• Tags: LED, photodiode, solar-cell, photovoltaic
• Difficulty: Moderate

LED (Light Emitting Diode): forward-biased p-n junction where electron-hole recombination releases energy as photons instead of heat. Photon wavelength lambda = $\frac{hc}{E_g}$, where $E_g$ is the band gap. GaAs: $E_g$ = 1.4 eV (IR, lambda = 886 nm). GaAsP: $E_g$ = 1.9 eV (red, 654 nm). GaN: $E_g$ = 3.4 eV (blue-UV, 365 nm). LEDs are forward-biased at 1.5-3 V; they are more efficient than incandescent bulbs.

Photodiode: reverse-biased p-n junction. Incident photons with E > $E_g$ create electron-hole pairs in/near the depletion region. The built-in field sweeps them, generating photocurrent proportional to light intensity. Operates in the third quadrant of I-V characteristic. Response time is very fast (~ns).

Solar cell: unbiased large-area photodiode. Photovoltaic effect: light generates a voltage across the junction. Open-circuit voltage ~0.5-0.6 V per Si cell. Short-circuit current proportional to intensity. Power = V*I; maximum at the "knee" of the I-V curve. Efficiency ~15-25% for commercial Si cells.