Geometry Considerations for Thermal Design of Microwave Heterojunction Bipolar Transistors

An efficient general algorithm for three-dimensional thermal simulation of semiconductor chips and its exemplary application to AlGaAs/GaAs heterojunction bipolar transistors (HBT) is presented. Firstly, the assumption of stationary heat sources as a boundary condition is validated by pulsed scattering parameter measurements. Secondly, the simulation method based on the fast Fourier transform is introduced. The model is verified by liquid crystal thermography, infrared thermography and comparison to another simulation method. In the third part of the paper, the model is applied to the self-heating problem in multifinger HBTs. Geometrical design rules for minimization of the maximum temperature and temperature gradients between the individual emitters are deduced. A tradeoff between the emitter size, the number of emitters, the power level and cooling measures such as substrate thinning must be carried out in order to control the thermal properties with respect to safe operation and reliability.