Defining the safe operating area for HBTs with an InGaP emitter across temperature and current density

One responsibility of the reliability engineer is to define the operating region of a component, providing designers and customers with a map of where the reliability is acceptable. This is often called the safe operating area (SOA). In this paper, SOA has been determined in terms of the FIT rate for an HBT across temperature and current density. Careful consideration was given to the temperature dependence of self-heating temperature (T_sh) in order to improve predictions. Two techniques were used to determine the confidence level on the FIT rate. These two different methods produced similar answers, giving the customer or the designer a good estimate of the margin available in the design. The procedure outlined here equips the reliability engineer with a sensible way to define the safe operating area of a device across temperature and current density. This reliability "map" furnishes the information required when choosing the operating conditions of the HBT and making compromises between current density, junction temperature and duty cycle