The JWST must have great sensitivity to detect the first light in the universe and learn how galaxies first formed. This goal requires that JWST detect sources as faint as magnitude 33 - which implies less than one photon per second at the detector! For this, JWST must employ detectors more sensitive than any flown on previous missions.
This figure shows that the JWST detector dark current goals and requirements are very low to make optimal use of the very low background provided by the JWST observatory. Note how broadband observations will be barely background limited (solid, red line), while spectroscopy will be mostly detector limited (dashed, blue line).
Rockwell Scientific H2RG will be used in NIRCam, NIRSpec, and the Guider system, These are 2k x 2k HgCdTe arrays, sensitive through the 0.6-5 micron wavelength region. The development systems have reached dark currents of less than 0.001 electrons per second and less than 10 electrons rms read noise per pixel in a 1000 sec total exposure. These detectors have also very low latency of less than 0.02%. Ten of these detectors will be used in NIRCam, four in the guider.
MIRI will employ 1k x 1k Si:As detectors being developed by Raytheon Vision Systems in collaboration with the NASA Ames Research Center (PI Craig McCreight). The detectors are sensitive in the 5-28 micron wavelength region. Two detectors will be needed for the medium resolution spectroscopy, one for imaging.
