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A hybrid infrared detector bonds a photosensitive semiconductor array to a separate silicon readout circuit. Each pixel has its own electrical connection; charge need not be transferred across the array as in a charge-coupled device.

An H2RG detector is a hybrid infrared detector: a photosensitive array is bonded, pixel by pixel through indium contacts, to a separate silicon readout circuit. In infrared use the absorber is normally mercury cadmium telluride, whose composition sets the absorption cutoff. The H2RG readout architecture can also be used with other absorbing materials, so the readout name does not fix the detector’s wavelength response. The manufacturer’s H2RG specification gives a $2048\times 2048$ grid and $18\mu \mathrm{m}$ pixel pitch, together with reference-pixel and guide-window capabilities.

Each illuminated pixel contains a photodiode connected to its own readout node. Absorbed photons generate carriers; their accumulated charge changes the node voltage, with $|\Delta V|=|\Delta Q|/C$ for node capacitance $C$. A reset establishes the baseline; row and column addressing and multiplexed output amplifiers sample the pixel voltage. This is not the serial transfer of charge packets through adjacent pixels used by a charge-coupled device.

Nondestructive detector readout allows repeated measurements during an integration. Correlated double sampling removes the reset baseline, Fowler sampling differences groups of reads at the beginning and end, and up-the-ramp sampling estimates the signal from its slope and can flag cosmic-ray steps or saturation. The “R” denotes detector reference pixels, which monitor electronic offsets; “G” denotes a programmable guide window that can be read rapidly between full-array operations. Reference pixels do not measure sky background. Cooling suppresses dark current and thermal background. Read noise, nonlinearity, persistence and interpixel coupling still require calibration.