Conductors, insulators, and semiconductors differ in one key way: how much energy an electron needs before it can break free and carry current. Metals need almost none, insulators need a lot, and semiconductors such as silicon sit in between. That middle ground is what makes silicon controllable, and controllability is the foundation of every logic gate you have ever drawn.
12 min read | Beginner Level
What You'll Learn
- The difference between the valence band and the conduction band
- Why copper conducts freely and silicon dioxide does not
- How the band gap sets a material's behaviour, using proportions instead of equations
- Where each material class appears on a real chip
What Makes a Material Conduct?
Electric current is simply charge on the move. For a material to conduct, it needs free carriers - charged particles that can drift when a voltage is applied. In solids, those carriers are electrons that have escaped the pull of their parent atoms.
Every material has plenty of electrons. The question is how many of them are free to move at room temperature. That single number separates the three classes:
| Material | Free electrons per cm3 (room temp) | Resistivity (Ω·cm) | Class |
|---|---|---|---|
| Copper | ~8.5 × 1022 (about one per atom) | ~1.7 × 10-6 | Conductor |
| Pure silicon | ~1 × 1010 (about one per 5 trillion atoms) | ~2.3 × 105 | Semiconductor |
| Silicon dioxide | Effectively zero | > 1014 | Insulator |
Copper conducts about a hundred billion times better than pure silicon. Silicon, in turn, conducts vastly better than the oxide grown on top of it. The rest of this tutorial explains where those huge differences come from.
Valence Band vs. Conduction Band
An isolated atom has electrons sitting at fixed energy levels. When billions of atoms pack together in a crystal, those levels smear out into continuous ranges called energy bands. Two bands matter for conduction:
- Valence band - the energy range of electrons that are tied up in bonds between neighbouring atoms. These electrons hold the crystal together and cannot wander.
- Conduction band - the energy range where electrons are released from bonds and can move freely through the crystal.
Between the two bands there may be a range of energies that no electron is allowed to have. This forbidden range is the band gap, measured in electron-volts (eV). One eV is the energy an electron picks up when it moves across a 1 V potential difference - a convenient unit at atomic scale.
The conduction story then becomes a simple chain of cause and effect:
Cause-and-effect chain for conduction
band gap size
-> how hard it is for a bonded electron to break free
-> how many electrons reach the conduction band
-> how many free carriers exist
-> how much current flows for a given voltage
smaller gap -> more free carriers -> lower resistance
larger gap -> fewer free carriers -> higher resistance
Conductors: Why Copper Conducts Freely
In a metal like copper, each atom has one loosely held outer electron. In the crystal, the valence and conduction bands overlap, so there is no gap at all. The outer electrons are already free before you apply any voltage. They form a "sea" of mobile charge shared by the whole crystal.
Because the carrier count is already at its maximum, you cannot meaningfully change a metal's conductivity with a small voltage or a little impurity. Copper is excellent at carrying current, but it is not controllable. That makes it perfect for wires and useless as a switch.
Where Are Conductors Used on a Chip?
The metal interconnect stack on a modern chip is mostly copper, with some tungsten or cobalt in the smallest local contacts. These wires distribute power and carry every signal between standard cells. Even though copper conducts extremely well, the wires are so thin that their resistance becomes a major source of delay and voltage drop in advanced nodes.
Insulators: Electrons Locked in Place
In an insulator such as silicon dioxide (SiO2), every electron is locked into a strong bond. The band gap is large - around 9 eV for SiO2. At room temperature, the random thermal energy available to an electron is only about 0.026 eV. That is hundreds of times smaller than the step it would need to take, so practically no electron ever makes the jump.
Insulators are just as important on a chip as conductors. They separate wires from one another, isolate neighbouring transistors, and form the thin layer between a transistor's gate and its channel.
Semiconductors: Why Silicon Sits in Between
Silicon has four outer electrons, and each one forms a bond with a neighbouring silicon atom. The band gap is about 1.12 eV. That is small enough that a few electrons gain enough thermal energy to break free at room temperature, but large enough that the vast majority stay bonded.
This "in between" position is the whole point. Pure silicon is a poor conductor on its own, but its carrier count can be raised by factors of millions through small, deliberate changes:
- adding a tiny amount of impurity atoms (doping)
- applying an electric field (the principle behind the MOSFET gate)
- shining light on it (the principle behind image sensors and solar cells)
- raising the temperature (usually unwanted, and the root cause of leakage)
A material whose conductivity can be dialled up and down on command is exactly what you need to build a switch. That is the defining feature of a semiconductor - not "medium resistance", but controllable resistance.
| Material | Band gap (eV) | Class | Where it appears in chip design |
|---|---|---|---|
| Copper | None (bands overlap) | Conductor | Interconnect wires, power grid |
| Germanium | ~0.66 | Semiconductor | Mixed with silicon (SiGe) to speed up PMOS channels and RF transistors |
| Silicon | ~1.12 | Semiconductor | Transistors in almost every digital IC |
| Gallium arsenide | ~1.42 | Semiconductor | High-frequency RF amplifiers |
| Gallium nitride | ~3.4 | Wide-gap semiconductor | Power converters, fast chargers, RF power |
| Silicon nitride | ~5 | Insulator | Spacers and etch-stop layers |
| Silicon dioxide | ~9 | Insulator | Isolation between transistors and wires |
The boundaries between classes are conventions, not physical laws. A rough working rule: overlapping bands means conductor, a gap of roughly 0.5 to 3.5 eV means semiconductor, and materials above about 4 eV usually behave as insulators. Some ultra-wide-gap materials, such as diamond (~5.5 eV), can still be doped and used as semiconductors in specialised devices.
Quick Check
Q: Pure silicon has a band gap of 1.12 eV and room-temperature thermal energy is about 0.026 eV. Why does silicon conduct at all?
