An 18-Gb/s fully integrated optoelectronic integrated circuit for short-distance communications is realized in the TSMC 65-nm CMOS process. The system consists of a CMOS on-chip photodetector, an inverter-based cascode transimpedance amplifier, a DC offset cancellation buffer, a main amplifier, a three-stage tunable continuous-time linear equalizer, a two-stage modified limiting amplifier, a DC offset cancellation network, an adaptive equalization loop, a low dropout regulator, and a 50-ω termination output buffer. The CMOS P-Well/Deep N-Well on-chip photodetector improves bandwidth and responsivity without technology modification. Moreover, the adaptive cascaded equalization further compensates for the limited bandwidth of the on-chip photodetector with a 5-10-dB/dec roll-up frequency response. The electrical measurement results show a transimpedance gain of 102 dBω and a bandwidth of 12.5 GHz. Furthermore, the optical measurement results demonstrate a fully integrated solution with (1) standard mode: data traffic of 9 Gb/s for 2-1 PRBS with 10 BER, -4.2-dBm optical input sensitivity, and 5.33-pJ/b efficiency; (2) avalanche mode: data traffic of 18 Gb/s for 2-1 PRBS with 10 BER, -4.9-dBm optical input sensitivity, and 2.7-pJ/b efficiency. The chip occupies a core area of 0.23 mm and dissipates 48 mW from a 1/1.2-V voltage supply.