Thermal switchback in high f t epitaxial transistors

To investigate the reasons for second breakdown 1 - or thermal switchback--and then to extend the safe operating area of power transistors, the behavior of an array of twenty small devices, mounted in parallel on a common heat sink, first without, then with an emitter series resistance, was evaluated. The first configuration suffers from β changes occurring at relatively low currents, an abrupt decrease of the base-to-emitter voltage, and failure to dissipate power. The configuration with emitter resistance shows a very tight collector-current base-current distribution, and normal base-to-emitter voltage characteristic. The same effect was obtained on a single large area device by employing a distributed thin-film resistor in series with the emitter. The uniform current distribution and the better thermal spreading of the heavily inter-digitated epitaxial device compensate for the small increase in saturation voltages due to the emitter resistance. Improved operating current levels and safe operating area are results observed on relatively small chip sizes.