This paper presents a continuous frequency switching zig-zag voltage-controlled oscillator (VCO) to overcome the frequency discontinuity problem during the band switching process in multi-band wideband VCOs. Complementary PMOS and NMOS varactors are explored to realize opposite VCO tuning gains with high linearity. The VCO reduces the nonlinearity of the transmitted chirp in frequency-modulated continuous-wave (FMCW) radars, improving the range resolution. Implemented in a 40-nm CMOS technology, the proposed zig-zag multi-band VCO shows a maximum peak frequency error of 10 MHz over 23.4-27.6 GHz frequency range, a -106.6 dBc/Hz phase noise at 1 MHz offset, and a -183.1 dBc/Hz FoM while consuming 12.69 mW power.