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 a small amount of indium, a group III element, is added to pure selenium, a p-type semiconductor is formed. This is because indium acts as an acceptor impurity, introducing positive holes in the selenium lattice structure, making positive holes the majority charge carriers.

Step-by-step solution

Understand Semiconductor Types

Semiconductors have two main types based on their majority charge carriers: n-type (negative) and p-type (positive). N-type semiconductors are created by introducing donor impurities that can provide free electrons, while p-type semiconductors are created by introducing acceptor impurities which create positive holes.

Consider Selenium's Semiconductor Properties

Selenium is a semiconductor, which means it has an energy band gap that allows it to conduct electricity when energy is supplied. In its pure form, selenium has a balanced number of electrons and holes, making it an intrinsic semiconductor. Doping introduces an impurity that will skew the balance between electrons and holes, transforming the semiconductor into either n-type or p-type.

Identify Indium's Properties

Indium, the impurity added to selenium in this exercise, is a group III element in the periodic table. Group III elements are known to serve as acceptor impurities because they have one less electron in their outer shell than the semiconductor atoms they replace, leaving a hole in the lattice structure.

Determine the Type of Semiconductor Formed

Given that indium is a group III element and serves as an acceptor impurity, adding a small amount of indium to pure selenium will create a p-type semiconductor. This is because indium introduces positive holes in the selenium lattice structure, making the positive holes the majority charge carriers. In summary, the type of semiconductor formed when a small amount of indium impurity is added to pure selenium is a p-type semiconductor.