Show Answer
Thermal energy is an average. Electrons constantly exchange energy with the vibrating crystal, and a very small fraction briefly gain far more than the average - enough to cross the 1.12 eV gap. That tiny fraction, about one electron per 5 trillion atoms, gives pure silicon its small but non-zero conductivity.
The Band Gap as a Barrier Height
A useful way to visualise the band gap is as a step an electron has to climb. The valence band is the floor where bonded electrons live. The conduction band is a ledge above it where electrons can roam freely. Thermal energy gives electrons random kicks upward.
- In a metal, the floor and the ledge are at the same level - no step to climb.
- In silicon, the ledge is a modest step up. Most kicks fall short, but a rare strong one makes it.
- In an insulator, the ledge is so high that no realistic kick ever reaches it.
The key insight is that the number of electrons that make it does not fall in a straight line as the step gets taller. It falls steeply. At room temperature, every additional 0.1 eV of band gap cuts the number of thermally freed carriers by roughly a factor of seven.
Proportion rule: band gap vs. free carriers (room temperature)
gap +0.1 eV -> free carriers roughly / 7
gap +0.2 eV -> free carriers roughly / 50
gap +0.5 eV -> free carriers roughly / 16,000
Germanium (0.66 eV) vs. silicon (1.12 eV):
pure germanium has thousands of times more free carriers
-> much higher leakage, a major reason silicon replaced it
Why Did Silicon Win?
Early transistors were made of germanium, but its small band gap produced high leakage that got worse quickly with heat. Silicon's larger gap keeps leakage low at normal operating temperatures. Silicon also grows a high-quality insulating oxide on its own surface, which made the MOSFET and the planar manufacturing process practical. That combination is why nearly every digital chip today is silicon.
Temperature: Metals and Semiconductors Respond Oppositely
Heat affects every material, but not in the same direction:
| Material | What heat does | Net effect on resistance |
|---|---|---|
| Metal (copper) | Carrier count is already fixed; stronger lattice vibrations scatter the moving electrons more often | Resistance goes up |
| Pure semiconductor (silicon) | Many more electrons gain enough energy to cross the band gap | Resistance goes down, sharply |
| Insulator (SiO2) | Gap is so large that the extra thermal energy barely matters | Stays extremely high |
This difference shows up directly in chip design. Hot copper wires get slightly slower, while hot silicon produces more stray carriers - which appear as leakage current in transistors that are supposed to be OFF.
Where Are Wide-Band-Gap Semiconductors Used?
Power converters in electric-vehicle chargers, solar inverters, and compact laptop chargers increasingly use gallium nitride or silicon carbide. Their wide band gap keeps leakage low even at high temperatures and lets them withstand much higher voltages than silicon for the same size. The same property that makes them great power switches makes them expensive and less practical for dense digital logic.
Quick Check
Q: A chip heats up from 25 °C to 100 °C. Does the resistance of its copper wires and of its undoped silicon regions move in the same direction?
Show Answer
No. Copper resistance rises because electrons scatter more off the vibrating lattice. Silicon resistance falls because many more electrons cross the band gap and become free carriers.
Common Mistakes
Mistake #1: Thinking "semiconductor" means "medium resistance"
A semiconductor is defined by its band gap and the fact that its conductivity can be changed by doping, fields, light, or heat. Some semiconductors conduct very poorly when pure, and heavily doped silicon is only about 500 times more resistive than copper - over 100 million times more conductive than pure silicon. What matters for chip design is that the conductivity is controllable, not that it happens to sit in the middle.
Mistake #2: Picturing the band gap as a physical gap in space
The band gap is a range of energy, not a distance. Electrons in the valence band and the conduction band occupy the same physical crystal. An electron "crossing the gap" means it gained enough energy to leave its bond, not that it moved across an empty region.
Mistake #3: Assuming insulators have no electrons
Silicon dioxide has as many electrons per atom as any other material. They are all locked into strong bonds, so none are free to carry current. Conduction depends on free carriers, not on the total electron count.
Mistake #4: Assuming heat always increases resistance
That rule holds for metals, but semiconductors do the opposite: heat frees more carriers and lowers resistance. Carrying the metal intuition over to silicon hides the real problem at high temperature, which is extra leakage current.
Practice Exercise
Challenge: Pick the Material for the Job
You are choosing materials for three parts of a design. For each one, pick copper, silicon, gallium nitride, or silicon dioxide, and justify your choice using the band-gap picture:
- A layer that must stop any current flowing between two stacked wires
- A switch that must turn ON and OFF billions of times per second in dense logic
- A power switch in a charger that runs hot and blocks several hundred volts
Hint
Ask two questions for each part: how many free carriers do I want, and do I need to change that number on command?
Solution
1. Silicon dioxide - large band gap (~9 eV), effectively zero free carriers, so it blocks current between wires. 2. Silicon - moderate band gap (~1.12 eV). Few free carriers at rest, but the count can be raised on command, which makes a switch possible. It is also cheap to manufacture at very high density. 3. Gallium nitride - wide band gap (~3.4 eV). Leakage stays low even when hot, and the material withstands high voltages.
Copper is the only option that is never a switch: its carrier count is fixed and cannot be controlled.
Summary
In this tutorial, you learned:
- Conduction depends on free carriers, not on how many electrons a material has in total
- The valence band holds bonded electrons; the conduction band holds free ones
- The band gap is the energy step between them: no gap for metals, a moderate gap for semiconductors, a large gap for insulators
- Free carrier count falls steeply as the band gap grows - roughly 7× fewer for every extra 0.1 eV at room temperature
- Silicon's value lies in controllable conductivity, which is what makes transistors possible
- Heat raises metal resistance but lowers semiconductor resistance, which is the root of leakage
