A monolithic constant excess bias control circuit for single-photon avalanche diode (SPAD) in standard 0.18-μ m CMOS technology is reported. The proposed real-time breakdown monitoring technique is incorporated into a passive-quenching-active-reset circuit. A sample-and-hold circuit samples the breakdown level of SPAD through well-defined sampling phase. Following the sample-and-hold circuit, the level shifter with voltage shifting defined by predetermined excess bias provides the reset voltage of SPAD. The design operates the SPAD under constant excess bias and effectively mitigates the impact of the process-voltage-temperature variation by maintaining the optimal excess bias condition.