Semiconductor Materials

Introduction

Semiconductor materials are substances with electrical conductivity between that of conductors and insulators. The choice of semiconductor material profoundly impacts device performance, determining parameters like operating speed, temperature range, breakdown voltage, and optical properties. Silicon dominates the industry, but compound semiconductors serve critical roles in specialized applications.

Elemental Semiconductors

Silicon (Si)

Silicon is the most widely used semiconductor, comprising over 95% of all semiconductor devices manufactured worldwide.

Advantages of Silicon:

• Abundant (second most common element in Earth's crust)
• Excellent native oxide (SiO₂) for insulation and passivation
• Mature fabrication technology (60+ years of development)
• Wide operating temperature range (-55°C to 175°C)
• Good mechanical strength

Germanium (Ge)

Germanium was the first semiconductor used commercially (1940s-1960s) before being largely replaced by silicon.

Current applications of Ge:

• High-speed SiGe BiCMOS transistors
• Infrared photodetectors
• Multi-junction solar cells (in combination with GaAs)
• Gamma-ray spectroscopy

Compound Semiconductors

III-V Compounds

II-VI Compounds

IV-IV Compound

Direct vs Indirect Band Gap

Direct band gap: Electron can transition directly by emitting/absorbing a photon. Efficient for LEDs and lasers.

Indirect band gap: Requires a phonon (momentum change) in addition to photon. Inefficient for light emission but fine for electronic devices.

Crystal Growth Methods

Czochralski (CZ) Process

• Produces wafers up to 300mm (12-inch) diameter
• Pull rate: 1-2 mm/min
• Contains oxygen impurities from quartz crucible

Float Zone (FZ) Process

• Higher purity than CZ (no crucible contact)
• Used for power devices and detectors
• Limited to smaller diameters (~200mm)

Semiconductor Purity Requirements

Wide Band-Gap Semiconductors

Silicon Carbide (SiC)

Gallium Nitride (GaN)

Numerical Example

Problem: Calculate the intrinsic resistivity of GaAs at 300K given:

• n_i = 1.8 × 10⁶ /cm³
• µ_n = 8500 cm²/V·s
• µ_p = 400 cm²/V·s

Compare with Silicon.

Solution:

Step 1: Calculate conductivity of GaAs

Step 2: Calculate resistivity

Step 3: Compare with Silicon (ρ_Si ≈ 2.3 × 10⁵ Ω·cm)

GaAs is about 1700 times more resistive than intrinsic silicon due to its larger band gap resulting in far fewer intrinsic carriers.

Interview Questions

1. Why is silicon dioxide (SiO₂) important for silicon technology?

SiO₂ forms a high-quality, stable, thin insulating layer on silicon surfaces. It serves as gate insulator in MOSFETs, passivation layer for protection, masking layer during fabrication, and inter-metal dielectric. No other semiconductor has such a convenient native oxide.

1. What makes GaN and SiC suitable for power electronics?

Both have wide band gaps (3.4 and 3.26 eV), giving 10× higher breakdown fields than silicon, enabling smaller devices at the same voltage. Higher thermal conductivity allows operation at elevated temperatures. Lower on-resistance reduces conduction losses.

1. Explain the difference between direct and indirect band gap materials.

In direct band-gap materials, the conduction band minimum and valence band maximum occur at the same crystal momentum. Electrons can transition directly by emitting a photon. In indirect materials, a phonon is also needed, making optical transitions inefficient.

1. Why has germanium made a comeback in modern ICs?

SiGe alloys provide higher carrier mobility than pure silicon, enabling faster transistors for RF and high-speed applications. Germanium's strain effects improve silicon MOSFET performance. Ge is also excellent for infrared photodetection.

1. What purity levels are needed for semiconductor-grade silicon and why?

Electronic-grade silicon requires 99.999999999% purity (11 nines). Even parts-per-billion impurities significantly affect carrier concentration. Unintentional doping at 10¹² /cm³ would alter intrinsic silicon's electrical behavior measurably.

Summary

The choice of semiconductor material determines device capabilities. Silicon remains dominant due to cost, oxide quality, and mature processing. Compound semiconductors (GaAs, GaN, InP) serve specialized high-speed and optoelectronic applications. Wide band-gap materials (SiC, GaN) are revolutionizing power electronics with higher efficiency and temperature tolerance.