A process is reported for monolithic integration of heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDs). The InGaAs/AlInAs HBT layers and the InGaAs/AlAs/InAs RTD layers are grown on an InP substrate in one molecular beam epitaxy growth run. The devices are separated by an InP etch stop layer grown between the two materials. The devices are planarized with polyimide and interconnected by second-level metal. Only minor modifications to the existing HBT process were required to incorporate the RTDs. Laser reflectometry was used to accurately control the polyimide etch depth fur both etch steps. Process simulation was used prior to wafer fabrication to predict the required etch depths and reduce process development time. A two-step polyimide etch back allows both devices to be contacted directly by second-level metal. This allows both devices to be fabricated with the minimum design rule dimensions, thereby improving speed and reducing power consumption. An additional InP etch stop layer was included in one wafer between the emitter superlattice and the base spacer layer. The etch stop layer improved beta uniformity but decreased the unity current gain cutoff frequency (F-t) by 11% and the maximum frequency of oscillation (F-max) by 18%.