III-V antimonides (InSb, GaSb, AlSb) are narrow-gap zinc-blende semiconductors used as infrared photodetector and laser materials. Their bandgap narrows as temperature rises, following the empirical Varshni relation:
Eg(T) = Eg(0) - α·T² / (T + β)
InSb: Eg(0)=0.235 eV α=6.0e-4 eV/K β=500 K
GaSb: Eg(0)=0.813 eV α=3.78e-4 eV/K β=94 K
AlSb: Eg(0)=1.696 eV α=4.2e-4 eV/K β=140 K
A photon of wavelength λ carries energy Eph = 1.2398 / λ(µm) eV. It is absorbed — promoting an electron across the gap — only if Eph ≥ Eg(T). That threshold defines the detector's cutoff wavelength λc = 1.2398 / Eg(T) eV: any photon longer than λc carries too little energy to be detected.
Dark current ∝ T^1.5 · exp(-Eg(T) / (2·k·T))
Even with no light, thermally-agitated electrons occasionally jump the gap on their own, producing unwanted dark current that competes with the real photosignal — this is why real InSb detectors are cooled (often to 77 K) to shrink that thermal term, even though cooling also shrinks the bandgap slightly further and pushes the cutoff to longer wavelengths.
- Material buttons — swap between InSb (narrowest gap, mid-wave IR, highest electron mobility), GaSb (near/mid-IR lasers) and AlSb (widest gap, used as a barrier layer).
- Temperature slider — narrows or widens the gap via Varshni's equation; watch the band separation and the dark-current bar respond exponentially.
- Photon λ slider — falling photons in the 2D band diagram are absorbed (teal flash, electron jumps the gap) when their energy clears Eg(T), or fall straight through when it doesn't.
- Photon flux slider — controls how many photons per second arrive, scaling the photocurrent readout.
- Drag / scroll — pan the band diagram and zoom in on the gap region.