GaAs HBT RELIABILITY

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Abstract
This paper reviews recent studies on the reliability of GaAs heterojunction bipolar transistors (HBTs), mainly carbon-doped-base AlGaAs/GaAs HBTs, and discusses degradation modes in these devices. The carbon-doped-base AlGaAs/GaAs HBTs generally have more stable current-voltage characteristics than beryllium-doped-base ones. However, even carbon-doped-base HBTs show a decrease in the current gain due to an anomalous increase in the base current that occurs under high-current stress. The estimated ohmic contacts of these HBTs show that their stability is high enough under actual operating conditions. Bias stress testing has shown that two factors dominate the reliability of carbon-doped-base AlGaAs/GaAs HBTs: the material and the passivation. HBTs with an AlGaAs surface passivation layer on the extrinsic base region exhibit a longer life than those without one. The base current ideality factor of short-lived HBTs increases to larger than 2 during bias stress testing, suggesting that the origin of the excess base current cannot be simply explained by a space-charge recombination process. The reported activation energies are distributed around 0.5 eV, which is almost the same as those for AlGaAs/GaAs laser diodes. Cross-sectional TEM analysis has revealed that current-induced defects are formed in the base layer. The indium-incorporated-base HBTs show a median life of longer than 15,000 hours at a junction temperature of 215°C with a collector current density of 50 kA/cm 2, implying that GaAs HBTs have sufficient reliability for very-high-speed communication systems.

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