Bipolar device design can be considered in two parts. The first part deals with designing bipolar transistors in general, independent of their intended application. In this case, the goal is to reduce as much as possible, consistent with the start-of-the-art fabrication technology, all the internal resistance and capacitance components of the transistor. The second part deals with designing a bipolar transistor for a specific circuit application. In this case, the optimal device design point depends on the application. The design of a bipolar transistor in general is covered in this chapter, and the optimization of a transistor for a specific application is discussed in Chapter 8.
Design of the Emitter Region
It was shown in Section 6.2 that the emitter parameters affect only the base current, and have no effect on the collector current. In theory, a device designer can vary the emitter design to vary the base current. In practice, this is rarely done, for two reasons. First, for digital-circuit applications, as long as the current gain is not unusually low or the base current unusually high, the performance of a bipolar transistor is rather insensitive to its base current (Ning et al., 1981). For many analog-circuit applications, once the current gain is adequate, the reproducibility of the base current is more important than its magnitude. Therefore, there is really no particular reason to tune the base current of a bipolar device by tuning the emitter design, once a low and reproducible base current is obtained. Second, as can be seen in Appendix 2, the emitter is formed towards the end of the device fabrication process. Any change to the emitter process to tune the base current could affect the doping profile of the other device regions and hence could affect the other device parameters. As a result, once a bipolar technology is ready for manufacturing, its emitter fabrication process is usually fixed. All that a device designer can do to alter the device and circuit characteristics in this bipolar technology is to change the base and the collector designs, which often can be accomplished independently of the emitter process and hence has no effect on the base current.