Anomalous current distributions in power transistors

Current and temperature distributions of multi-emitter power transistors are analysed. The thermal properties, including thermal coupling, are described by a thermal resistance matrix. Anomalous current distributions over the emitter fingers which may lead to second breakdown are shown to be associated with the eigenstates of this matrix. Voltage and current-controlled second breakdown correspond to different eigenstates; the maximum power is inversely proportional to the relevant eigenvalue λ. The presence of emitter (or base) series resistance R gives rise to a thermal breakdown voltage BVTH∞ R/λ below which the device is always stable. The influence of the spacing between emitter fingers and of the quality of the thermal contact between chip and heat sink on the power handling capability is computed. Experimental evidence is presented which supports the theory.