Thermal design of multifinger bipolar transistors

In this paper, design guidelines are provided to improve the thermal stability of three- and four-finger bipolar transistors. Experiments and simulations are first performed on silicon-on-glass (SOG) three-finger bipolar junction transistors (BJTs) with self-heating and mutual thermal resistances varying in a large range of values, depending on the silicon area, presence of heat spreaders, isolation, and distance between fingers. To avoid strong asymmetries between the mutual thermal resistances of two adjacent fingers as compared to nonadjacent fingers, a "hexagonal" topography is proposed. It is demonstrated that, due to the nature of this solution, the onset of thermal instability can be shifted to a higher value of dissipated power. The key role of metal lines and bondpads in fully isolated devices is also highlighted. Subsequently, the electrothermal behavior of SOG four-finger BJTs is investigated and the thermally induced current nonuniformity over the individual fingers is experimentally monitored for the first time. Thermal-only and electrothermal simulations are employed to conceive and analyze more thermally robust layout topographies, referred to here as "rhombus" and "square."