HgCdTe Tunable-Bandgap Infrared Detector

Composition & Temperature

Cd fraction x — Hg_1-xCd_xTe 0.20

Temperature T (K) 80

Incident photon λ (μm) 10.0

Band Structure

Bandgap E_g

—

Cutoff λ_c

—

Detected

0 / 0

Regime

—

Cd cation site Hg cation site Te anion site Photon (absorbed = flash)

How it works

Hg_1-xCd_xTe is a pseudo-binary alloy: Cd substitutes for Hg on the cation sublattice of the zinc-blende lattice, tuned continuously by the mole fraction x. Its bandgap follows the empirical Hansen relation used industrially to design infrared detectors:

Eg(x,T) = -0.302 + 1.93x - 0.810x² + 0.832x³ + 5.35e-4·T·(1-2x)   [eV]
λc = 1.24 / Eg   [μm]  (cutoff wavelength, hc/e ≈ 1.24 eV·μm)

x — Cd content. At x=0 (pure HgTe) Eg is negative: a zero-gap semimetal. Around x≈0.2-0.4 the gap opens into the long/mid-wave infrared range used by real thermal-imaging detectors.
T — raising temperature widens the gap slightly (positive dEg/dT here) and shifts λc to shorter wavelengths; detectors are cooled to reduce thermal noise, not primarily to shift the cutoff.
λ slider / Fire photon — a photon of energy E = 1.24/λ eV is launched at the crystal. If E ≥ Eg the crystal absorbs it (an electron is excited across the gap, the event a real detector would register); if E < Eg the gap is transparent to it and it passes through.
Auto-scan — sweeps λ continuously so you can watch the absorption/transmission boundary track λ_c live.

This composition tunability is why HgCdTe is the dominant material for cooled infrared focal-plane arrays (military, astronomy, thermal imaging): one growth recipe, continuously adjustable cutoff, from short-wave out past 14 μm.

E_g = — eV λ_c = — μm

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