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Analytical expressions for the electrothermal parameters governing thermal instability in bipolar transistors, i.e., thermal resistance RTH, critical temperature Tcrit and critical current JC,crit, are established and verified by measurements on silicon-on-glass bipolar NPNs. A minimum junction temperature increase above ambient due to selfheating that can cause thermal breakdown is identified and verified to be as low as 10-20°C. The influence of internal and external series resistances and the thermal resistance explicitly included in the expressions for Tcrit and JC,crit becomes clear. The use of the derived expressions for determining the safe operating area of a device and for extracting the thermal resistance is demonstrated.