Hot-carriers in MOSFETs are responsible for timedependent near-infrared emission, synchronous with the switching transitions in CMOS circuits. Fast electrical waveforms propagating through integrated circuits can be effectively measured by means of high sensitivity solid-state photodetectors with sharp time-resolution. Thanks to a time jitter of less than 30ps, we obtained an equivalent analog bandwidth of about 30GHz. We developed a photoemission model in SPICE in order to simulate the luminescence waveforms. By comparing measured and simulated optical waveforms, in-depth insight of circuit behavior is reported, leading to a powerful identification of defects and failures.