Question

Zinc oxide is a semiconductor. Its conductivity increases when it is heated in a vacuum but decreases when it is heated in oxygen. Account for these observations.

Short answer

Heating zinc oxide in a vacuum increases free electron concentration, enhancing conductivity, while heating in oxygen creates oxygen ions that capture free electrons, reducing the conductivity.

Step-by-step solution

Understanding the Nature of Semiconductors

Recognize that the conductivity of a semiconductor depends on the availability of free charge carriers which can be electrons or holes. The concentration of these charge carriers is affected by temperature and the chemical environment.

Effect of Heating in a Vacuum

Understand that when zinc oxide is heated in a vacuum, the increased temperature provides energy to the semiconductor, allowing more electrons to jump from the valence band to the conduction band, increasing the number of free charge carriers and thereby increasing conductivity.

Effect of Heating in Oxygen

When heated in oxygen, oxygen molecules react with the free electrons to form negative oxygen ions. These ions capture free electrons from the conduction band, which reduces the number of free charge carriers, decreasing the conductivity of zinc oxide.

Key concepts

Zinc Oxide as Semiconductor

Zinc oxide (ZnO) plays a vital role in the world of semiconductors due to its unique electrical properties. It is classified as a II-VI semiconductor, which means it's made from a combination of elements from group II and group VI of the periodic table. This material possesses a wide bandgap, enabling it to operate at high voltages and temperatures without undergoing breakdown.

In semiconductors like zinc oxide, electrical conductivity can be intricately modulated, making them versatile for a variety of electronic applications, from transistors to solar cells. Unlike metals, where conductivity decreases as temperature increases due to increased scattering of electrons, the conductivity in semiconductors like ZnO increases with temperature. This is because heat energy helps more electrons to overcome the bandgap, transitioning them from the valence band where they are bound, to the conduction band where they can freely move and contribute to electrical current.

Charge Carriers in Semiconductors

Charge carriers are the particles that carry charge through a semiconductor. In semiconductors like zinc oxide, these primarily include electrons and holes. Electrons are negative charge carriers that move through the conduction band, while holes are basically the absence of an electron in the valence band, acting as positive charge carriers.

The behavior and concentration of these charge carriers are what dictate the conductivity of a semiconductor. Doping, which is the process of adding impurities to a semiconductor, can increase the availability of one type of charge carrier over the other. For instance, an n-type semiconductor has more free electrons due to doping with elements that have more valence electrons, whereas a p-type semiconductor has more holes due to doping with elements that have fewer valence electrons. The delicate balance and interaction between these charge carriers are essential for the semiconductor's function in electronic devices.

Effect of Temperature on Semiconductors

Temperature has a significant impact on the behavior of semiconductors. As temperature rises, the thermal energy shakes the atomic lattice, which can cause more electrons to leap into the conduction band - a phenomenon we refer to as thermal excitation. This increase in charge carriers leads to higher conductivity.

In zinc oxide, a semiconductor with a broad energy bandgap, heating it in a vacuum causes a notable increase in conductivity. Why? Because the elevated temperatures provide enough energy for a substantial number of electrons to transition from the valence band to the conduction band. Consequently, we observe an increased number of charge carriers, resulting in enhanced conductivity; a principle that is crucial for understanding semiconductors' behavior under various temperature conditions.

Semiconductor Conductivity in Different Environments

The conductivity of semiconductors like zinc oxide is not only affected by temperature, but also by the environment in which they are present. When zinc oxide is heated in oxygen, a distinct decrease in conductivity is observed. This happens because the oxygen molecules in the environment react with the free electrons to form negative oxygen ions.

These newly formed ions essentially capture the free electrons from the conduction band, reducing the number of available charge carriers and, hence, the conductivity. This environmental sensitivity is vital when designing sensors and other devices that rely on semiconductors' conductive properties. Understanding the interplay between a semiconductor's conductivity and its environment is essential for developing robust and reliable electronic components.