Re: Different Channel lengths
Generally because that is the only processes and device models the foundries offer. It take a considerable amount of time and money to do parametric extraction to set up the device models for each generation. So offering too many does not make sense.
You probably know of 180nm 130nm and 90nm. What you probably have not heard of are well established processes at 150nm, 110nm, 100nm and 80nm that IDMs have internally. 150nm was a squeeze on the 180nm using the same lithography equipment that was not capable of resolving 130nm, so a way of shrinking die size at no added capital costs to the fab. 90nm required a new type of UV lithography wavelength and was very expensive. So 130nm fabs developed squeeze technologies at 120, 110 and 100nm but you never get to hear about them.