Thermal stability analysis of multiple emitter finger microwave AlGaAs/GaAs heterojunction bipolar transistors

A numerical electrothermal model for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) has been developed, taking into account the nonuniform junction temperature rise due to self-heating. The model simulates both the DC current-voltage (I-V) characteristics and the microwave performance of multiemitter finger devices. The linear active region of the common-emitter I-V characteristics exhibits ´current crush´, where the collector current rapidly decays with increasing collector voltage due to the formation of highly localized hot spots within the device. It is also shown that a rapid fall-off in the cut-off frequency and maximum frequency of oscillation occurs corresponding to the onset of this thermal instability. Methods of overcoming this instability using emitter ballast resistors and higher-thermal-conductivity substrates are discussed along with corresponding effects on the DC and microwave performance of the device.<>