Transit-time and series-resistance effects in heavily doped Schottky-barrier diodes operating in the thermionic mode are evaluated by applying the laws of vacuum tube electronics to the space-charge region and by incorporating the treatment of the series-resistance effect into the discussion. It is assumed that the field distribution in the space-charge region is linear, that collisions in the space-charge region can be neglected, and that the signals are so small thatr the equations of motion can be linearized. The case of uniform field distribution is also briefly dealt with; the results are not significantly different from the linear field case. At a frequency equal to three times the plasma frequency of the bulk semiconductor the detection and mixing efficiency is about 0.25 times the low-frequency value. The effect of the high-frequency conductance of the space-charge region turns out to be quite small when the series-resistance effect is taken into account; the performance is then determined by a single transit-time factor ‖g (jω) ‖2 and a series-resistance factor that is independent of transit time. The effect of tunneling on the device performance is discussed briefly. Satisfactory detection and mixing performance seems feasible for the 10–40-μm wavelength range.