A theoretical study has been made of the initial electron flow when a signal voltage V(t) is applied across an idealized planar diode. The analysis is based upon three fundamental postulates: Poisson's equation, Newton's equation, and the continuity equation. This quasi‐stationary study includes both the growth and motion of the space‐charge cloud of electrons across the interelectrode space and the transient build‐up of the current in the external circuit. The analytical forms and numerical plots of the current build‐up under space‐charge‐limited and temperature‐limited operation are compared for various ``large'' and ``small'' signal voltages.