How it works
In a direct bandgap semiconductor (GaAs, InP, GaN), electrons at the conduction band minimum and holes at the valence band maximum have the same crystal momentum — radiative recombination requires only a photon, with no phonon involved. Silicon and germanium have indirect bandgaps; recombination requires a phonon to conserve momentum, making radiation probability ~10⁶× lower. A PIN photodetector reverse-biased at −5 V has a wide intrinsic (I) region that provides a large depletion volume for photon absorption and fast carrier sweep-out; BPW34 silicon PIN photodiode responds from 430–1100 nm. Solar cell open-circuit voltage Voc = (nkT/q)·ln(IL/I0+1); short-circuit current ISC scales with illumination intensity. Laser diodes achieve lasing above threshold current density by stimulated emission in a Fabry-Perot cavity formed by cleaved facets.
Key points to remember
External quantum efficiency of a high-quality InGaN blue LED is above 80% in 2024-era devices — this was unthinkable before the work that earned the 2014 Physics Nobel Prize. Photodiode responsivity R = Iph/P_opt (A/W); for a silicon photodiode at 850 nm, R ≈ 0.6 A/W. Solar cell fill factor FF = Pmax/(Voc·ISC), typically 0.7–0.85 for silicon cells. Threshold current density for a Fabry-Perot InGaAsP laser diode at 1310 nm is typically 1–2 kA/cm². The avalanche photodiode (APD, e.g., Hamamatsu S2384) provides internal gain through impact ionisation in a high reverse bias (typically 150–200 V), enabling detection of very weak signals at the cost of added noise (excess noise factor F(M) = kM + (1−k)(2−1/M)).
Exam tip
The examiner always asks you to explain why silicon is not used for LED fabrication — state that silicon has an indirect bandgap, so radiative recombination probability is very low because momentum conservation requires phonon participation, making quantum efficiency too poor for practical light emission.