Abrupt Mg doping in thin graded base GaAs/GaAlAs heterojunction bipolar transistors

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Abstract
Abrupt Mg doping profiles have been realized in npn GaAs/GaAlAs heterojunction bipolar transistor (HBT) structures with compositional grading in the base region. The layers were grown by metalorganic vapor-phase epitaxy (MOVPE). Acceptor concentrations in the base are 1×1019 cm−3. Mg is also incorporated in the emitter and cap layers due to the Mg memory effect, and in the collector layer due to dopant diffusion. The incorporation of Mg is found to depend on the Si-donor concentration in these layers. Annealing experiments at 840 °C show only little broadening of the Mg doping profile. High-temperature process steps will consequently not change the Mg concentration significantly. We will demonstrate that neither the Mg memory effect nor Mg dopant diffusion limits the use of Mg for MOVPE-grown HBT layer sequences.