Analytical model for the AlGaAs/GaAs multiemitter finger HBT including self-heating and thermal coupling effects

An analytical model which can be used to predict the thermal as well as electronic behaviour of the multiple emitter heterojunction bipolar transistor (HBT) is presented. The model is developed from a knowledge of device make-up (doping concentrations, layer thicknesses etc.), and relevant physics (such as the effects of graded heterojunction, self-heating, thermal coupling and ballast emitter resistance) is included in a unified manner. Thermal runaway phenomenon observed in the multifinger HBT at high current levels has been successfully described. Experimental evidence obtained from six-finger and four-finger HBTs are included in support of the model. The thermal runaway phenomenon is caused by the uneven increase of the base and collector currents at elevated temperatures due to the thermal effect.