the substrate modification on thermal instability is investigated. It is shown that thermally conducting substrates are needed to electro-thermally stabilize bipolar transistors.