Intrinsic vs Extrinsic

Part of The Transistor

Intrinsic semiconductors are pure materials with equal numbers of electrons and holes; extrinsic semiconductors have been deliberately doped with impurities to create an excess of one carrier type — the foundation of all practical devices.

Why This Matters

Pure silicon is nearly useless for electronics. It conducts poorly, it does so equally in all directions, and its conductivity cannot be controlled. The transformation from useless pure crystal to the basis of all computing technology happens through doping — intentionally contaminating the silicon with precise amounts of specific impurities.

Understanding the difference between intrinsic (pure) and extrinsic (doped) semiconductors explains why transistors work at all. It explains why one side of a junction is positive and the other negative, why current flows only one way through a diode, and why a tiny base current can control a large collector current. These are not arbitrary behaviors — they flow directly from the physics of intrinsic versus extrinsic material.

For anyone attempting to fabricate semiconductors from salvaged or raw materials, this distinction also determines which materials are worth processing and which impurities must be avoided or deliberately introduced.

Intrinsic Semiconductors

An intrinsic semiconductor is a perfectly pure semiconductor crystal. In silicon, each atom forms four covalent bonds with its neighbors. At absolute zero (−273°C), all electrons are locked in these bonds — the material is a perfect insulator.

At room temperature, thermal energy breaks a small fraction of these bonds, freeing electrons. Each freed electron leaves behind a “hole” — a missing bond that acts as a mobile positive charge carrier. Because electrons are always freed in pairs with holes, an intrinsic semiconductor always has equal concentrations of both carriers.

Key properties of intrinsic semiconductors:

Property	Silicon at 25°C	Germanium at 25°C
Carrier concentration	~1.5 × 10¹⁰ cm⁻³	~2.4 × 10¹³ cm⁻³
Resistivity	~2,300 Ω·cm	~47 Ω·cm
Band gap	1.12 eV	0.67 eV

The carrier concentration numbers look large but are tiny compared to the total atom density (~5 × 10²² atoms/cm³). Only about 1 in every 10 trillion silicon atoms has a free electron at room temperature — hence the near-insulating behavior.

Germanium has a smaller band gap, meaning more bonds break thermally. This gives germanium better conductivity at room temperature but also more problematic leakage current as temperature rises.

Extrinsic Semiconductors: The Doped State

Extrinsic semiconductors have been doped — carefully contaminated with tiny amounts of impurity atoms that either donate extra electrons or create extra holes. This completely transforms the electrical properties.

N-Type Material (Electron-Rich)

N-type doping uses Group V elements (phosphorus, arsenic, antimony). Each dopant atom has five valence electrons. When it sits in the silicon lattice in place of a silicon atom (which has four valence electrons), four bonds form with neighbors — and one electron is left over. This extra electron is very loosely held and can move freely through the crystal with minimal thermal activation.

Result: many more free electrons than holes. Electrons are the majority carriers; holes are the minority carriers.

The conductivity increase is dramatic. Adding just 1 part per million phosphorus to silicon increases electron concentration from ~10¹⁰ to ~5 × 10¹⁶ cm⁻³ — a factor of 5 million. Resistivity drops from 2,300 Ω·cm to about 0.1 Ω·cm.

P-Type Material (Hole-Rich)

P-type doping uses Group III elements (boron, gallium, indium, aluminum). Each dopant atom has three valence electrons. When placed in the silicon lattice, it can only form three bonds, leaving one unsatisfied bond — a hole. Neighboring electrons can jump into this hole, effectively moving the hole through the crystal as a mobile positive charge carrier.

Result: many more holes than free electrons. Holes are the majority carriers; electrons are the minority carriers.

Parameter	Intrinsic Si	N-Type Si	P-Type Si
Free electrons	~10¹⁰ cm⁻³	~10¹⁶–10¹⁸ cm⁻³	~10⁴ cm⁻³
Holes	~10¹⁰ cm⁻³	~10⁴ cm⁻³	~10¹⁶–10¹⁸ cm⁻³
Net charge	Neutral	Neutral	Neutral
Majority carrier	Equal	Electrons	Holes

The Material Stays Electrically Neutral

Despite having more of one carrier type, both n-type and p-type material are electrically neutral overall. The extra electrons in n-type are balanced by the positive charge of the phosphorus ions locked in the lattice. The holes in p-type are balanced by negative boron ions. Current flow, not static charge, is what changes.

Temperature Dependence: A Critical Difference

Intrinsic and extrinsic materials behave very differently with temperature:

Intrinsic material: Conductivity rises steeply with temperature as more bonds break. At high enough temperatures, the material acts essentially like a conductor. This is a problem — intrinsic silicon at 150°C has radically different properties than at 25°C.

Extrinsic material at low-to-moderate temperatures: Conductivity is dominated by the dopant, which is fully ionized at room temperature. Conductivity actually decreases slightly as temperature rises (like a metal) because carrier mobility decreases. The behavior is relatively predictable.

Extrinsic material at high temperatures: As temperature rises, thermally generated intrinsic carriers eventually outnumber the dopant-supplied carriers. The material reverts to intrinsic-like behavior. For silicon, this becomes significant above ~150–200°C. For germanium, the problem starts above ~70–80°C — a major reason germanium transistors were replaced by silicon.

Identifying Intrinsic vs Extrinsic Material

Four-Point Probe Resistance Measurement

Measure resistivity: intrinsic silicon has very high resistivity (thousands of ohm-cm), while doped silicon is orders of magnitude lower.

Hot-Point Probe Test

To determine carrier type in an unknown semiconductor:

1. Heat a metal probe to ~200°C
2. Touch it to the semiconductor surface alongside a cold probe
3. Connect a voltmeter between probes
4. N-type: hot probe is negative relative to cold (electrons diffuse toward cold end)
5. P-type: hot probe is positive relative to cold

Hall Effect Measurement

A magnetic field deflects moving carriers — electrons to one side, holes to the other. By measuring the transverse voltage produced when current flows through a semiconductor in a magnetic field, you can determine both carrier type and concentration. This requires a sensitive voltmeter and a reasonably strong permanent magnet.

Practical Implications for Fabrication

If you are attempting to grow semiconductor crystals from scratch:

• Starting material purity is everything. Even parts-per-billion of unwanted impurities produce uncontrolled doping. Intrinsic silicon requires refining to 9N purity (99.9999999%).
• Germanium is more forgiving. Natural germanite ore is relatively rare but germanium can be refined more easily than silicon. Early transistors used germanium because ultra-pure silicon was harder to produce.
• Controlled doping is the goal. The transition from useless intrinsic material to useful extrinsic material requires adding impurities in controlled amounts — parts per million to parts per billion. This is the core fabrication challenge.

Summary

Intrinsic vs Extrinsic — At a Glance

• Intrinsic semiconductors are pure materials with equal, small concentrations of electrons and holes — near-insulating and impractical
• Extrinsic semiconductors are doped with controlled impurities: n-type (Group V) adds free electrons, p-type (Group III) adds holes
• Doping increases carrier concentration by millions of times, reducing resistivity from thousands to fractions of an ohm-cm
• Both types remain electrically neutral — it is mobile carrier concentration, not charge, that changes
• Temperature causes extrinsic material to revert to intrinsic behavior above ~150°C (silicon) or ~80°C (germanium)
• Hot-point probe test identifies carrier type; resistivity measurement distinguishes intrinsic from extrinsic material