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Comment by bogomipz | Hacker News Reader

The picture under the heading "The structure of the chip" has two arrows pointing to polysilicon regions. Does anyone know why the polysilicon region that the upper arrows points to is os much larger than the polysilicon regions that the lower arrow points to? They seem like a couple of order of magnitude larger.

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Looking at one of the labeled diagrams [0], it looks like all the large areas of polysilicon are either serpentine structures that are probably resistors, or they're adjacent to one of the power rails. The ones you're referring to seem to be at the top of the instruction register section and right below the Vdd (-9V) line. So we may simply be looking at larger transistors to handle more current and voltage.

[0] https://lh3.googleusercontent.com/-nHUr8PLBS-8/WF4ffgi2aiI/A...

I know slightly less than nothing about this topic, but I remember reading this from a while back:

"Notes for reversers Electron holes have more effective mass, and therefore less mobility. P-type regions are thus made bigger than N-type regions to compensate. The size disparity often differentiates P-type and N-type silicon."

Source: http://siliconzoo.org/tutorial.html

Maybe that is helpful...?

What a great link, thanks! I had not seen this before.

The "extra" polysilicon region is a capacitor made of silicon and polysilicon. These capacitors were a "bootstrap load", essentially a way to drive a transistor harder to get a stronger output. The capacitors you're looking at are for the instruction register to drive the signals through the instruction decoder (basically a PLA). Since the decoder is fairly large, the signals into it need to be boosted. I haven't looked at this for several years, so I may be wrong about some of this.

http://www.intel4004.com/btstrp.htm

Your link has a great pic of the original schematic print out. Might you or anyone else know if there is a high-res version of the printout online anywhere? Cheers.

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