Experimental Research On Reliability Of Si/SiGe/Si Heterojunction Bipolar Transistors (HBTs) Under Thermal And Electrical Stress

The rapid progress of technology for SiGe Heterojunction Bipolar Transistor(HBT) based on Si technology has been made in recent decades. The performance ofSiGe HBT is superior to that of traditional Si bipolar junction transistor (BJT), andoutperforms AlGaAs/GaAs HBT,GaAs MESFET in some aspects. Therefore, SiGeHBT has a wide range of applications. The precondition for device to be used widelyis its reliability. Currently the researches mostly focus on the improvement of deviceparameters, while there is little information on the reliability. The failure mechanismsand failure phenomena remain unclear. In this paper, the reliability of SiGe HBTs isinvestigated under thermal and electrical stress. The main work is as follows. First, three reverse-bias electrical stress methods are adopted. The first method isopen-circuit collector and reverse-bias emitter-base junction (OC stress method ),thesecond is forward-bias collector-base junction and reverse-bias emitter-base junction(FC stress method),the third is short-circuit collector-base junction and reverse-biasemitter-base junction (SC stress method). Experiments show that the degradation ofthe devices depends on the magnitude, energy and the type of injected carriers and onthe recombination enhanced impurity diffusion. Secondly, the thermal stress and the dual-stress of thermal and OC electrical areperformed respectively also. Under thermal stress, as the increase of time,the currentgain decreases, the turn on voltage of base-collector junction (BC junction) increases,but the base-emitter junction (BE junction) I-V characteristics change little. Thesemay be related to base impurities out-diffusion at BC junction. Underthermal-electrical stress, as the increase of time, the decrease of the current gain isfaster than that under only thermal stress, which is resulted from the increase of thebase current. This may be related to the decrease of minority carrier diffusion lengthcaused by the increase of defects in the base region. Lastly, the temperature characteristics of SiGe HBT are also studied. Thevariations of the device performance parameters between 23oC～260oC are given. Thetemperature coefficient of the BE junction voltage of Si/Si0.85Ge0.15/Si HBT is about-1.5mV/K, which is smaller than -2mV/K of Si BJT's. According to it, we haveobtained that the Si0.85Ge0.15 band gap width at 0K is Eg-SiGe＝1.09 eV, and we havealso proved that the SiGe HBT is suitable for microwave power applications.