Semiconductors

Learning Objectives

By the end of this page, you should be able to:

• Explain why a semiconductor's conductivity sits between a conductor and an insulator, and why that alone doesn't define it
• Distinguish intrinsic from extrinsic semiconductors and explain the role of doping
• Describe how n-type and p-type materials are created and how they differ
• Relate temperature and light to carrier generation in semiconductors
• Connect semiconductor fabrication steps to the transistors and chips built from them

Quick Answer

A semiconductor is a material — most commonly silicon — whose electrical conductivity lies between that of a conductor and an insulator, and, crucially, can be deliberately controlled. In its pure ("intrinsic") form, silicon barely conducts at room temperature because its 1.12 eV band gap keeps most electrons locked in the valence band. Add tiny, controlled amounts of impurity atoms — a process called doping — and you create n-type material (extra free electrons) or p-type material (extra "holes," or missing electrons). Joining n-type and p-type regions creates a p-n junction, the building block of diodes, transistors, and every integrated circuit. This controllability, not the resistivity value itself, is what makes semiconductors the foundation of modern electronics.

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What Makes a Semiconductor Different

Silicon has a 1.12 eV band gap — small enough that thermal energy at room temperature promotes a handful of electrons into the conduction band, but large enough that pure silicon is still a poor conductor. What makes semiconductors technologically important isn't this middling conductivity; it's that engineers can dial it up or down by orders of magnitude using three independent knobs: temperature, light, and doping. No conductor or insulator offers this kind of control — copper's conductivity barely changes with light, and glass doesn't become a useful conductor no matter how much you dope it.

Common Misunderstanding: Many students think "semiconductor" simply means "resistivity between a conductor and an insulator." That's necessary but not sufficient — the defining trait is controllability. A material that happens to sit at 10⁻² Ω·m but can't be doped or modulated wouldn't earn semiconductor status in the way silicon does.

Intrinsic vs. Extrinsic Semiconductors

Intrinsic Semiconductors

Definition: Pure semiconductor material with no significant impurities — silicon (Si) and germanium (Ge) are the classic examples.

Explanation: In intrinsic silicon, every electron that gets thermally excited into the conduction band leaves behind a "hole" (an absence of an electron that behaves like a positive charge carrier) in the valence band. Electron and hole concentrations are always equal in an intrinsic semiconductor.

Example: At room temperature, intrinsic silicon has only about 1.5 × 10¹⁰ free carriers per cm³ — compare that to copper's roughly 10²³ free electrons per cm³. This is why pure silicon alone is nearly useless for building devices.

Real-World Example: Solar cells use lightly doped silicon layers, but the concept of intrinsic carrier generation is what governs a photodiode's "dark current" — the small current that flows even without light, purely from thermally generated electron-hole pairs.

Why It Matters: Intrinsic behavior sets the baseline. Every doping and temperature effect discussed below is measured relative to this thermally-generated carrier population.

Extrinsic Semiconductors (Doped)

Definition: Semiconductor material with controlled amounts of impurity atoms added to increase and control the number of free charge carriers.

Explanation: Doping introduces atoms with either one more or one fewer valence electron than silicon (which has 4).

• N-type: Doped with donor atoms (Group V — phosphorus, arsenic) that contribute an extra free electron. Majority carriers are electrons.
• P-type: Doped with acceptor atoms (Group III — boron, gallium) that create a "hole" that accepts an electron. Majority carriers are holes.

Example: Doping silicon with 1 phosphorus atom per million silicon atoms can increase conductivity by a factor of roughly 10,000 — a dramatic, controllable jump.

Real-World Example: Every transistor in your phone's processor is built from precisely patterned n-type and p-type regions, created by ion implantation of dopants at specific depths and concentrations across the silicon wafer.

Why It Matters: Without doping, there would be no way to create the p-n junctions that form diodes, transistors, and integrated circuits — doping is the single most important process step in semiconductor manufacturing.

