A microplasma transistor has been realized by injecting electrons into the sheath of a rare gas plasma with a low voltage (|Vb|<25 V), controllable electron emitter. Integrating a solid state emitter with a 500 μm diam. cylindrical microcavity plasma yields a three terminal current-controlled device capable of modulating the conduction current and light intensity generated by the microplasma. For an emitter voltage of Vb = −10 V, the rms charge carried by the conduction current of a Ne microplasma is tripled relative to the value measured for no current injection. Similarly, the wavelength-integrated visible emission is increased by 2.7 and 4 dB for Vb = −5 and −25 V, respectively. From the continuity equation for charged particle flux in the sheath, the electron density at the edge of the sheath is determined to be ns = (3±1)×1012 cm−3 for an electron temperature in the 1–5 eV range. Energizing the electron emitter is estimated to reduce the ratio of the ion to electron number densities at the cathode surface from 25 to 14. A parameter βp, defined as the microplasma transistor conductance normalized to that for the conventional plasma device (i.e., Vb = 0), is introduced and found to be ∼ 40 for this unoptimized device when |Vb| = 5 V.