Methods of according to the present disclosure can include: providing a substrate including: a first semiconductor region, a second semiconductor region, and a trench isolation (TI) laterally between the first and second semiconductor regions; forming a seed layer on the TI and the second semiconductor region of the substrate, leaving the first semiconductor region of the substrate exposed; forming an epitaxial layer on the substrate and the seed layer, wherein the epitaxial layer includes: a first semiconductor base material positioned above the first semiconductor region of the substrate, and an extrinsic base region positioned above the seed layer; forming an opening within the extrinsic base material and the seed layer to expose an upper surface of the second semiconductor region; and forming a second semiconductor base material in the opening.

A semiconductor device, including: a first layer including monocrystalline material and first transistors, the first transistors overlaid by a first isolation layer; a second layer including second transistors and overlaying the first isolation layer, the second transistors including a monocrystalline material; where the second layer includes at least one through layer via to provide connection between at least one of the second transistors and at least one of the first transistors, where the at least one through layer via has a diameter of less than 200 nm; a first set of external connections underlying the first layer to connect the device to external devices; and a second set of external connections overlying the second layer to connect the device to external devices.