Question

Selenium is a semiconductor used in photocopying machines. What type of semiconductor would be formed if a small amount of indium impurity is added to pure selenium?

Short answer

When indium (In) is added as an impurity to selenium (Se), it replaces some of the selenium atoms, resulting in a lack of three electrons and creating an excess of "holes." As a result, the majority carriers in the doped material will be holes, making the type of semiconductor formed a p-type semiconductor.

Step-by-step solution

Understanding Semiconductors and Doping

Semiconductors are materials that have electrical conductivity between that of insulators and conductors, usually achieved by adding impurities (doping) to the material. There are two types of semiconductors: n-type and p-type. N-type semiconductors are formed when the doping material introduces an excess of electrons, making the majority carriers of the semiconductor electrons. In contrast, p-type semiconductors are formed when doping creates a lack of electrons or an excess of "holes," which are essentially positive charge carriers. Thus, the majority carriers in p-type semiconductors are holes.

Investigating Selenium and Indium Properties

Selenium (Se) is a group 16 element in the periodic table and has six valence electrons. Indium (In) is a group 13 element with three valence electrons. When doping occurs, impurity atoms replace some of the original semiconductor atoms, causing an excess or lack of electrons in the material. To determine which type of semiconductor is formed when indium is added to selenium, we need to analyze how the number of valence electrons changes as a result of the doping process.

Analyzing the Doping Process

When indium (In) is added as an impurity to selenium (Se), it replaces some of the selenium atoms. Since indium has three valence electrons, whereas selenium has six, the addition of indium results in a lack of three electrons where an indium atom replaces a selenium atom. In this case, there will be a lack of electrons or an excess of "holes," which are essentially positive charge carriers. As a result, the majority carriers in the doped material will be holes.

Determining the Type of Semiconductor

Considering the excess "holes" created through doping, we can conclude that the type of semiconductor formed when indium is added to pure selenium is a p-type semiconductor, as the majority carriers in the material are holes.