The design and measurement of semiconductor devices in terms of thermal behaviour

When a new design of transistor or its modification is under consideration, it is highly desirable that the thermal resistance be estimated in advance of measurements. It is shown how the thermal resistance of the various heat paths from the transistor junction to the surrounding air can be calculated, and how charts may be constructed for convenience in evaluation. The setting-up of an electrical resistance analogue of the transistor heat paths is described, enabling the total thermal resistance of the transistor to be obtained and the effect of varying specific elements to be observed. The procedure is described for a typical transistor configuration, where the agreement between theory and measurement is shown to be within 20%. To measure the thermal resistance of a transistor it is convenient to use the temperature variation of one of the device parameters as an indicator of junction temperature. The parameters of a transistor are surveyed, with the emphasis on their suitability for use as a thermometer. The parameter chosen may depend on the type of transistor being considered, but in general a preference is expressed for using the variation in forward emitter-base voltage as the basis of the measurement. An apparatus for making such measurements is described.