Operation of the junction transistor as a fast low-level switch

The paper considers the conditions necessary for a transistor to realize both good steady-state performance and high switching speed when operating as a low-level switch. It is found that, when functioning in this manner, the device is not adequately represented by the familiar simple model in terms of two independent charge stores, or by the equivalent definition of switching behaviour by charge control parameters. A more complete device model is proposed in which the distributed nature of the base charge is taken into account to the extent of representing the property that charge stored in relatively remote regions of the base may have quite a different time dependence from that of the charge immediately between the emitter and collector and under their direct control. Methods of measuring the relevant parameters of this modified device representation, which involves four independent charge stores, are given. When switching a low-level transistor switch to the ?off? state a tendency to switch partially ?on? again is observed. This effect, believed to be due to delayed in-flow of remote base charge, is explained by the proposed device model but not by the simple two-charge-store representation. It is shown that a base switching network may be designed by means of which such remote charge flow is closely compensated by an appropriate base drive current; high switch-off speed may then be achieved. An analytical design of the switching network is presented though its experimental derivation is more convenient in practice. By suitable design, the switching speed of the voltage-driven transistor switch may be approached without excessive transients in a circuit which, for good definition of d.c. conditions, is essentially a base-current-driven switch in the steady state. Although transistors of uniform-base type are discussed throughout the paper, the approach to the characterization of switching performance and the operation of the low-level switch at high speed is als- o applicable to devices of graded base type.