The bias dependence of the single-port microwave reflection gain of 15 m-diameter Si Esaki tunnel diodes, grown by molecular beam epitaxy, was studied as a function of frequency. A simple equivalent circuit accurately modeled the data and yielded the forward-bias junction capacitance, which cannot be obtained by conventional low frequency capacitance-voltage techniques. The diodes were highly-doped step p-i-n junctions and exhibited a peak current density of 16 kA/cm 2 . The microwave reflection gain and cutoff frequency were 12 dB and 1.6 GHz, respectively, with a speed index (slew rate) of 7.1 V/ns.