8.28 Sspd_chapter 6_part 9_caliberating athena for typical bipolar  (Page 2/4)

1. Tuning Base and Collector Currents – All Regions

2. Tuning the Base Current – All Regions

3. Tuning the Collector Current – All Regions

4. The Base Current Profile – Medium Injection

5. The Base Current Profile – Low Injection

6. Conclusions

If you follow this order, there should be a reasonable correlation between measured and simulated data. Most of the tuning parameters, however, have some degree of interdependency to the extent of which is also device design specific. Therefore, some degree of iteration of the tuning parameters is to be expected.

When tuning bipolar transistors, there is a greater emphasis to accessing tuning parameters by using the device simulator, ATLAS, compared to optimizing MOSFETs, where most tuning parameters are process-related. A powerful combination is the tuning of a BiCMOS process where you can use the MOSFET part of the process flow to tune the process parameters, while using the Bipolar part of the flow to tune ATLAS. This technique should yield a high degree of predictability in the results.

Tuning the process simulator parameters in ATHENA are mainly required to model effects, such as the implantation induced defect enhanced diffusion responsible for the Emitter Push Effect, which is essential to obtain the correct depth of the base-collector junction. The correct process modeling of the out diffusion of dopant from the poly-emitter into the mono-crystalline substrate is also critical to obtaining well-matched I-V curves. Another critical process modeling area is the base implant, because it is essential to match measured and modeled base resistance for correct modeling of the collector current. These and other issues are discussed in these sections.

7.9.1: Tuning Base and Collector Currents – All Regions

The most important parameter to model the general level of base and collector currents is the device measurement temperature. The base and collector currents are strongly influenced by temperature changes, as small as a few degrees centigrade. A significant effort should be made to determine the exact temperature of the device during measurements before calibration is attempted. This temperature should be input into ATLAS in the MODELS statement using the TEMPERATURE=<n>parameter. An increase in temperature will cause an increase in base and collector currents.

7.9.2: Tuning the Base Current – All Regions

A critical region for poly-emitter bipolar devices is the interface between the poly-emitter and the mono-crystalline silicon. This region is difficult to process simulate directly as the interface between the polysilicon emitter and single crystalline silicon usually consists of a thin, uneven and possibly non-continuous film of oxide. This is simulated by calibrating the overall effect of this interface with ATLAS. The tuning parameter is the surface recombination velocity at this interface for electrons (VSURFN for PNP devices) or holes (VSURFP for NPN devices). This will only be effective for thin emitters where at least a fraction of the holes (for NPN devices) can reach the emitter before recombination.