Common Misunderstanding: Students sometimes think n-type material is negatively charged and p-type is positively charged overall. Both are electrically neutral — "n" and "p" describe the majority charge carrier type (electron or hole), not a net charge on the material.

How Semiconductors Conduct

1. At room temperature, most electrons occupy the valence band, below the band gap.
2. Thermal energy, an applied field, doping, or absorbed light can promote electrons into the conduction band.
3. Each promoted electron leaves a hole behind; both electrons (in the conduction band) and holes (in the valence band) contribute to current.
4. In doped material, the majority carrier type (electrons in n-type, holes in p-type) dominates conduction, while the minority carrier type still exists in much smaller numbers.

From Material to Device: The Fabrication Chain

1. Growing a single-crystal silicon ingot (Czochralski process)
2. Slicing the ingot into thin wafers and polishing them
3. Photolithography to pattern circuit features
4. Doping specific regions (ion implantation or diffusion) to create n-type and p-type zones
5. Depositing metal interconnects and insulating layers
6. Dicing the wafer into individual chips and packaging them

Real-World Example: A modern CPU may have billions of transistors on a single die, each one relying on precisely controlled doping profiles measured in nanometers — a testament to how far the doping principle from a single p-n junction has been scaled.

Visual: Semiconductor Classification and Carrier Types

Key Terms

Term	Definition
Band gap	Energy difference between valence and conduction bands (1.12 eV for Si)
Intrinsic semiconductor	Pure semiconductor with equal electron and hole concentrations
Extrinsic semiconductor	Semiconductor deliberately doped to increase carrier concentration
Doping	Adding controlled impurity atoms to modify a semiconductor's conductivity
Donor atom	Impurity with an extra valence electron, creates n-type material
Acceptor atom	Impurity with one fewer valence electron, creates p-type material
Majority carrier	The more abundant charge carrier type in a doped semiconductor
Minority carrier	The less abundant charge carrier type in a doped semiconductor
P-n junction	Interface between p-type and n-type regions; basis of diodes and transistors

Common Mistakes

Misconception 1: "N-type material has a net negative charge and p-type has a net positive charge." Why it's wrong: Doping only changes the type of majority carrier, not the overall charge balance — both regions remain electrically neutral because the doped atoms themselves are still bonded into the crystal lattice. Correct understanding: "N" and "p" describe the dominant carrier (electron or hole), not a net electrical charge on the bulk material.

Misconception 2: "More doping always means a better semiconductor." Why it's wrong: Excessive doping can degrade carrier mobility (dopant atoms scatter carriers) and can push a semiconductor toward metal-like "degenerate" behavior, losing the controllability that makes it useful. Correct understanding: Doping concentration is engineered precisely for the target application — light doping for high-voltage devices, heavy doping for low-resistance contacts.

Misconception 3: "A semiconductor conducts because it's 'halfway' between a metal and an insulator, like a partial conductor." Why it's wrong: This implies a fixed, static conductivity, when the entire point of a semiconductor is that its conductivity can shift by orders of magnitude under doping, light, or temperature — none of which changes a conductor's or insulator's behavior nearly as dramatically. Correct understanding: The band gap size (0.1–3 eV) allows external control of carrier population — that controllability, not a fixed "in-between" resistivity, is the defining feature.

Comparison and Connections

Aspect	Intrinsic Semiconductor	N-type	P-type
Carrier source	Thermal generation only	Donor impurities (P, As)	Acceptor impurities (B, Ga)
Majority carrier	Electrons = holes	Electrons	Holes
Conductivity at room temp	Very low	High, tunable by doping level	High, tunable by doping level
Typical use	Base wafer material	One side of a diode/BJT/MOSFET	Other side of a diode/BJT/MOSFET

Practice Questions

Recall

1. What is the band gap of silicon, and what does it represent physically?
2. Name the majority carrier in n-type and in p-type material.

Understanding 3. Explain why doping silicon with phosphorus increases its conductivity by orders of magnitude. 4. Why is "controllability" a better definition of a semiconductor than "resistivity between a conductor and insulator"?

