Experimental comparison of base recombination currents in abrupt and graded AlGaAs/GaAs heterojunction bipolar transistors

The relative importance of the base bulk recombination current and the base-emitter junction space charge recombination current is examined for AlGaAs/GaAs HBTs with different grading schemes in the base-emitter junction. Experimental results demonstrate that, in abrupt HBTs, the base bulk recombination current is larger, and the base current increases with the base-emitter bias with an ideality factor of approximately 1. In contrast, in graded HBTs, the space charge recombination current dominates and the base current ideality factor is approximately 2. These experimental results agree well with a published theoretical calculation.