Experimental Research On Reliability Of GeSi/Si Heterojunction Bipolar Transistors (HBTs)

With the development of GeSi epitaxy technology, the performance of GeSi HBTs is already superior to Si BJTs, and to AlGaAs/GaAs HBTs and GaAs MESFETs in some aspects, so the GeSi HBTs have much prospect for wide applications. The devices reliability is the precondition for their applications. Currently the researches are more focused on the improvement of device parameters, while little information exists on the reliability. In this thesis, the GeSi HBT's reliability is investigated.First of all, a new GeSi HBT process flow was designed. With this technology flow, the stability, reliability and the performance of GeSi HBTs were improved significantly. In order to study the reliability of the devices efficiently, an automatic program-controlled experiment system and a data processing system were developed. Those systems had the features of quickness, convenience and high precision.In this thesis two reverse-bias electrical stress methods were used to investigate the reliability of the devices. One is a typical of OC stress method with collector open and reverse-bias emitter-base junction. Another one, a new technique, is FC stress method with forward-bias collector-base junction and reverse-bias emitter-base junction. Experimental results indicated that for the two reverse-bias stresses the degradation of the devices depended on magnitude and energy of the injected carriers. FC stress condition may speed up the degradation of devices and shorten the time of evaluating the devices life.In order to evaluate reliability of the devices under thermal stress condition, we took some high temperature storage experiments and combination experiments of temperature and electricity. We found that the device characteristic parameters fluctuated under the thermal stress conditions. In addition, we found that the combination of temperature and electricity stress accelerated the devices degradation more quickly.On the purpose of explaining the devices degradation mechanism, a model for devices degradation was presented. The model successfully explained the experimental phenomena.In the experiment we also observed negative differential resistance characteristics of GeSi HBTs with heavily doped base at high collector-emitter voltage and high current. A new interpretation to this phenomenon was given. Thiscan be attributed to the negative feedback of thermoelectricity.The results of the reliability tests offer valuable feedback for SiGe HBT design in order to improve the long-term reliability of the device and circuits made with them.