Thermal impedance modeling of GaAs multi-finger HBTs from low frequency s-parameter measurements

Self heating in GaAs based heterojunction bipolar transistors affect the microwave performance and needs to be taken into account in circuit design. It can be modeled by the addition of a thermal impedance to a compact nonlinear transistor model. We developed thermal impedance parameters that can be scaled for multi-finger devices, using the measured low frequency s-parameters of 1, 2, 4, and 6-finger transistors. The model gave very good fit to the measurements at all bias points.