in order to avoid thermal runaway bipolar transistors , emitters are provided with ballast resistors . elongate ballast resistors may be used , part of the lengths being connected for obtaining suitable resistance and design variability . the emitters are split up into a plurality of emitter portions , each with a separate emitter ballast resistor . the collector and base are correspondingly split up . the transistor is split up into unit cells , each comprising an emitter , a ballast resistor , a base , and a collector , which are respectively connected via respective common leads . this structure may advantageously be realized in a soi technique , the galvanic isolation enabling unproblematic mixing of digital and analog and power devices in the same chip .