CURRENT STATUS OF RELIABILITY OF INGAP GAAS HBTS/

We review the current status of two major reliability issues in GaAs-b ased heterojunction bipolar transistors (HBTs), particularly InGaP/GaA s HBTs: degradation in current gain (beta) and variation of turn-on vo ltage (V-bc) In the case of AlGaAs/GaAs HBTs, the beta gradually decre ased, then drastically degraded. After degradation, the device exhibit s an increase in base current Ibr which has an ideality factor n simil ar 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 de gradation mode is similar. The estimated E-a and time to failure for I nGaP/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 revi ew previously proposed degradation mechanisms for GaAs-based HBTs; hyd rogen reactivation, microtwin-like defect formation, dark defect forma tion and carbon precipitation. TEM observation of a degraded InGaP/GaA s HBT indicated that there are at least two possible degradation mecha nisms: formation of carbon precipitates in the base region and migrati on of metallic impurities from the base electrode to the base region. The second issue is concerned with the exponential increase in V-be wi th operating time. The mechanism for the increase in V-be was clarifie d based on reactivation of passivated carbon accepters in the base reg ion during operation. If the device suffers from H+ isolation, V-be de creases rapidly at the initial stage, then exponentially increases. Th e first stage of V-be variation can be explained by fact that a high d ensity of hydrogen atoms migrating from the region to the intrinsic ba se 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.