In Chapter 7, the design of the individual regions and parameters of a bipolar transistor was discussed. It was noted that, during device operation, an individual device region is not isolated from and independent of the other device regions. Optimization of one device parameter often adversely affects the other device parameters. Thus, optimization of the design of a bipolar transistor is a tradeoff process . This design tradeoff should be done at the circuit and/or chip level, for the optimum design of a transistor is a function of its application and environment. In this chapter, we will first discuss some figures of merit for evaluating a bipolar transistor for typical digital and analog circuit applications, and then discuss the tradeoffs in the design of a bipolar transistor for these applications.

When we consider the performance of a circuit, the wires connecting the transistors and elements that make up the circuit and connecting the output of the circuit to the input of another circuit must be included. The resistance and capacitance as well as the signal propagation delays associated with the interconnect wires have been discussed in Section 5.2.4 in connection with CMOS circuits. The reader is referred to that subsection for details. In this chapter, the wire capacitance which acts as a load on a bipolar circuit is included when we consider the performance and optimization of bipolar transistors and circuits.

In practice, the choice of a particular device design point is often dictated by many nontechnical factors. These...