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We utilize medium-resolution JWST/NIRSpec observations of 164 galaxies at$z=2.0-9.3$ from the Cosmic Evolution Early Release Science (CEERS) survey toinvestigate the evolution of the excitation and ionization properties ofgalaxies at high redshifts. Our results represent the first statisticalconstraints on the evolution of the [OIII]/H$\beta$ vs. [NII]/H$\alpha$,[SII]/H$\alpha$, and [OI]/H$\alpha$ ``BPT'' diagrams at $z>2.7$, and the firstanalysis of the O32 vs. R23 diagram at $z>4$ with a large sample. We divide thesample into five redshift bins containing 30-40 galaxies each. The subsamplesat $z\sim2.3$, $z\sim3.3$, and $z\sim4.5$ are representative of themain-sequence star-forming galaxy population at these redshifts, while the$z\sim5.6$ and $z\sim7.5$ samples are likely biased toward high specificstar-formation rate due to selection effects. Using composite spectra, we findthat each subsample at $z=2.0-6.5$ falls on the same excitation sequence in the[NII] and [SII] BPT diagrams and the O32-R23 diagram on average, offset fromthe sequences followed by $z=0$ HII regions in the same diagrams. The directionof these offsets are consistent with high-redshift star-forming galaxiesuniformly having harder ionizing spectra than typical local galaxies at fixednebular metallicity. The similarity of the average line ratios suggests thatthe ionization conditions of the interstellar medium do not strongly evolvebetween $z\sim2$ and $z\sim6$. Overall, the rest-optical line ratios suggestthe $z=2.7-9.3$ CEERS/NIRSpec galaxies at log($M_*/M_{\odot})\sim7.5-10$ havehigh degrees of ionization and moderately low oxygen abundances($\sim0.1-0.3~Z_{\odot}$), but are not extremely metal poor ($<0.1~Z_{\odot}$)even at $z>6.5$.

Excitation and Ionization Properties of Star-forming Galaxies at z=2.0-9.3 with JWST/NIRSpec