Current status of reliability of InGaP/GaAs HBTs

We review the current status of two major reliability issues in GaAs-based heterojunction bipolar transistors (HBTs), particularly InGaP/GaAs HBTs: degradation in current gain (beta) and variation of turn-on voltage (V-bc) In the case of AlGaAs/GaAs HBTs, the beta gradually decreased, then drastically degraded. After degradation, the device exhibits an increase in base current Ibr which has an ideality factor n similar to 2 in the Gummel plot. The activation energy for the degradation was estimated to be 0.6 +/- 0.1 eV. However, in InGaP/GaAs HBTs, much higher reliability than in AlGaAs/GaAs HBTs was achieved though the degradation mode is similar. The estimated E-a and time to failure for InGaP/GaAs HBTs are 2.0 +/- 0.2 eV and 10(6) h at T-j = 200 degrees C, respectively, which are the highest values ever reported. We also review previously proposed degradation mechanisms for GaAs-based HBTs; hydrogen reactivation, microtwin-like defect formation, dark defect formation and carbon precipitation. TEM observation of a degraded InGaP/GaAs HBT indicated that there are at least two possible degradation mechanisms: formation of carbon precipitates in the base region and migration of metallic impurities from the base electrode to the base region. The second issue is concerned with the exponential increase in V-be with operating time. The mechanism for the increase in V-be was clarified based on reactivation of passivated carbon accepters in the base region during operation. If the device suffers from H+ isolation, V-be decreases rapidly at the initial stage, then exponentially increases. The first stage of V-be variation can be explained by fact that a high density of hydrogen atoms migrating from the region to the intrinsic base region, passivate the carbon atoms at the initial stage. From these results, one can expect that the use of He+ as an implant instead of H+ can solve this problem. (C) 1997 Elsevier Science Ltd.