Heterojunction bipolar transistors (HBTs) offer high power and linearity for microwave and mixed-signal applications. The large-signal performance of an HBT is frequently limited by the maximum operating current, which is set by thermal considerations. Hence, an accurate determination of the junction temperature in HBT devices and the corresponding microwave/millimeter-wave monolithic integrated circuits (MMICs)/DAC is critical to achieving maximum performance while maintaining reliable operation. An original accurate closed-form expression is presented for the thermal resistance of HBT structures from which the maximum junction temperature can be determined. The model's validity is verified by numerical simulations and experimental data. The model agrees with two sets of experimental data to within 2% and 6%. The closed-form solution offers device and MMIC designers a fast assessment of the performance and reliability of devices and circuits. Circuit optimization for required performance can be achieved efficiently.