Properties of AlGaAs/GaAs heterojunction bipolar transistor with AlGaAs ledge bypass capacitor

This paper proposes a device structure for enhancing thermal stability of a multi-finger power heterojunction bipolar transistor (HBT) with little sacrifice on RF properties. The proposed structure has an embedded base-ballast resistor and a bypass capacitor on each finger, which can be fabricated using a normal HBT process. It uses a p ⁺-base layer and a depleted n-AlGaAs ledge to implement the base-ballast resistor and the bypass capacitor, respectively. Because the bypass capacitor uses the depleted AlGaAs ledge as a dielectric layer, no additional process step, except an ion-implantation process, is necessary to fabricate the structure. It is demonstrated experimentally that the thermal stability of an HBT with the proposed structure is enhanced significantly with little degradation on the RF properties.