Transit time effects in diodes, in pictorial form

In order to obtain a pictorial representation of what happens in thermionic valves at frequencies at which the transit times of the electrons play an important part, electron positions and velocities, in a plane parallel diode, were calculated for a succession of times. In the case of a saturated diode, the voltage was assumed and the current obtained from the diagram. The result shows that there is an alternating component of electron current which leads the applied voltage and appears in the circuit as an addition to the geometrical capacitance current. The valve, therefore, appears as a finite resistance in parallel with an effective capacitance greater than the geometrical capacitance. In the case of the space-charge-limited diode the current was assumed and the voltage obtained from acceleration diagrams. The result shows that the valve behaves as a resistance in parallel with a capacitance smaller than the geometrical capacitance. This method has two advantages over purely analytical methods. It describes the mechanism of the effects, and it shows the presence of harmonics which usually disappear in the approximations made in analysis.