Conductors, Insulators, Semiconductors — Core Principles

Core Principles

Materials are classified as conductors, insulators, or semiconductors based on their ability to conduct electricity, which is fundamentally explained by the energy band theory. In solids, atomic energy levels broaden into energy bands: the valence band (VB) and the conduction band (CB).

The VB contains electrons involved in bonding, while the CB contains free electrons that contribute to current. The energy difference between the VB and CB is the forbidden energy gap ($E_g$). Conductors have overlapping VB and CB or a partially filled CB, meaning $E_g \approx 0$, allowing high conductivity.

Insulators have a large $E_g$ (typically $>3 \text{ eV}$), preventing electrons from easily moving to the CB, resulting in very low conductivity. Semiconductors have a small $E_g$ (typically $0.5 \text{ eV}$ to $3 \text{ eV}$).

At room temperature, some electrons can jump this gap, creating electron-hole pairs and enabling moderate conductivity that increases with temperature. This band gap concept is crucial for understanding material behavior in electronics.

Important Differences

vs Insulators and Semiconductors

Aspect	This Topic	Insulators and Semiconductors
Forbidden Energy Gap ($E_g$)	Very large ($>3 \text{ eV}$)	Small ($0.5 \text{ eV}$ to $3 \text{ eV}$)
Valence Band at 0 K	Completely filled	Completely filled
Conduction Band at 0 K	Completely empty	Completely empty
Conductivity at Room Temp.	Extremely low	Moderate (between conductors and insulators)
Effect of Temperature on Conductivity	Negligible increase (until breakdown)	Increases significantly
Charge Carriers	Virtually none	Electrons and holes
Examples	Glass, Rubber, Wood, Diamond	Silicon, Germanium, Gallium Arsenide

The fundamental distinction between insulators and semiconductors lies in the magnitude of their forbidden energy gap ($E_g$). Insulators possess a very large $E_g$ (typically $>3 \text{ eV}$), making it nearly impossible for electrons to jump to the conduction band at normal temperatures, hence their extremely low conductivity. Semiconductors, conversely, have a much smaller $E_g$ (typically $0.5 \text{ eV}$ to $3 \text{ eV}$). This allows a measurable number of electrons to gain thermal energy and cross the gap at room temperature, creating both electrons and holes as charge carriers, leading to moderate conductivity that is highly sensitive to temperature changes. Both have filled valence bands and empty conduction bands at absolute zero.