SiGe Heterojunction Bipolar Transistor (SiGe HBT) Research And Design

Due to the development of modern mobile communications and microwave communications, demands are improving rapidly for microwave semiconductor devices with higher power and lower noise. Traditional Si devices are unable to meet the new requirements of higher performance in high frequency and lower cost. So the SiGe/Si devices have been proposed and investigated [1,2], whose fabrication process is compatible with traditional Si process so as to reduce fabrication costs. It has been shown that the performance of SiGe Heterojunction Bipolar Transistor (SiGe HBT) in high frequency is much better than that of Si Bipolar Junction Transistor (Si BJT), and is better than that of AlGaAs/GaAs Metal Semiconductor Field Effect Transistor (AlGaAs/GaAs MESFET) in some aspects. Thus, the SiGe technology is very promising.The research in this thesis focuses on the design of SiGe devices. The main job includes the design of transistor structure and the optimal design of the base buffer. In the design of the transistor structure, the structure and performance of SiGe HBT are first simulated with ISE TCAD. Then, taking into consideration the actual process [3], the optimization of the design is performed. In theory, with the vertical structure and size unchanged, the performance of device can significantly improve by improving the fabrication process and reducing the lateral size. So, based on the former samples, the layout is re-designed.Another emphasis of this thesis is the design of base buffer. As is known to all, one of the most important characteristics of SiGe HBT is the strained SiGe base growing on the Si substrate. Generally, the base is required to be very thin so as not to cause the base SiGe crystal lattice mismatching in subsequent annealing process. Also, in order not to increase the thermal noise of device, the base is always heavily doped. Thus, it is believed that in subsequent high-temperature process, the base boron will outdiffuse very easily into emitter and collector, which will form a parasitic barrier for the electrons when moving from emitter into base and result in performance degradation of the device. One of the typical solutions to this problem is to set buffers on both sides of the base. This thesis investigates this problem carefully. Different buffers are set according to different fabrication processes. And,by doing so, the optimized performance is obtained. In the last part of this thesis, the available samples are analyzed.And deficiency in the previous designs and problems of previous fabrication process are found by the analysis of the output characteristics of devices. Much reasonable advice is brought forward, with the hope that future researches will benefit from it.