• $summary_{type}$: concept
• $word_{count}$: 170

LED: forward-biased junction where recombination energy is emitted as light (not heat). Photon energy ≈ band gap: lambda = $\frac{hc}{E_g}$ = 1240/$E_g$(eV) nm. Material determines color: GaAs (IR, 1.4 eV), GaAsP (red-yellow, 1.8-2.0 eV), GaN (blue, 2.7 eV; UV, 3.4 eV). LEDs operate at 1.5-3 V forward bias, are more efficient than incandescent bulbs, and have lifetimes of 50,000+ hours. Photodiode: reverse-biased junction. Photons create electron-hole pairs in the depletion region; the built-in field sweeps them, adding to reverse current. Photocurrent is linearly proportional to light intensity. Response time ~ns (very fast). Used in optical communication, light meters, barcode scanners. Solar cell: large-area unbiased junction (photovoltaic mode). Light generates voltage (~0.5-0.6 V per Si cell) and current. Cells connected in series increase voltage; in parallel increase current. Fill factor = $\frac{V_mp*I_mp}{(V_oc*I_sc)}$ measures output quality. Si solar cell efficiency: 15-25%.