The N+ rectangle near the bottom of the transistor is to reduce resistance because the heavily doped N+ conducts better than lightly doped N. The resistance fro

The N+ rectangle near the bottom of the transistor is to reduce resistance because the heavily doped N+ conducts better than lightly doped N. The resistance from the collector through the N region to the junction could be several kohms. The N+ layer reduces this to a few ohms. Why don't they use N+ instead of N everywhere? Less doping in the collector-base junction improves the breakdown voltage. It's also possible to extend the N+ region under the collector until it meets the buried N+ layer, which reduces resistance even more, but that takes another processing step. So it's all tradeoffs.