Semiconductors Explained — Bands, Doping and the pn Junction
Band gaps, n-type and p-type doping, and how the pn junction turns silicon into diodes and transistors — the physics inside every chip.
Quick answer
A semiconductor conducts only when electrons gain enough energy to jump the band gap into the conduction band — about 1.1 eV in silicon. Adding trace impurities (doping) precisely controls how many mobile electrons or holes exist, turning an insulator-like crystal into the switchable material of all electronics.
Unit: electronvolt (eV) for band gaps
Example: Phosphorus has 5 valence electrons to silicon's 4. Doping silicon with phosphorus donates one free electron per atom — n-type material whose conductivity can be a million times higher than pure silicon's.
What you will learn
- Define the band gap and quote values for Si, Ge, and GaAs
- Explain why heating increases a semiconductor's conductivity
- Distinguish n-type from p-type doping
- Describe the depletion region of a pn junction
- Explain in one paragraph how a transistor controls current
Between conductors and insulators
Metals conduct because electrons roam freely; insulators lock electrons to atoms. Semiconductors sit between — and their conductivity is wildly tunable: it rises steeply with temperature (the opposite of metals) and explodes with tiny impurity additions.
That tunability is the whole game. A material whose conductivity you can dial from insulating to conducting, and switch with voltage, is a material you can build logic from.
Energy bands and the band gap
In a crystal, atomic energy levels smear into bands. The filled valence band holds bound electrons; the empty conduction band above it holds mobile ones. Between them lies the forbidden band gap — the toll every electron must pay to conduct.
Metals have overlapping bands (no toll). Insulators have huge gaps (several eV — unaffordable). Semiconductors have small gaps: silicon eV, germanium eV, gallium arsenide eV. Room-temperature thermal energy is only ~0.026 eV, so just a trickle of electrons makes the jump — until you help them.
Intrinsic behavior: heat helps
In pure (intrinsic) silicon, heat occasionally kicks an electron across the gap, leaving a hole — a missing electron that behaves as a positive charge carrier, drifting the opposite way. Each excitation creates one electron and one hole, so conductivity grows exponentially with temperature.
This is backwards from metals, where heating increases atomic jiggling and hinders electron flow. If a material conducts better hot than cold, you are looking at semiconductor physics.
Doping I: n-type
Doping replaces a few silicon atoms with neighbors from the periodic table. Phosphorus (group V) has five valence electrons to silicon's four: four join the crystal bonds, and the fifth sits in a donor level just below the conduction band, needing only a whisper of thermal energy to break free.
The result is n-type material: current carried mostly by negative electrons. Doping levels are absurdly small — one impurity per million silicon atoms is heavy doping — yet conductivity can jump by factors of a million.
Doping II: p-type
Boron (group III) has only three valence electrons, so one bond goes begging — an acceptor level just above the valence band eagerly grabs a neighboring electron, leaving a mobile hole behind. Now the majority carriers are positive: p-type material.
Holes are not bookkeeping fiction. They have effective mass, they drift in electric fields, and they carry real current — just think of a bubble rising through water while the water sinks around it.
The pn junction: where the magic happens
Put p-type against n-type and electrons diffuse across, filling holes near the boundary. This creates the depletion region — a zone swept clean of mobile carriers, with uncovered ions setting up a built-in electric field that stops further diffusion.
Apply voltage forward (pushing carriers toward the junction) and current flows; apply it reverse and the depletion region widens, blocking current. That one-way behavior is a diode — rectification, the basis of every power supply.
A pn junction conducts in forward bias and blocks in reverse bias. Every LED, solar cell, and rectifier is this junction with different decorations.
Transistors in one paragraph
Sandwich a thin p-type layer between two n-type regions (npn) and you get a transistor. A small current into the middle (base) layer lets a large current flow between the outer (collector and emitter) layers — the base voltage opens or closes the floodgate. As a switch it makes digital logic; as a valve it makes amplifiers.
Modern chips pack billions of these switches onto a fingernail of silicon, and every one is controlled doping plus pn junctions.
Light in, electricity out: the photovoltaic effect
A photon with energy above the band gap can kick an electron across it, creating an electron-hole pair. Build a pn junction around that process and the built-in field sweeps the carriers apart before they recombine — current flows, and you have a solar cell.
Photons below the gap pass through uselessly; photons far above it waste the excess as heat. That is why solar efficiency has a theoretical ceiling (about 33% for single-junction silicon) and why multi-layer cells stack different band gaps.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Band gap | eV | Energy to promote an electron to the conduction band | |
| Electronvolt | eV | Energy an electron gains crossing 1 volt ≈ 1.6×10⁻¹⁹ J | |
| Carrier concentrations | per m³ | Densities of electrons / holes | |
| Donor impurity | — | Group-V atom donating a free electron (n-type) | |
| Acceptor impurity | — | Group-III atom creating a mobile hole (p-type) |
Common mistakes
Holes drift, carry current, and have measurable effective mass. In p-type material they are the majority carrier, as real as electrons for every circuit calculation.
Doping is controlled to parts per million with atomic precision. It is the most deliberate impurity addition in all of engineering, not dirt.
The mechanism differs completely: conduction needs thermal excitation across a gap, so conductivity rises with temperature — the opposite of metals. Different physics, not weaker physics.
Real-world applications
Microprocessors and memory chips are billions of transistors on silicon.
Solar cells convert sunlight via the photovoltaic effect at pn junctions.
LEDs and laser diodes emit light when carriers recombine across the gap.
Temperature and light sensors exploit the gap's sensitivity to heat and photons.
Frequently asked questions
Why silicon and not germanium?
Silicon's 1.1 eV gap gives low leakage current at room temperature (germanium's 0.7 eV leaks more), it forms a superb native oxide (SiO₂) for insulating layers, and it is the second most abundant element in Earth's crust. Germanium ruled the 1950s; silicon won on all three counts.
What is a hole, really?
When an electron leaves the valence band, the vacancy behaves as a positive particle: neighboring electrons hop into it, so the vacancy moves opposite to electron flow. It has charge +e and an effective mass set by the band structure. For circuit math, treat it as a genuine positive carrier.
How does a diode block reverse current?
Reverse bias pulls carriers away from the junction, widening the carrier-free depletion region. With no mobile charges there, current can't cross — only a tiny leakage flows. Forward bias floods the region with carriers instead, and current flows freely.
What does the band gap physically mean?
It is the minimum energy an electron needs to break free of its atomic bonds and roam the crystal. Below the gap, electrons are bound and the material insulates; above it, they conduct. The gap's size decides whether a crystal is a metal, semiconductor, or insulator.