Ken Shirriff wrote an article on reverse engineering the 555 timer chip, He writes:
This article explains how the LMC555 timer chip works, from the tiny transistors and resistors on the silicon chip, to the functional units such as comparators and current mirrors that make it work. The popular 555 timer integrated circuit is said to be the world’s best-selling integrated circuit with billions sold since it was designed in 1970 by analog IC wizard Hans Camenzind. The LMC555 is a low-power CMOS version of the 555; instead of the bipolar transistors in the classic 555 (which I described earlier), the CMOS chip is built from low-power MOS transistors. The LMC555 chip can be understood by carefully examining the die photo.
Reverse engineering the popular 555 timer chip – [Link]