Zero dark current in H2RG detectors: it is all multiplexer glow

We present analysis of James Webb Space Telescope (JWST) near infrared H2RG detectors ul{with a 5$upmu$m cutoff} showing that at temperatures below 60K there is no measurable dark current. Instead, the observed signal in dark exposures is almost entirely due to multiplexer glow that arises as each pixel is selected. We are able to separate the per sample glow from the time dependent dark current by comparing the observed signal in both continuous and sparsely sampled dark exposures. We show that the glow is lower within the regions of the detector that are missing the epoxy back fill (voids). We also show that the magnitude of the glow is affected by the pixel source follower current, the pixel clocking rate, and the number of outputs running in parallel. Our measurement of an insignificant dark current shows that the detector noise is no longer limited by the quality of the HgCdTe layer but instead by the multiplexer and readout electronics.