Switching Time in Junction Diodes and Junction Transistors

The time in which a junction diode may be switched from forward to reverse conduction is of great importance in computing networks. By considering the behavior of the minority carriers in a diode In a representative switching circuit an approximate solution for the switching transient may be derived. The transient is separated into two phases: first, one of constant current, where the flow is limited by the external resistance, and second, a "collection" phase, where the current decays at a rate determined by the minority carrier lifetime and the dimensions of the diode. A critical parameter in the solution is the ratio of the short-circuit reverse current to the forward current before switching. The mathematical treatment is a boundary value solution of the minority carrier diffusion equations which is accomplished by the use of Laplace transformations. The duration of the two phases of current flow is determined for a planar junction, a hemispherical junction, and for a planar junction with junction-to-contact distance small compared to a diffusion length. The last treatment is extended to the junction transistor and the behavior of the collector current is calculated. The general results indicate that for a given nority carrier lifetime the last two of the three diode structures will give the smallest switching times. In addition it is found generally that the time is minimized by decreasing lifetime and increasing the ratio of reverse to forward current.