Thermal characteristics of InGaP/GaAs HBT ballasted with extended ledge

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 resistors. Both ballasted and unballasted devices were fabricated and compared. A small signal equivalent circuit gives us the magnitude of the effective ballast resistance. The thermal characteristics, including gain-collapsed I-V and V/sub be/ regression curve are shown and modeled. The temperature dependency of base sheet resistance and its influence on the device performance are also discussed.