In my design of Bandgap circuit I used at first ideal resistors, now I want to use the one from the sky130 pdk, could anyone 1-give a hint which type of resistor to choose and what is the criteria of choosing 2-say I have 10k ohm resistor what is the relation to find w and l of resistor in pdk to match the same value, thanks.

Poly resistors are generally the best to use. If you use Magic's device generator, you can plug in W and L values and see what the total resistance is; it calculates everything including the additional resistance of the contacts at the end. You can find the ohms/square resistance of each type in the readthedocs.build PDK documentation. Bandgaps will generally have very large resistances and so you'll probably want the ultra-high res 2k/ohms value. Since the resistance is high, you'll probably want to use the narrowest W available (0.35um). If the device mismatch is too large for your bandgap spec, then you'd need to increase the resistor W until it meets spec.

so @Eslam Morsie, according to @Tim Edwards suggestions you can try xschem's

sky130_fd_pr/res_xhigh_po_0p35.sym

poly resistor. (R ~ 2.6kΩ/[ ], w=0.35u) or the

sky130_fd_pr/res_xhigh_po.sym

poly resistor (R ~ 2.0kΩ/[ ], w=1u ). Bigger W needs far more layout space but has better matching characteristics.

@Stefan Schippers: Shouldn't both resistors be 2 kOhms/square? Or is the effective sheet resistance really that high at 0.35um?

@Tim Edwards @Stefan Schippers Thanks, I will try it.

@Eslam Morsie: I recommend the MiM capacitors; they're high value per unit area, linear, and you can put transistors and other components underneath them.

how to get the W and L if I use cap-mim m3-1 in sky130 ? @Tim Edwards

@Eslam Morsie: The MiM cap is 1pF/um^2. Is that what you're asking?

yes. In xschem there are two caps m3_1 , m3_2 what is the difference between them ?

They are on different metal layers, and they can be stacked on top of each other, so if you use both, you can get 2pF/um^2 total.

OK many thanks. @Tim Edwards

@Eslam Morsie: A single cap has a maximum limit of 30um x 30um. Beyond that, you can array as many of them together as you need to make up the total.

OK thank you very much. @Tim Edwards

@Eslam Morsie First, try not to want a 1nF capacitor - that'll be big! Sometimes you still do want one though. If your capacitor does not need to have a constant capacitance vs. bias voltage, then you can use gate oxide to make a capacitor, which might give better density (and you can put mimcaps on top too). You can use an ordinary MOSFET with S,D connected to one terminal and G the other one, though it needs to be biased enough for the channel to be on, so a zero-Vt device might be convenient. Otherwise there are usually devices specifically intended as capacitors using gate oxide, maybe like an nmos in an nwell. Be very careful with grounded n-wells or any grounded n-diffusions as they are latchup hazards, and need to be surrounded by a ring of grounded substrate contacts, and kept away from things that inject current into the substrate (like IO cells that get abused by end users of the chip).

@Chris Jones many thanks I will try the gate oxide.

@Tim Edwards the nominal square sheet resistance should be 2KOhm, but it looks to me that a 0.35/0.35 resistor has bigger resistance than 1/1 square. This matches with my working experience with poly sheet resistances on different processes. This is probably due to edge effects.