Application 5. A photodiode needs to detect faint light. Explain, using the concept of intrinsic carrier generation, why cooling the photodiode reduces its noise ("dark current"). 6. You need a semiconductor region with very low resistance for a metal contact pad. Would you use light or heavy doping, and why?

Analysis 7. Compare intrinsic and extrinsic silicon in terms of temperature sensitivity — which one's conductivity is more strongly affected by a 20°C rise, and why? 8. A device requires both n-type and p-type regions in immediate contact. Explain what forms at that boundary and why it is the foundation of a diode.

Answer Guidance: For Q5, cooling reduces the number of thermally generated electron-hole pairs (fewer carriers crossing the band gap from heat alone), lowering dark current. For Q7, intrinsic silicon's conductivity is far more temperature-sensitive because its carrier count depends entirely on thermal excitation, whereas heavily doped extrinsic silicon's carrier count is dominated by the fixed dopant concentration, which barely changes with temperature. For Q8, a depletion region forms at the p-n junction due to carrier diffusion and recombination, creating a built-in potential that only allows current to flow in one direction — this is the basis of diode rectification.

FAQ

Q1: Why is silicon used instead of germanium, even though germanium was used in the first transistors? Silicon has a wider band gap (1.12 eV vs. 0.67 eV), giving it much better performance at higher temperatures, and it naturally forms a stable, high-quality oxide (SiO₂) that is essential for building insulating layers directly on the chip — germanium's oxide is not nearly as useful.

Q2: Does doping change what element the semiconductor is made of? No — doping adds an extremely small concentration of a different element (often 1 part in a million or less) into the existing crystal lattice; the semiconductor is still overwhelmingly silicon (or germanium), just with engineered impurity sites.

Q3: Can a semiconductor be doped with both n-type and p-type impurities in the same region? Yes, in a process called compensation — but the net majority carrier is determined by whichever dopant concentration is higher. This isn't common in practice because it wastes control precision, but it can happen unintentionally during multiple processing steps.

Q4: Why do semiconductors conduct better as temperature increases, unlike metals? In a semiconductor, more heat means more electrons gain enough energy to jump the band gap, creating more carriers overall. In a metal, carriers are already abundant, and added heat just increases lattice vibration, which scatters electrons and increases resistance instead.

Q5: What's the difference between doping and alloying? Doping introduces an extremely small, carefully controlled concentration of impurity atoms specifically to modify electrical carrier behavior. Alloying combines much larger proportions of different elements, typically to modify mechanical, thermal, or chemical properties (as in steel or brass), not primarily electrical conductivity type.

Quick Revision

• Semiconductors have a band gap of 0.1–3 eV; silicon's is 1.12 eV, germanium's is 0.67 eV.
• The defining trait of a semiconductor is controllability via doping, temperature, or light — not just mid-range resistivity.
• Intrinsic semiconductors are pure, with equal electron and hole concentrations from thermal generation alone.
• Extrinsic semiconductors are doped: n-type (donor atoms, Group V, majority carrier = electrons) or p-type (acceptor atoms, Group III, majority carrier = holes).
• N-type and p-type materials are both electrically neutral overall — "n" and "p" describe carrier type, not net charge.
• Doping can increase conductivity by orders of magnitude with impurity concentrations as low as 1 in a million.
• A p-n junction (n-type region touching p-type region) is the fundamental building block of diodes and transistors.
• Semiconductor conductivity increases with temperature (opposite of metals) because more carriers are thermally generated.
• Fabrication involves crystal growth, wafer slicing, photolithography, doping, and metallization.
• Silicon dominates over germanium due to wider band gap and native oxide (SiO₂) quality.

Related Topics

Prerequisites: Conductors and Insulators (band theory fundamentals).

Related Topics: Material Characterization (how doping profiles and defects are measured); Advanced Material Technologies (compound and nanoscale semiconductors).

Next Topics: Magnetic Materials, followed by Dielectrics — both essential for the passive components that surround semiconductor devices in real circuits.