Numerical simulation of Silicon-Germanium Heterojunction Bipolar Transistor (HBT) in silvaco/atlas and analysis of HBT base transit time to achieve faster operation

The present analysis for base transit time of a modern high-speed npn bipolar transistor is done for Gaussian-doped base considering doping dependence of mobility, bandgap narrowing effect and carrier velocity saturation effect at base-collector junction using analytical process. This paper analyzes the base transit time of a Silicon-Germanium Heterojunction Bipolar Transistor (SiGe HBT) considering all the above parameters as well as concentration dependent lifetime and field mobility. To Improve operating time from the concept of solid state device physics, effects of changing various characteristics of base of the HBT were studied. Using modeling and numerical simulating software SILVACO/ATLAS, the minority carrier current density and electron density incorporating all the effects are numerically solved. Effects on the device are analyzed by varying various parameters such as base voltage, base width, standard deviation of the Gaussian profile, etc. The simulated devices are found out to be applicable for low level of injection. It was found that a device with base width of 100 ηm , and with a base voltage of 0.7v works very well when considering all effects and limitation providing a transit time as small as 3.5 ps. The simulated devices offer an in-depth insight into device operation and can be considered as useful tools in device design and optimization.