A InGaP/GaAs heterojunction bipolar transistor structure is proposed in which the base epi-layer underneath the extended ledge works as a base ballast resistor. The structure eliminates the critical alignment for a passivation ledge formation as well as additional process steps for external base ballast resistor. Both ballasted and unballasted devices were fabricated and compared. Small signal equivalent circuit gives us the magnitude of the effective ballast resistance. The thermal characteristics, including gain-collapsed I-V and Vb, regression curve are shown and modeled. The temperature dependency of base sheet resistance and its influence on the device performance are also discussed.