Structures for a heterojunction bipolar transistor and methods of fabricating such structures. A hardmask is formed that includes an opening over a first portion of a substrate in a first device region and a shape over a second portion of the substrate in a second device region. An oxidized region in the first portion of the substrate while the shape blocks oxidation of the second portion of the substrate. The oxidized region is subsequently removed from the first portion of the substrate to define a recess. A first base and a first emitter of a first heterojunction bipolar transistor are formed over the first portion of the substrate in the first device region, and a second base and a second emitter of a second heterojunction bipolar transistor are formed in the recess over the second portion of the substrate in the second device region.