Problem 65 - Chemistry An Atoms First Approach

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?

Step-by-Step Solution

Answer

When a small amount of indium (group III element with 3 valence electrons) impurity is added to pure selenium (group VI semiconductor with 6 valence electrons), it creates an acceptor level in the energy band structure and a hole in the valence band. Consequently, the doped semiconductor acquires p-type properties, meaning it exhibits a majority of positive charge carriers (holes). Therefore, a p-type semiconductor is formed.

Step 1: Identify the semiconductor material and the impurity element

The semiconductor material given in the problem is selenium, and the impurity element to be added is indium.

Step 2: Determine the group of semiconductor and impurity element

Selenium (Se) belongs to group VI of the periodic table, which means it has 6 valence electrons. Indium (In) belongs to group III and has 3 valence electrons.

Step 3: Analyze the impurity level and its effect

When a group III element, like indium, is added to a group VI semiconductor, like selenium, the impurity has one fewer valence electron than the host semiconductor atoms. This creates an acceptor level in the energy band structure and a hole (a missing electron) in the valence band. Consequently, the doped semiconductor acquires p-type properties, meaning it now exhibits a majority of positive charge carriers (holes).

Step 4: State the type of semiconductor formed

When a small amount of indium impurity is added to pure selenium, a p-type semiconductor is formed.

Key Concepts

Semiconductor Doping

Doping is a deliberate process used in semiconductor manufacture where impurities are added to control the electrical properties of the material. Think of it like a pinch of salt that changes the taste of a dish. In the context of semiconductors, these 'flavors' are the conductivity characteristics. Normal semiconductors have balanced numbers of electrons and holes, but doping disrupts this balance to improve the material's ability to conduct electricity.

There are two types of doping: n-type, where negatively charged electrons are the majority, and p-type, which has a majority of 'holes', or the absence of electrons that positively affect conduction. In our exercise, selenium, an element with six valent electrons, is doped with indium, which has only three, creating a doped material that lacks sufficient electrons to fill the valence band thus forming holes. This p-type doping creates more holes as charge carriers, effectively altering the electrical conductivity of selenium.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. They are crucial because they're the ones involved in forming bonds with other atoms. More importantly, in the context of semiconductors, these electrons are the ones that can gain enough energy to jump into the conduction band and carry an electric current. In pure semiconductors at room temperature, there's a delicate balance between electrons and the spaces they leave behind, known as holes, when they move to conduct electricity.

When we add an element like indium to selenium, we're introducing atoms with fewer valence electrons. Since indium has only three valence electrons, when it replaces a selenium atom in the crystal lattice, it brings fewer electrons to the table—specifically, one less in this case—creating a p-type semiconductor with 'holes' pining for electrons.

Energy Band Structure

In solid-state physics, the energy band structure of a material depicts the ranges of energy that an electron within the material may have. It's divided into three primary parts: the conduction band, the valence band, and the forbidden gap in between them. The valence band is full of electrons at low energy states, while the conduction band is where electrons can move freely, conducting electricity.

However, electrons need enough energy, usually from heat or light, to leap from the valence band to the conduction band across the forbidden gap—a no-man's land for electron energy levels. Doping alters the energy band structure by introducing new energy states—donor or acceptor levels—near the conduction or valence bands. In p-type doping as seen in the selenium-indium case, acceptor levels are created above the valence band, which makes it easier for valence electrons to jump into these new states, leaving behind beneficial holes to enhance the material's conductivity toward a positive charge.