There is a chip in every AF lens. Apparently the one in your particular model of the Sigma 105 is incompatible. This is a pretty big surprise, because in general when this happens it's pretty subtle and doesn't affect the big picture. In fact, this is a pretty common lens and the fact that it's the first time that it's come up here at Nikonians suggests that it may be a problem with your individual lens rather than the model as a whole. But either way, Sigma has a good reputation for getting this sorted out.
As an example in the past, when the D200 came out, Sigma HSM lenses worked for the most part, but if one had the original D200 firmware AND older HSM lenses, then the AF-ON button sometimes did not engage properly. Sigma offered free rechipping for that, and quite a number of people had that done before Nikon issued a firmware upgrade on the D200 that also addressed the problem. That was the biggest and most visible error that I can remember.
Rechipping requires sending the lens in, and from what I recall it takes a couple of weeks.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!