Optical & Thermal Properties

Light and Semiconductors

The bandgap determines how a semiconductor interacts with light:

• Absorption: Photons with energy ≥ Eg can excite electrons across the bandgap. Silicon absorbs visible light (which is why silicon wafers are opaque and dark-colored).
• Transparency: Photons with energy < Eg pass through. Silicon is transparent to infrared light with wavelength > 1.1 µm.
• Emission: When electrons fall from conduction to valence band, they can emit photons. Efficient in direct bandgap materials (LEDs), inefficient in silicon.

Applications based on light-semiconductor interactions:

• Solar cells: Silicon absorbs sunlight and generates electron-hole pairs → electricity
• Image sensors: Each pixel is a photodiode that converts light to charge (your phone camera)
• LEDs: GaN and GaAs emit light when carriers recombine across the direct bandgap

Thermal Properties

Heat management is critical in semiconductor devices:

• Thermal conductivity: Silicon conducts heat reasonably well (150 W/m·K) — better than most semiconductors but worse than metals like copper (400 W/m·K). SiC (490 W/m·K) and diamond (2000 W/m·K) are much better.
• Thermal runaway: As temperature rises, leakage current increases, generating more heat, which increases temperature further. This positive feedback can destroy a chip if not managed.
• Self-heating: Transistors generate heat during switching. At billions of transistors switching billions of times per second, total heat dissipation can reach 100–300W in high-performance processors.