PH-03 Applications Summary — Semiconductor Devices in Technology — Summary

Zener diode as voltage regulator: Maintains constant output voltage $V_Z$ despite fluctuating input; series resistor absorbs voltage excess; used in all DC power supplies, battery chargers, and reference voltage circuits
Photodiode in optical fiber communication: Reverse-biased photodiode at receiver end converts modulated light pulses back to electrical signals; fast response time (nanoseconds) essential for data rates exceeding gigabits per second
Photodiode in medical devices: Pulse oximeters use paired red and infrared photodiodes to measure blood oxygen saturation ( $SpO_{2}$ ) by comparing light absorption through tissue
LED in displays: LED arrays form pixel displays in traffic signals, digital billboards, and TV backlighting; color determined by semiconductor band gap (blue: GaN, green: GaP, red: GaAsP)
LED in optical communication transmitters: Infrared LEDs (GaAs, λ ≈ 870 nm) modulate high-frequency signals for short-range fiber optic links and TV remote controls
Solar cells in power generation: Crystalline silicon solar cells ( $E_g$ = 1.1 eV) convert sunlight to electricity; multiple cells in series (panel) produce practical voltages; used from calculators to satellite power systems
Bridge rectifier in power supplies: Four-diode bridge configuration in every AC adapter (phone charger, laptop charger) converts 50 Hz AC mains to pulsating 100 Hz DC, which is then filtered by capacitors
NAND gates in computing: Modern CPU chips contain billions of NAND gates; any logic operation (addition, comparison, memory) is ultimately built from NAND gate combinations
NOR gates in programmable logic: Early ROMs (Read-Only Memories) and PALs were implemented using NOR gate arrays; NOR flash memory in USB drives derives its name from the NOR gate structure
Logic gates in digital calculators: The arithmetic unit of every calculator implements addition via XOR gates (sum) and AND gates (carry); these are built from NAND gates at the transistor level
Zener diode overvoltage protection: Transient voltage suppressor diodes (TVS) — Zener variants — protect USB ports, HDMI connectors, and automotive electronics from voltage spikes
Photovoltaic effect historical context: Einstein explained the photoelectric effect (photon energy quantization) in 1905; the photovoltaic effect in solar cells is the solid-state analog — both demonstrate E = hf