InP/InGaAs Heterojunction Bipolar Transistors (HBTs) have demonstrated excellent high-frequency performance [1-4] and are widely used for optical fiber transmission [5-7]. However, the current mesa HBT structure utilizes a very thick, highly doped n+InGaAs layer for the subcollector contact. This added mesa height makes multi-level interconnection processes more difficult, which impedes the capability of fabricating compact integrated circuits. In addition, InP has a much higher thermal conductivity than InGaAs, so heat dissipation may be a problem for densely packed circuits with the above structure. This paper reports on InP/InGaAs HBTs grown on Ge/P co-implanted substrates by Metal-Organic Molecular Beam Epitaxy (MOMBE). This embedded subcollector HBT structure offers several advantages for the fabrication of large-scale integrated circuits on InP substrates.