Reduction of the Base-Collector Capacitance of Heterostructure Bipolar Transistors Using Regrowth Over a Patterned Subcollector

For the standard triple mesa process typically used in the fabrication of III-V heterostructure bipolar transistors (HBT's) the base-collector capacitance becomes a limiting parasitic as the emitter size is reduced. Undercutting of the collector mesa is sometimes used to reduce the capacitance, however this is very uncontrollable especially as the dimensions are reduced. Here we report on the use of regrowth over a patterned subcollector to achieve a substantial reduction in the base-collector capacitance, C sub (BC), for InP/InGaAs latticed matched to InP based HBT devices. After patterning the sub-collector, an initial MOCVD regrowth is done followed by MOMBE regrowth of the remainder of the structure. All of the lithography was done using a stepper. We have developed a process to protect the initial alignment features during the regrowth steps to ensure proper realignment. This is a critical issue particularly for hte smaller devices where the regrowth technique can have and advantage.