Optimization of polysilicon emitters for BiCMOS transistor design

Practical aspects of designing polysilicon emitters for large-scale integrated circuit manufacture are presented. It is shown that by choosing the emitter dose such that the minimum base saturation current density is observed, the highest product of common emitter gain and Early voltage is obtained. In practice, this allows high values of both parameters to be chosen, giving an increased bipolar output impedance. This reduces the need for elaborate Early voltage compensation during BiCMOS circuit design. In addition, the increase in the integrated base charge gives a reduction in the punched base resistance and hence the bipolar noise figure. The increase in the total base charge permits the use of a deeper, wider base region. Therefore, maximizing the product of common emitter gain and Early voltage gives transistors with optimized performance and improved reproducibility