Semiconductor Basics — the foundational principles of how semiconductor materials and devices work, forming the basis of all modern electronics.

What Is a Semiconductor?

Semiconductors are materials with electrical conductivity between conductors (metals) and insulators (glass). Silicon (Si) is the dominant semiconductor material because it is abundant, forms a stable oxide (SiO2), and its conductivity can be precisely controlled through doping. Other semiconductors include germanium (Ge), gallium arsenide (GaAs), silicon carbide (SiC), and gallium nitride (GaN) for specialized applications.

Band Theory

• Valence Band: The highest energy band fully occupied by electrons at absolute zero.
• Conduction Band: The next higher energy band where electrons can move freely and conduct current.
• Band Gap: The energy difference between valence and conduction bands. For silicon, $E_g = 1.12$ eV at room temperature. Insulators have large band gaps (>4 eV), metals have overlapping bands, and semiconductors sit in between.
• Doping: Introducing impurity atoms to control conductivity. N-type doping (phosphorus, arsenic) adds extra electrons as majority carriers. P-type doping (boron) creates holes as majority carriers.

The PN Junction

When P-type and N-type materials meet, a depletion region forms at the junction — a zone depleted of free carriers that creates a built-in potential barrier (~0.7V for silicon). This is the fundamental building block of all semiconductor devices:

• Forward Bias: External voltage reduces the barrier, current flows freely.
• Reverse Bias: External voltage increases the barrier, only tiny leakage current flows.
• Diode Behavior: Current flows easily in one direction but is blocked in the other.

The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

The MOSFET is the fundamental building block of modern digital circuits. It has four terminals: Gate, Source, Drain, and Body (substrate).

• Gate Voltage controls whether current flows between Source and Drain by creating (or not) a conductive channel.
• Threshold Voltage ($V_{th}$): The minimum gate voltage needed to turn the transistor ON.
• NMOS: Conducts when gate voltage is HIGH (electron channel).
• PMOS: Conducts when gate voltage is LOW (hole channel).
• CMOS: Complementary pairing of NMOS and PMOS — the foundation of all modern logic circuits.

Key Semiconductor Concepts

• Wafer: A thin slice of crystalline silicon (~300mm diameter) on which chips are fabricated. One wafer yields hundreds to thousands of individual dies.
• Die (Chip): A single integrated circuit cut from the wafer after fabrication.
• Transistor Scaling: Moore's Law observation that transistor density doubles roughly every two years. Modern nodes (3nm, 2nm) pack billions of transistors per chip.
• Photolithography: Using light to pattern circuit features onto the wafer. EUV (Extreme Ultraviolet) lithography at 13.5nm wavelength enables sub-7nm features.
• Yield: The percentage of functional dies per wafer. Yield is a critical economic metric — even small improvements translate to millions in revenue.

Fabrication Overview

1. Wafer Preparation: Grow single-crystal silicon ingots (Czochralski process), slice into wafers, polish to atomic smoothness. 2. Oxidation: Grow thin SiO2 layers for insulation and gate oxides. 3. Deposition: Add thin films of materials (metals, dielectrics) using CVD, PVD, or ALD. 4. Lithography: Pattern features using photoresist and light exposure. 5. Etching: Remove material selectively using plasma (dry) or chemical (wet) etching. 6. Ion Implantation: Precisely introduce dopant atoms to control electrical properties. 7. Metallization: Create metal interconnect layers (copper) that wire transistors together. 8. Packaging: Encapsulate the die, connect it to external pins, and mount on substrate.

Semiconductor Basics provide the essential foundation for understanding chip design, fabrication processes, and the physics that enables modern computing — from smartphones to data center GPUs.