Hi guys, I have a beginner question regarding poly resistor, according to the https://skywater-pdk.readthedocs.io/en/main/rules/device-details.html, it is recommended to use poly precision resistors for analog design. But a quick comparison of poly resistor and N+ diffusion sheet resistance seems to suggest, N+ diffusion to be more accurate with tighter LSL and USL. So what's the rationale to use poly precision resistor in analog design?

The poly resistor has a decent low TCR, and is quite linear, with a linear parasitic capacitance to the substrate (since sitting on SiO2). This makes it a prime candidate for analog design, however, required an extra mask, so sometimes not available if designing for cost. N-well resistor is always available (as soon as there is a PMOS), but has a strong nonlinear parasitic cap (a reverse PN junction) to the substrate. However, since it sits in the substrate, thermal dissipation is good, which is the only downside (besides cost) of the poly resistor. BTW, the silicided poly-resistor is always available when there is a PMOS/NMOS (with the exception of newer technologies with metal gates). What is usually meant is the silicide-blocked high-Rsquare (ca. 300Ohm/sq) poly resistor. The suppression of the silicide requires the extra mask, and often a higher Rsquare is also available (ca. 1-2kOhm/sq).