Key Semiconductor Materials

Why This Matters

Understanding semiconductor materials is fundamental to analyzing how electronic and optoelectronic devices function—and why engineers choose specific materials for specific applications. You're being tested on your ability to connect material properties like bandgap energy, carrier mobility, and thermal stability to device performance and real-world applications. The physics here bridges quantum mechanics (band theory) to practical engineering decisions.

Don't just memorize bandgap values and material names. Know why a direct bandgap matters for LEDs, how wide-bandgap materials enable high-power applications, and what trade-offs engineers face when selecting materials. When you can explain the underlying physics connecting material properties to device behavior, you're thinking like the exam wants you to think.

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Elemental Semiconductors: The Foundational Materials

These single-element semiconductors were the first to be developed and remain workhorses of the industry. Their relatively simple crystal structures make them easier to purify and process, though their indirect bandgaps limit some optical applications.

Silicon (Si)

• Bandgap of 1.12 eV (indirect)—this moderate value makes it versatile for both electronics and photovoltaics at room temperature
• Native oxide ($SiO_2$) formation provides an excellent insulating layer, which revolutionized MOSFET fabrication and integrated circuit manufacturing
• Exceptional thermal stability and abundant raw materials make it the cost-effective standard for over 95% of semiconductor devices

Germanium (Ge)

• Higher electron mobility than silicon—electrons move faster through the lattice, enabling better high-speed performance
• Smaller bandgap of 0.66 eV makes it sensitive to infrared radiation but causes higher leakage currents at elevated temperatures
• Historical significance as the first transistor material; now used in fiber optics, infrared detectors, and as a silicon substrate enhancer

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III-V Compound Semiconductors: Direct Bandgap Champions

Compounds formed from Group III and Group V elements offer direct bandgaps, meaning electrons can transition between bands by emitting or absorbing photons efficiently. This makes them essential for light-emitting and light-detecting devices.

Gallium Arsenide (GaAs)

• Direct bandgap of 1.42 eV—electrons recombine radiatively, making it ideal for LEDs, laser diodes, and solar cells
• Electron mobility roughly 6× higher than silicon enables faster switching speeds in high-frequency RF applications
• Higher production cost than silicon limits use to applications where performance justifies expense (telecommunications, space systems)

Indium Phosphide (InP)

• Direct bandgap of 1.34 eV corresponds to wavelengths used in fiber-optic communications (near 1.55 μm window)
• Exceptional high-frequency performance with low noise characteristics makes it the standard for telecommunications amplifiers
• Lattice matching with other III-V compounds allows sophisticated heterostructure device fabrication

Aluminum Gallium Arsenide (AlGaAs)

• Tunable bandgap achieved by varying aluminum content ($Al_xGa_{1-x}As$), ranging from 1.42 to 2.16 eV
• Lattice-matched to GaAs enables high-quality heterojunction interfaces essential for quantum well lasers
• High electron mobility combined with bandgap engineering makes it critical for laser diodes and high-electron-mobility transistors (HEMTs)

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Wide-Bandgap Semiconductors: High Power and High Temperature

Materials with bandgaps exceeding ~2 eV can operate at higher temperatures, voltages, and frequencies than conventional semiconductors. The wider bandgap means more energy is required to excite carriers, reducing thermal generation of unwanted carriers.

Silicon Carbide (SiC)

• Wide bandgap of 3.26 eV enables operation at temperatures exceeding 600°C and breakdown voltages above 10 kV
• Thermal conductivity 3× higher than silicon allows efficient heat dissipation in power electronics
• Dominant in electric vehicle inverters and renewable energy systems where efficiency at high power is critical

Gallium Nitride (GaN)

• Bandgap of 3.4 eV combined with high electron saturation velocity enables efficient high-frequency power amplifiers
• Two-dimensional electron gas (2DEG) formation at heterojunction interfaces provides exceptional mobility for RF applications
• Revolutionizing power conversion with smaller, more efficient chargers and adapters; also essential for blue LEDs

Zinc Oxide (ZnO)

• Wide bandgap of 3.37 eV makes it transparent to visible light while conducting electricity—ideal for transparent electrodes
• Piezoelectric properties enable sensing and actuation applications beyond typical semiconductor uses
• Non-toxic and earth-abundant offers a sustainable alternative to other wide-bandgap materials

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Optoelectronic Specialists: Optimized for Light

These materials are engineered specifically for efficient light emission or absorption, with bandgaps tuned to target wavelengths.

Indium Gallium Nitride (InGaN)

• Continuously tunable bandgap from 0.7 eV (InN) to 3.4 eV (GaN) by adjusting indium content, spanning IR through UV
• Enables blue and green LEDs—the missing colors that, combined with red, made white LED lighting possible (2014 Nobel Prize)
• High internal quantum efficiency despite significant crystal defects, a phenomenon still being researched

Cadmium Telluride (CdTe)

• Direct bandgap of 1.5 eV nearly optimal for single-junction solar cell efficiency (close to Shockley-Queisser ideal)
• High optical absorption coefficient means thin films (~1 μm) capture most sunlight, reducing material costs
• Cadmium toxicity concerns require careful lifecycle management but haven't prevented large-scale photovoltaic deployment

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Quick Reference Table

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Self-Check Questions

1. Which two materials share the property of having tunable bandgaps through composition variation, and what spectral ranges does each cover?

2. Explain why silicon dominates the semiconductor industry despite having lower electron mobility than GaAs and an indirect bandgap.

3. Compare and contrast SiC and GaN for power electronics applications—under what operating conditions would you choose each?

4. A device engineer needs to build a laser operating at fiber-optic communication wavelengths (~1.55 μm). Which material system would you recommend and why?

5. Why do direct-bandgap semiconductors outperform indirect-bandgap materials in LED applications? Identify one material from each category to support your explanation.