Understanding and Equivalence of Response Time Measurements in Photodetectors

The response time of a photodetector is a key figure of merit that determines its ability to follow rapid variations in optical intensity, impacting applications from high-speed communication and time-of-flight ranging to ultrafast imaging and time-resolved spectroscopy. Yet reported response times often differ widely because commonly used measurement techniques probe fundamentally distinct physical processes. This Viewpoint systematically analyzes three widely used measurement techniques─the square-pulse response, the ultrafast-pulse transient response, and the 3 dB bandwidth method─and clarifies the specific detector dynamics each technique measures. We identify the conditions under which these methods are formally equivalent and the regimes where their results necessarily diverge. In particular, we emphasize that the detector response time is not an intrinsic material constant but rather a quantity that depends on the device operating conditions, including excitation intensity, carrier density, and device operating regime. By establishing a unified framework linking time- and frequency-domain characterizations, this work clarifies long-standing confusion surrounding response-time reporting and provides practical guidance for selecting appropriate measurement strategies across diverse photonic and optoelectronic applications.