8.28 Sspd_chapter 6_part 9_caliberating athena for typical bipolar  (Page 3/4)

The surface recombination velocity parameter not only affects the base current, it also affects the base current in all of the operating regions. Therefore, it is a powerful parameter to approximately match the base current and gain throughout the full operating range. In some cases, the base current may be less affected in the very high and very low injection regions by changes in the surface recombination velocity, and adding some scope to fine tuning the profile of the base current versus base-emitter voltage curve.

It is important to define the poly-emitter as an electrode so it can define the interfacial surface recombination velocity, VSURFN and VSURFP, using the CONTACT statement. This is in contrast to the MOSFET calibration text where we strongly advise you not to define the polygate as an electrode. Be sure not to get these two confused. The parameter that activates the recombination velocity is SURF.REC, which is also in the CONTACT statement. For example, an NPN BJT statement would be:

CONTACT NAME=emitter N.POLYSILICON SURF.REC VSURFP=1.5e5

A lower value of recombination velocity, VSURFP, will reduce the base current and increase the gain, hfe. The reverse is also true.

7.9.3: Tuning the Collector Current – All Regions

Figure 7.43 shows the parameter that affects the collector current over the entire range is the intrinsic base resistance. The base resistance is primarily determined by the dose of the base implant(s). An increase in the base implant dose will decrease the intrinsic base resistance and decrease the collector current in all injection regions. In some cases, however, the collector current may be affected a little in the very high injection region, giving scope for fine tuning the profile of collector current versus base- emitter voltage.

Figure 7.43: Effect of base doping profile on low injection base current in BJT

If the pinched or intrinsic base sheet resistance is a measured parameter, the simplest way to match measured and simulated data is to make slight changes to the base implant dose so that the simulated dose is not outside the expected error in actual implanted dose in conjunction with the error in percentage activation.

In some designs, where the base contact is close to the collector contact or the base contact is the substrate or is generally wide, the collector current can also influence all current injection regions by specifying a surface recombination velocity at the base contact. For a typical design with a buried n+ collector and surface contacts, the surface recombination velocity at the base contact may have little affect on the collector current.

7.9.4: The Base Current Profile – Medium Injection

In ATLAS, there are two major parameters that have a significant affect on the base current in the medium injection regime. These parameters are the Poly-emitter Work Function and the Bandgap Narrowing Effect. These parameters are described below.

Poly-emitter work function

If the poly-emitter is described as N.POLYSILICON in the CONTACT statement for an NPN device, as already described, the Poly-emitter Work Function is then set to 4.17 V and is correct for saturation doped n++ polysilicon. But if the poly-emitter is not saturation-doped, the work function will differ from this ideal and have a pronounced affect on the base current and current gain in the medium injection regime as shown in Figure 7.44. The work function of the poly-gate can vary from 4.17 V for n++ poly-silicon to (4.17 V + Eg) for p++ polysilicon, depending on the position of the Fermi-Energy. Changing the work function of the poly-emitter by just 0.1 V from 4.17 V to 4.27 V can often reduce the current gain in half in the medium injection regime, so it’s very important to assign the correct value. The CONTACT statement below assigns a work function of 4.27 eV to the poly-emitter, while keeping the other parameters the same as before.