Problem 115 - Chemistry: The Central Science in SI Units, Global Edition

Question

Unlike metals, semiconductors increase their conductivity as you heat them (up to a point). Suggest an explanation.

Step-by-Step Solution

Answer

Semiconductors increase their conductivity as temperature rises (up to a point) because the increased temperature provides necessary energy to transition more electrons from the valence band to the conduction band. This increase in the number of charge carriers (electrons) enhances conductivity, unlike metals where lattice vibrations interfere with electron movement at higher temperatures. However, this behavior is observed only up to a certain temperature, where other factors may counteract this effect.

Step 1: Understand semiconductors properties

Semiconductors are materials that have electrical conductivity between that of insulators and conductors (e.g., metals). Their valence electrons are not as tightly bound as in insulators but are not freely moving like in metals. The most common example of semiconductor materials is silicon (Si) and germanium (Ge).

Step 2: Band structure

The energy and position of electrons in materials are typically arranged in energy bands. There are two main energy bands in semiconductors - the valence band that contains the valence electrons responsible for bonding within the material and the conduction band, which is separated from the valence band by a bandgap. Electrons in the conduction band are free to move around the material, and hence, contribute to electrical conductivity. For good electrical conductivity, the electrons must be able to transition from the valence band to the conduction band.

Step 3: Temperature effects

As the temperature increases, the energy of the electrons in the material also increases. This added energy can cause some electrons in the valence band to gain enough energy to overcome the bandgap and transition into the conduction band. When more electrons are in the conduction band, the conductivity of the semiconductor increases.

Step 4: Explain the phenomenon

Semiconductors increase their conductivity as you heat them (up to a point) because the increased temperature provides the necessary energy to move more electrons from the valence band to the conduction band. Unlike metals, whose lattice vibrations increase with temperature, leading to a decrease in conductivity due to more frequent electron scattering, the dominant effect in semiconductors is the increase in the number of charge carriers (electrons). However, this effect is only observed up to a certain temperature, beyond which other factors can counteract this behavior.

Key Concepts

Bandgap

The bandgap is a crucial characteristic of semiconductors. It represents the energy difference between the valence band and the conduction band. Electrons must overcome this energy barrier to move from the valence band to the conduction band where they can contribute to electrical conductivity.
Typically, semiconductors like silicon have a bandgap around 1.1 eV (electron volts).
This bandgap is not too wide, allowing electrons to jump across it when provided with enough energy.

• Unlike in insulators, the bandgap in semiconductors is small enough to permit electron flow under certain conditions.
• A smaller bandgap means it requires less energy for electrons to jump to the conduction band.

Understanding the bandgap is essential as it defines the material's ability to conduct electricity.

Conductivity

Conductivity in semiconductors is the measure of a material’s ability to allow the flow of electric current. For semiconductors, this heavily depends on electron movement between the valence and conduction bands.
When electrons gain access to the conduction band, they can move freely, thereby increasing conductivity.

• Conductivity is typically lower in semiconductors at low temperatures due to the fewer electrons in the conduction band.
• As temperature rises, more electrons gain energy to cross the bandgap, enhancing conductivity.

The balance between electron availability and movement across bands defines the effective conductivity of semiconductors.

Temperature Effects

Temperature has a substantial impact on the behavior of semiconductors. Unlike metals, where increasing temperature often leads to decreased conductivity, semiconductors exhibit an increase up to a certain point.
As temperature rises, electrons in the semiconductor gain kinetic energy.

• This energy enables more electrons to overcome the bandgap and enter the conduction band.
• More electrons in the conduction band increase the overall charge carriers, thus enhancing conductivity.

However, this trend only continues until a specific point; beyond that, excessive thermal agitation may disrupt the orderly flow of electrons.

Valence Band

The valence band is an energy band in semiconductors where valence electrons reside. These electrons are associated with atom bonding and determining the material's electronic properties.
At absolute zero, the valence band is fully occupied.

• Electrons in the valence band are not free to move, limiting their role in conductivity.
• Additional energy is required for electrons to escape the valence band and contribute to electrical conduction by entering the conduction band.

Understanding the dynamics within the valence band helps predict how a semiconductor will react under varying thermal conditions.

Conduction Band

The conduction band is where electrons reside after they gain sufficient energy to surpass the bandgap in semiconductors. This band is crucial for the material's ability to conduct electricity.
Electrons in the conduction band are what allow electrical current to flow through the semiconductor.

• To reach the conduction band, electrons require an energy boost, often provided by heat.
• Once in the conduction band, these electrons are free to move, carrying electric charge with them and enhancing conductivity.

The ability of electrons to transition and remain in the conduction band is what sets semiconductors apart from insulators and metals in terms of conductivity.