Substrate leakage current in InGaP/GaAs heterojunction bipolar transistors (HBTs) has been observed. For a substrate thickness of 100 mm and an emitter area of 60 mm 2 , the parasitic current is on the order of 10 À8 A. The current flowing between the collector terminal and the substrate can be decomposed into linear and parabolic components. The parabolic dependence is also observed in Medici simulation. A theoretical derivation based on electron drift in the substrate shows parabolic dependence on applied voltage, consistent with our measurement and Medici simulation. In addition, it has been found that the current is inversely proportional to the cube of substrate thickness. To account for this substrate parasitic effect, a modification of the vertical bipolar inter-company (VBIC) model is proposed.

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