Debugged a chip and one of its IO was not functioning. We suspected IO pad might be burned. So we used IV curve tracer to get this IO’s IV curve. It came back as the graph shown. Turned out this kind of waveform was expected. The ESD protection window defines the trigger voltage (Vt1) of ESD protection circuit to be smaller than both junction and gate-oxide breakdown voltages of internal circuits (VBD, Internal), so that ESD protection circuit can effectively clamp down the ESD transient voltage. The snapback holding voltage (Vh) also needs to be larger than VDD to avoid latch-up issue. So this curve indicated a good IO pad.
Yes IV curve tracing is a good way to check if io pad is burned out. Using gpio can confirm the issue but not necessarily pinpoint if pad is bad. Some pads may not mapped to gpio as well such as analog pad.