im Forschungsverbund Berlin e.V.

Published in:
Göttingen: Cuvillier Verlag, 2002.
Zugl.: Dissertation Universität Karlsruhe, 2002.
ISBN 978-3-89873-553-7

Abstract:
GaAs-based Heterojunction Bipolar Transistors (HBTs) have the potential for becoming key components in present and future microwave applications. HBTs combine good high power and high frequency characteristics, making them suitable for rf power amplifier or oscillator circuits.
This thesis deals with the development of optimized growth procedures for GaAs-based HBTs in Metalorganic Vapor Phase Epitaxy (MOVPE), which is one of the most sophisticated production methods for heterostructure compound semiconductors. The result of the optimization carried out is demonstrated by characterization of processed GaInP/GaAs-HBT devices with regard to dc and rf performance as well as device lifetime. Correlation of fundamental material properties depending on MOVPE growth conditions and resulting HBT device performance are the foundation of the presented results.
Device theory shows that recombination processes in the highly doped base layer play the major role for the maximum achievable current gain. Bulk properties of the emitter material or emitter-base interface characteristics mainly influence the turn-on voltage, as long as the band gap of the emitter material remains unchanged. Despite the very low emitter-base doping ratio the emitter injection efficiency is close to unity as a result of the large valence bandoffset in the GaInP/GaAs material system. Numerical device simulation is used for predicting basic device behaviour showing that the highly doped base layer sets the most stringend demands on material properties.
The presented growth results for emitter, collector and contact layers meet device requirements regarding n-type doping, surface morphology and homogeneity of thickness and material composition (4 inch). The HBT base layer requires both a very high and stable p-type doping, favouring carbon as an acceptor due to its low dopant diffusivity and high solubility in GaAs. Carbon doping up to 6×10¹⁹ cm^-3 is achieved either via trimethylgallium (TMG) and the group V precursor only (intrinsic doping) or using an extrinsic carbon source (carbontetrabromide, CBr4). Structural, morphological and electrical layer properties are used to qualify the base growth conditions. Despite comparable structural and majority carrier characteristics differences in minority carrier properties depending on base doping method are observed. These results correlate with the measured dc current gains of processed large area devices. Base doping with TMG and arsine only results in HBT current gains which are among the best published to date. Improvement of the intrinsic base doping homogeneity turned out to be one major optimization task. Adjustment of growth conditions resulted in a very uniform base sheet resistance within a 5% range over 4 inch. Important factors for reproducibility are identified and specific characterization tools have been developed.
The incorporation of hydrogen in the MOVPE growth process results in passivation of carbon acceptors. High-temperature annealing steps turn out to be unsuitable to remove hydrogen from the base due to the limited thermal stability of base layer properties. However, using optimized growth conditions hydrogen indiffusion during after base growth could be significantly reduced. Consequently, the short time current gain degradation owing to the electrical activation of carbon acceptors is below 5%. Longterm degradation behaviour has been investigated at high current densities. Extrapolation of lifetime measurements predicts a time-to-failure of more than 106 hours at usual junction temperatures for optimized devices.
As the result of the growth development GaInP/GaAs-HBT power cell devices with very good output power, microwave gain and efficiency at the targeted frequency are demonstrated.

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