We report growth by MBE and fabrication of state-of-the-art AlGaAs/GaAs selectively doped heterostructure transistors (SDHT's) and ring oscillators on Si substrates. In MBE growth, use of minimum As4/Ga flux ratio during initial nucleation combined with in-situ thermal cycles gave a marked improvement in material quality. Although undoped GaAs buffer layers were highly resistive and were fully depleted under a Schottky contact, a parallel n-type conduction path confined in a thin region (<0.1 pm) near the GaAs/Si interface was sometimes observed whose sheet density (1012 - 1013 cm−2) and mobility (600-900 cm2V−1 s−1) were independent of temperature between 77K and 300K. This parallel conduction did not affect the dc behavior and switching speed of SDHT's, and it was successfully prevented by doping the GaAs with Be near the interface. For a 2DEG sheet density of 1012 cm−2, a mobility as high as 53,500 cm2V−1s−1 at 77K was obtained, as against a mobility of ∼70,000 cm2 V−1 s−1 for a similar structure on GaAs substrates. For 1 μm-gate-length devices, maximum transconductances of 220 and 365 mS/mm were measured at 300K and 77K, respectively. A minimum propagation delay time, Td, of 28 ps/stage was measured at 300K for direct coupled FET logic (DCFL) ring oscillators for 1.1 mW/stage power dissipation. rd reduced to 17.6 ps/stage at 77K. From microwave S-parameter measurements at 300K, current gain and power gain cutoff frequencies of 15 GHz and 18 GHz, respectively, were measured. These results are comparable to the SDHT technology on GaAs substrates.