Zener Diode — Voltage Regulation

The Zener diode maintains a constant output voltage regardless of input voltage fluctuations. In a circuit with series resistor R_s:

• Load voltage: V_L = V_Z (constant at breakdown)
• Series resistor drops excess voltage: V_R = V_in − V_Z
• Current through Zener: I_Z = (V_in − V_Z)/R_s − I_L

Real-world applications: Voltage references in power supplies, overvoltage protection, ADC reference voltages.

Photodiode — Light-to-Current Conversion

Operating in reverse bias, the depletion region widens, creating a large sensitive volume. Photons with energy > E_g generate electron-hole pairs. The resulting photocurrent is:

$I_{ph} \propto \text{Illuminance (lux)}$

Response time is very fast (nanoseconds) in reverse bias because the built-in field rapidly sweeps carriers apart.

Real-world applications: Optical fiber receivers, CDs/DVDs, barcode scanners, medical pulse oximeters.

LED — Current-to-Light Conversion

In forward bias, electrons injected from n-side recombine with holes in the p-region, releasing energy as photons:

$\lambda = \frac{hc}{E_g} \approx \frac{1.24 \text{ eV·μm}}{E_g}$

Real-world applications: Traffic lights, smartphone displays (OLEDs), indicator lights, optical communication transmitters.

Solar Cell — Light-to-Voltage Conversion

The photovoltaic effect: photons create e-h pairs; the junction's internal field separates them, driving electrons to n-side and holes to p-side, creating an EMF (~0.5–1 V per cell). Multiple cells in series create practical voltages.

Real-world applications: Rooftop solar panels, calculator batteries, space satellite power.