Improving collector-current uniformity in emitter-graded AlGaAs/GaAs heterojunction bipolar transistors

To investigate the effect of a graded layer on collector-current uniformity, two types of HBTs were fabricated by metalorganic chemical vapor deposition (MOCVD). One type had a bandgap graded layer at the emitter-based interface to eliminate the conduction-band spike. The other type was a conventional HBT (heterojunction bipolar transistor) with an abrupt heterojunction fluctuated due to the fluctuation of the barrier energy from the emitter to the base. The bandgap-graded layer drastically suppressed the fluctuation of the collector current. the standard deviation of the threshold voltage was improved from 3.03 to 0.42 V by adopting bandgap grading at the emitter-based interface.<
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