Match each optoelectronic device to the effect that makes it work — note that two devices emit or control light and two detect it.

A → IV, LED: electroluminescence. Under forward bias, electrons and holes are injected across the junction and recombine radiatively, each recombination releasing a photon of energy roughly equal to the bandgap:

\(h\nu \approx E_g \Rightarrow \lambda(\mu m)\approx\dfrac{1.24}{E_g(eV)}\)

Producing light from an electric current is precisely electroluminescence. Direct-bandgap materials such as GaAs, GaAsP and GaN are used because silicon's indirect gap makes radiative recombination very inefficient.

B → III, LCD: dynamic scattering of light. A liquid crystal does not generate light at all; it modulates light that is already there (backlight or ambient). An applied field re-orients the rod-like molecules, changing how they scatter or rotate the polarisation of light passing through, so a pixel appears bright or dark. That is why LCDs consume so little power — they steer light rather than create it.

C → I, LDR: photoresistive effect. A light-dependent resistor is a slab of cadmium sulphide or similar semiconductor whose resistance falls when illuminated, because absorbed photons generate extra electron–hole pairs and raise the conductivity. It has no junction, is non-polar, and is slow (milliseconds), but it is cheap and is used in street-light and camera-exposure controls.

D → II, photodiode: photoconductive effect. A reverse-biased junction operated in photoconductive mode: absorbed photons create carriers in the depletion region, and the reverse current becomes proportional to the incident optical power. Compared with an LDR it is far faster and highly linear. (The same device with zero bias works in photovoltaic mode, which is how a solar cell operates.)

Note the neat contrast being tested: LED converts electricity to light; LCD merely modulates existing light; LDR and photodiode convert light to an electrical quantity, the first as a resistance change without a junction, the second as a current through a junction.

Hence, the correct matching is A-IV, B-III, C-I, D-II.