Reading an avalanche photodiode’s reverse I–V curve
What a dark reverse-bias sweep can tell you about leakage, breakdown, and the limits of the measurement.
Why sweep in the dark?
An avalanche photodiode (APD) uses a strong electric field to multiply charge carriers. In a dark sweep, the instrument steps the reverse bias and records current while the device is shielded from light. That gives you a baseline for leakage and its dependence on bias. The curve alone cannot give you optical gain. For that, you also need a controlled illumination measurement and a reference condition.
Separate leakage from the knee
Plot the magnitude of current on a logarithmic axis and keep the original signed data. Compare leakage at a stated reverse bias below breakdown. As bias increases, look for the region where current rises much more steeply. A useful breakdown estimate comes from a declared rule, such as a specified current criterion, rather than from where the curve looks steepest on a particular chart.
Make the estimate reproducible
Record the temperature, voltage step, settling time, and current compliance alongside the trace. If the source reaches compliance, flag those points before extracting a breakdown value. If your criterion falls between samples, report how you interpolated. Comparing two devices makes sense only when the measurement conditions and extraction rule are comparable.