ABSTRACT
Understanding the properties of the various base current components facilitates the description of the measured device characteristics, such as the β vs. IC curve of Figure 17.9. The previous analysis of the transistor currents implicitly assumes that the transistor operates in the normal bias condition, under which the transistor gain is seen to increase steadily with the collector current. However, IC cannot increase indefinitely without adverse effects. Figure 17.9 reveals that, after a certain critical IC is reached while VBC is kept constant, the current gain plummets, rendering the device useless. For high-power applications, such as a power amplifier for wireless communications, HBTs are biased at a large current (with a current density on the order of 104 A/cm2), not only because the output power is directly proportional to IC, but also because the high frequency performance is superior at large IC (but before the current gain falls). Therefore, it is imperative to understand the factor setting the maximum IC level, below which the current gain is maintained at some finite values.
