8.28 Sspd_chapter 6_part 9_caliberating athena for typical bipolar

SSPD_Chapter 6_Part 9_Caliberating ATHENA for Bipolar Process.

7.9. Calibrating ATHENA for a Typical Bipolar Process Flow.

As with MOS calibration text, we assume you are familiar with the mechanics of making an input file and using the correct methods and models (see Section 2.4:“Choosing Models In SSUPREM4”). For example, incorrect selection of diffusion models defined in the METHOD statement would invalidate the remainder of the following section.

Calibrating a bipolar process flow entails matching the two parameters, base current and collector current versus base emitter voltage to measure results throughout the full operating range of the device. By implication, the current gain of the device (Ic/Ib) will also be matched. All of the following paragraphs refer to the standard plot of collector and base currents measured against the base-emitter voltage, Vbe, unless it’s specifically stated otherwise. This standard I-V graph is usually referred to as the Gummel Plot.

Another way of plotting the same information in a different format that can prove useful is a plot of current gain, hfe, versus the log of the collector current. This graph, however, is a derivation of the same information that makes it less clear as to which current is increasing or decreasing for each change. Therefore, a less useful graph when it comes to understanding exactly what is happening to the collector and base currents.

The full operating range of a bipolar junction transistor (BJT) consists of three general regions defined by the current density injected into the base. These three operating regions are usually described as low, medium, and high current injection regimes. The medium injection region is the most important part of the curve to model correctly as this represents the typical operating condition of the BJT. Each of the three operating regions is dominated by a different physical phenomenon. Therefore, successful modeling of a BJT involves matching both the base and collector currents in each of the three general operating regions, making a total of six areas for calibration. The derived parameter, hfe, is also a good parameter to monitor, since this is sensitive to errors in the ratio of collector to base current.

The following text suggests an approach and describes which of the six regions are effected by each change. The general technique is to calibrate the parameters that have the greatest effect on device performance in all regions first and then to move on to more subtle phenomenon that effect certain parts of the base or collector currents or both. In general, matching the collector current for all injection regions is less problematic than matching the base current at the extremes of the injection regions. Consequently, there are more sections on tailoring these parts of the curve. The text is divided into the following sections: