What are Pinch resistors?

In the early 70s, making large resistors was expensive in silicon area. The diffusion used to make the bases of NPN transistors had the highest resistance per silicon area. When making NPN transistors, the emitter is diffused over the top of a base region. The resistance in this sub-surface base region which is "pinched" between an N doped epi collector below it and a N doped emitter region above it can have 10 times the normal base resistance. It was often used in non critical applications to save on silicon die size.