Infrared Array Detectors

Infrared array detectors are the cryogenically cooled semiconductor arrays that image heat radiation, extending electronic detection beyond the silicon cutoff into the near and mid infrared.

Definition

An infrared array detector is a two-dimensional array of infrared-sensitive photodiodes or photoconductors hybridised to a silicon readout circuit and cooled to cryogenic temperatures, used to image wavelengths beyond the roughly one-micron limit of silicon CCDs.

Scope

This topic covers detector materials such as mercury cadmium telluride and indium antimonide and doped silicon for longer wavelengths, hybrid architectures bonded to silicon readout multiplexers, non-destructive and up-the-ramp sampling, dark current and the need for deep cooling, and the bad-pixel and persistence behaviour characteristic of infrared arrays.

Core questions

• Why can silicon CCDs not detect most infrared light?
• What materials and architectures are used for infrared arrays?
• Why must infrared arrays be cooled so deeply?
• How do readout schemes reduce noise in infrared detectors?

Key theories

Bandgap and material selection

A detector responds to photons energetic enough to bridge its bandgap, so longer infrared wavelengths require narrow-gap materials such as mercury cadmium telluride or doped silicon.

Hybrid detector architecture

The infrared-sensitive layer is bonded pixel by pixel to a separate silicon multiplexer, allowing the photodetector material and the readout electronics to be optimised independently.

Non-destructive readout and sampling up the ramp

Because infrared pixels can be read without erasing their charge, repeated sampling during an exposure lets noise be reduced and cosmic-ray hits identified.

Clinical relevance

Infrared arrays enable imaging and spectroscopy of dust-obscured star formation, cool stars and brown dwarfs, exoplanets, and high-redshift galaxies; they are the heart of instruments on facilities such as the James Webb Space Telescope.

History

Single infrared detectors gave way to small arrays in the 1980s as hybrid technology matured, and formats grew rapidly through the 1990s and 2000s. Mercury cadmium telluride and indium antimonide arrays now reach millions of pixels and dominate ground-based and space infrared instrumentation.

Key figures

• Frank Low
• Craig McCreight

Seminal works

• rieke2003
• mclean2008

Frequently asked questions

Why can a normal CCD not be used for infrared astronomy?

A silicon CCD only detects photons energetic enough to cross silicon's bandgap, which corresponds to wavelengths shorter than about 1.1 microns. Longer infrared photons pass through unabsorbed, so infrared work needs detectors made of narrower-bandgap materials.

Why are infrared arrays cooled far more than optical CCDs?

Narrow-bandgap infrared materials generate large dark currents at modest temperatures because even small thermal energy can free charge carriers. Cooling to tens of kelvin or lower suppresses this dark current so the array can detect faint astronomical infrared signals.